a next generation light source

a next generation light sourcea transformative tool for energy science

Proposal for approval of Conceptual Design (CD-0)Submitted to the U.S. Department of EnergyOffice of Basic Energy Sciences

December 2010

(TOC continued)

Cover — from a concept by Greg Engel

Disclaimer

This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information,

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CSO 21173-2

Table of contentsTable of contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiScientific and technical contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iv1 Needs for a next generation light source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Outline of the Current Proposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Overview of revolutionary X-ray science tools at NGLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1 Multi-dimensional X-ray Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2 Ultrafast Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3 Coherent Scattering and Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

3 NGLS – science drivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.1 Photosynthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93.2 Fundamental Energy and Charge Dynamics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.3 Advanced Combustion Science. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263.4 Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.5 Nanoscale Materials Nucleation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 433.6 Dynamical Nanoscale Heterogeneity in Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 493.7 Quantum Materials. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 563.8 Spin and Magnetization at the Nanoscale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 713.9 Biological Systems: Imaging Dynamics and Function. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

4 New techniques enabled by NGLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.1 Imaging structure and function in heterogeneous ensembles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 914.2 X-ray Imaging: From High Resolution to High Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1014.3 Multidimensional X-ray Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

5 Proposed facility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095.1 Capability Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

5.1.1 Requirements for the NGLS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1095.1.2 Capabilities of Present Facilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .110

5.2 Alternate Approaches. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1115.2.1 Conventional Pulsed Linacs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1125.2.2 Energy Recovery Linacs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1145.2.3 Third- and Fourth-Generation Storage Rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1145.2.4 HHG Laser Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .115

5.3 NGLS: A Transformative Tool for X-Ray Science . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1155.3.1 Machine Overview and Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1155.3.2 Layout, Conventional Facilities, and Utilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .118

5.4 Design Considerations and Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1195.4.1 Overview of FEL Physics and Technology Challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1195.4.2 Injector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1215.4.3 Linac . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1275.4.4 Beam Spreader . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1335.4.5 FEL Beamlines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1345.4.6 Beam Dumps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1445.4.7 Timing and Synchronization Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1455.4.8 Instrumentation and Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1465.4.9 Radiation Protection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

6 Experimental systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516.2 Overall Beamline Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1516.3 Mirror Damage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1536.4 Split and Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1536.5 Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1546.6 Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

7 Future upgrades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1578 Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

8.1 Cost Estimate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1598.2 Schedule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1608.3 Risk Management and R&D. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1608.4 Organization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1618.5 Environment, Safety and Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163Appendix 1 – X-ray Interactions and Non-Disruptive Probing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163Appendix 2 – Workshops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164Appendix 3 – List of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168

i

Scientific and technical contributors

Peter Abbamonte38

Paul Adams14

Musa Ahmed14

Caroline Ajo-Franklin14

A.P. Alivisatos14

Elke Arenholz14

Brian Austin23

William Bachalo4

Sam Bader2

Jill Banfield14

Ken Baptiste14

Ali Belkacem14

Alexis Bell14

James Berger30

Robert Bergman30

Uwe Bergmann25

Nora Berrah42

Jean-Yves Bigot12

Hendrik Bluhm14

Mike Bogan25

Axel Brunger26

Phillip Bucksbaum 25

John Byrd14

Jamie Cate30

Andrea Cavalleri7

Lorenz Cederbaum37

Henry Chapman7

Andrew Charman14

Lin Chen2

Yulin Chen14

Majed Chergui9

Yi-De Chuang14

C.L. Cocke13

Paul Corkum17

John Corlett14

Tanja Cuk14

Peter Denes14

Dan Dessau36

Thomas Devereaux25

Jim DeYoreo14

Lou DiMauro18

Larry Doolittle14

Hermann Durr25

Thomas Earnest14

Wolfgang Eberhardt10

Paul Evans29

Charles Fadley31

Roger Falcone14

Daniele Filippetto14

Peter Fischer14

Jim Floyd14

Steve Fournier14

Jonathan Frank22

Heinz Frei14

Bruce Gates31

Oliver Gessner14

Ben Gilbert14

Mary Gilles14

Steve Gourlay14

Michael Grass14

Chris Greene36

Jinghua Guo14

Joe Harkins14

M. Zahid Hasan20

Franz Himpsel29

Axel Hoffmann2

James Holton14

Malcolm Howells14

Greg Hura14

Nils Huse14

Zahid Hussain14

Enrique Iglesia14

Richard Jared14

Peter Johnson5

Chris Jozwiak14

Robert Kaindl14

Chi-Chang Kao25

Cheryl Kerfeld14

Steve Kevan40

Janos Kirz14

Chris Kliewer22

Alessandra Lanzara14

Wei-sheng Lee26

Dung-Hai Lee14

Steve Leone14

Nate Lewis6

Derun Li14

Mark Linne8

Zhi Liu14

Robert Lucht21

Jon Marangos11

Todd Martinez25

C.W. McCurdy14

Keith Moffat35

Joel Moore14

Shaul Mukamel32

Keith Nelson15

Anders Nilsson25

Dan Nocera15

Joe Orenstein14

David Osborn22

Abbas Ourmazd29

Howard Padmore14

C. Papadopoulos14

Chris Pappas14

Fulvio Parmigiani24

Claudio Pellegrini33

Gregg Penn14

Massimo Placidi14

Soren Prestemon14

Donald Prosnitz14

Ji Quiang14

R. Ramesh14

Theo Rasing28

Alex Ratti14

Kenneth Raymond30

Doug Rees6

Matthias Reinsch14

Eli Rotenberg14

Sujoy Roy14

Dilano Saldin29

Annette Salmeen14

Miquel Salmeron14

Fernando Sannibale14

Robin Santra7

Ross Schlueter14

Robert Schoenlein14

Andreas Scholl14

Andrew Sessler14

Zhi-Xun Shen25

Oleg Shpyrko34

Volker Sick39

Steve Singer14

Gabor Somorjai30

John Spence3

John Staples14

Jo Stohr25

Albert Stolow17

Craig Taatjes22

John Tainer14

Lou Terminello19

Neil Thomson23

Joachim Ullrich16

Marco Venturini14

Angela Violi39

Marc Vrakking1

Hai Wang41

Glenn Waychunas14

Russell Wells14

Russell Wilcox14

Kevin Wilson14

L. Andrew Wray14

Jonathan Wurtele14

Wilfred Wurth27

Vittal Yachandra14

Peidong Yang30

Junko Yano14

Linda Young2

A.A. Zholents2

Shuyun Zhou14

Max Zolotorev14

Peter Zwart14

ii

1AMOLF2Argonne National Laboratory3Arizona State University4ARTIUM Tech5Brookhaven National Laboratory6California Institute of Technology7CFEL DESY8Chalmers University9EPF Lausanne 10Helmholtz-Zentrum Berlin11Imperial College London12IPCM, Strasbourg13Kansas State University 14Lawrence Berkeley National Laboratory

15Massachusetts Institute of Technology16Max-Planck-Institut für Kernphysik 17National Research Council of Canada18Ohio State University,19Pacific Northwest National Laboratory20Princeton University ,21Purdue University,22Sandia National Laboratories,23Science and Technology Facilities Council, UK24Sinchrotrone Trieste25SLAC National Accelerator Laboratory26Stanford University27University of Hamburg28University of Radboud

29University of Wisconsin30University of California, Berkeley31University of California, Davis32University of California, Irvine33University of California, Los Angeles34University of California, San Diego35University of Chicago36University of Colorado37University of Heidelberg38University of Illinois39University of Michigan40University of Oregon41University of Southern California42Western Michigan University

1 Needs for a next generation light source

TheNextGenerationLightSource(NGLS)willbea

transformativetoolforenergyscience.Thishighrepeti-

tionrate,highbrightnessX-raylaserwillenablecinematic

imagingofdynamics,determinationofthestructureof

heterogeneoussystems,anddevelopmentofnovelnon-

linearX-rayspectroscopies.Theseuniquecapabilitieswill

leadtoanewunderstandingofhowelectronicandnucle-

armotionsinmoleculesandsolidsarecoupled,andhow

functionalsystemsperformandevolveinsitu.

NGLSwilldramaticallyimpactawiderangeofenergy

applications:fromnaturalandartificialphotosynthesis,

tocatalysts,batteries,superconductors,carbonseques-

tration, and biofuels. Solving the complex long-term

energy challenges facing the nation, and the world,

is the subject of a wide-ranging set of reports

produced by the scientific community together

with DOE’s Office of Basic Energy Sciences (BES)

(http://www .er .doe .gov/bes/reports/list .html) .These

reportshighlighttheurgentneedfordeeperunderstand-

ingofthebasicscienceunderpinningenergytechnolo-

giesinordertoensureasafeandsecureenergyfuture.

TheNGLS—withitscombinationofhighaveragepower,

ultrashortpulsesandcoherence— isa revolutionary

observational tool thatwillbridge thecriticalgaps in

ourunderstanding.

SincetheirfirstdiscoverybyRoentgen,X-rayshave

beenexploitedbyscientiststoanswerfundamentalques-

tionsaboutmoleculesandmaterials.Assourcesevolved

from X-ray tubes to synchrotron storage rings, three

broadclassesofX-rayexperimentshaveemerged:imag-

ing,structuraldetermination,andspectroscopy.NGLS

willtransformallthreeofthesetechniques,allowingusto

observe,inwaysneverbeforepossible,hownaturaland

artificialsystemsfunction—onmultipletimescalesand

downtonano-spatialscales.

Overthepast40years,DOE’slightsourcefacilities—

electron-storage-ring-basedX-raysynchrotrons,operated

acrossthenationbyBES—haveprovidedresourcesfor

ten thousand scientists annually, from universities,

nationallabs,andindustry.Researchershavereliedon

these facilities to answer fundamental questions in

diversefieldsofscience,andaddresscriticaltechnology

problemsinareasincludinghumanhealth,electronics

and informationprocessing,andenergy.Synchrotron

X-raylightsourceshaveenabledscientiststounravelthe

structuresofbiologicalmacromolecules,essentialforthe

designofnewdrugs;theyhaverevealedthepropertiesof

electronicmaterialsfordevicesthatunderlietheinforma-

tiontechnologyrevolution;andtheyhaveprovidedthe

firstglimpseofhowenergyconversionsystemsworkat

theatomiclevel.Whiletheseadvanceshavebeendra-

matic,thereismuchmoretolearn,andthearrayofX-ray

lasersatNGLSwillprovideafoundationformajorscien-

tificadvancesinthe21stcentury.

DOEhasbuiltuponits40-yearlegacyofX-raylight

sources,continuouslyupgradingexistingsynchrotron

facilitiestokeepthematthefrontier.Recently,aremark-

ablenewtool,theworld’sfirsthardX-raylaser—the

LinacCoherentLightSource(LCLS)attheSLACNational

AcceleratorLaboratory—hasstartedoperations.Ithas

exceededexpectations in termsofperformance,and

hascrackedopenthedoortotheX-raylaserera.While

earlyexperiments from theLCLSare illustrating the

promiseofX-raylasers,andestablishingastronguser

communityforthem,itisalsoalreadyclearthatanext

generationX-raylaserwillbeneededtorealizethefull

potentialofthisnewtool.Anextgenerationsource,built

usingamodernsuperconductinglinearaccelerator,and

takingadvantageofthelatestlaserseedingtechnolo-

gies,willhavethehighrepetitionrateandhighaverage

2

1 . NEEDS FOR A NEXT GENERATION LIGHTSOURCE

• tunabilityandpolarizationcontrol

• multicolorX-raypump-probeexperiments

• synchronizationtosub-femtosecondtimescales

• moderatepeakpower,highaveragepower,andthus

highpulserepetitionrate

To maintain global leadership in X-ray discovery

science — and the technologies enabled by those

discoveries— theUSmust remainat the frontierof

X-raylightsources.TheNGLSdesignisuniqueinbeing

abletomeettheseneeds.

1.1 OutlineoftheCurrentProposalInSection2,weprovideanoverviewoftherevolu-

tionarycapabilitiesofanextgenerationlightsource—

anX-raylaserthatproducesatrainofultrashortpulses

athighrepetitionrateandunprecedentedcoherentpower.

The capabilities we envision for the NGLS can be

viewedwithinasetofthreeoverarchingthemes:

• Multidimensional spectroscopy: Thisthemerefersto

aclassofmeasurementcapabilitiesthatincorporate

atime-orderedsequenceofpulsestoprepareand

probeevolvingcorrelatedstatesofsolid,liquid,and

gas-phasesystems.Thesetechniquesallowtheiden-

tificationofdynamic,chemicallyspecificinformation,

e.g.,ontheflowofenergyandcharge.Includedin

thisthemeareexperimentsthatutilizethehighpeak

andaveragepoweroftheNGLSX-raypulsesfornon-

lineartechniques.Also,thecoherenceoftheNGLS,

byitsnarrowbandwidthcapabilitiesinlong-pulse

operation,willallowunprecedentedhigh-resolution

spectroscopy,tounderstandimportantlow-energy

modesoffunctionalmaterials.

• Ultrafast dynamics:This theme refers to a class

ofcapabilities thatwillallowthemeasurementof

processes on timescales extending from those

of chemical reactions that might take seconds to

complete,downto the fundamental timescalesof

electron correlation that determine the behavior

ofpairsofelectronsinmaterialslikesuperconduc-

tors.ThenewestexistingX-raysourcescanprobesys-

tems on picosecond (10-12 second) or potentially

femtosecond(10-15second)timescales,whicharerel-

evanttochemicalreactions,asdeterminedbythe

ratiooftypicalatomicspacingstoatomicvelocities.

The NGLS will extend this capability to systems

evolving on the hundred-attosecond timescale

coherentpowerneededtogobeyondtheinitialstageof

X-raylasers,andenablescientiststoanswerfundamen-

talquestionsinawiderangeofdisciplines.Theadventof

X-raylasershasledtohundredsofscientistspublishing

importantworkfromLCLSandothersources,interna-

tionally.Thishasfocusedglobal interest,andset the

stageforthenextgeneration.

Thenecessityfornewobservationaltoolshasbeen

citedinseveralBESreports:

• Directing Matter and Energy: Five Challenges for

Science and the Imagination (2007) noted that

answering thecallof thegrandchallengeswould

necessitate“athree-foldattack:newapproachesto

trainingandfunding,development of instruments

more precise and flexible than those used up to now

for observational science,andcreationofnewtheories

andconceptsbeyondthosewecurrentlypossess.”

• New Science for a Secure and Sustainable Energy

Future(2008)describedacomprehensivesetofscien-

tificresearchthemes,andidentifiednewimplementa-

tionstrategiesandtoolsrequiredtoaccomplishthe

sciencedescribedinthetenBESBasicResearchNeeds

WorkshopsandintheGrandChallengesReport.These

included“…characterization tools probing the ultra-

fast and the ultrasmall…,”andthedevelopmentof

advancedtheoryandsimulationsforwhichexperi-

mentswouldprovidecriticalvalidation.

• Next-Generation Photon Sources for Grand

Challenges in Science and Energy(2008)identified

connections between new research opportunities

andthecapabilitiesofnextgenerationoflightsources,

with emphasis on energy-related research.

Itnoted that“…femtosecond time resolution and

high peak brilliance are required for following chemi-

cal reactions in real time, but lower peak brilliance

and high repetition rate are needed to avoid radiation

damage in high-resolution spatial imaging . . .”

The futureneedsof thescientificand technological

communitythatutilizesX-raylightcannotbemetsolelyby

upgradingexistinglightsources—asimportantasthose

sourceswillcontinuetobeoverthenextdecade.Scientific

andtechnologicalchallengesnowrequirenewcoherent

X-raysources—X-raylasers—tomeettherequirements

ofthemostincisiveexperiments.AfutureX-raylaserfacil-

itymustincorporatetechnologythatallows:

• simultaneousoperationofmultipleexperiments

• abroadrangeoftemporalandspectralproperties

3


• Spin and magnetism at the nanoscale,tounderstand

thefundamentalmechanismsofspinandmagne-

tism,andtodeterminetheultimatespeedandperfor-

manceofmagneticsystems

• Biological systems: imaging dynamics and function,

utilizingnovelmethods in the rapidlydeveloping

fieldofcoherentimagingofbiologicalsystemsunder

physiologicallyrelevantconditions

InSection4,werelatethesecapabilitiesandscientific

driverswithasummaryoftherevolutionarytechniques

thatwillbeenabledbyNGLS:

• Cinematic3Dchemicalimagingatthehighestspatial

resolution

• Imaging structureand function inheterogeneous

ensembles

• Multidimensional X-ray spectroscopy in rapidly

evolvingsystems

InSections5-6,therequirementsforanextgeneration

lightsourcearederivedfromthescientificneedsoutlined

inSection3,fromDOEworkshopsandresultingreports,

andfromLBNLworkshopsandreports.Wethendescribe

theproposedfacility,whichrespondstotheserequire-

ments,andcompareitwithothersources.

Thescientificchallengesdescribedcallforcapabilities

beyond those foundatanyexistingorplannedX-ray

source.Theyinclude:

• Higheraveragepowerwithanevenly-spaced,high-

repetition-ratetrainofcoherentpulses(torevealsub-

tleeffectsinawiderangeofcomplexmaterials)

• Shorterpulsedurations(toprobetherelevanttime-

scalesofphysical,chemical,andbiologicalfunction)

• Narrower bandwidths (to examine the important,

lowest-energymodesofcomplexsystems)

Addressingabroadrangeofscientificapplications,

andservingalargescientificcommunityrequiresmulti-

ple instruments—operatingsimultaneously—with

flexible means of delivering X-rays tailored to each

instrument and experiment. Synchronization of the

X-raypulseswithadditionalsources(THz,IR,oroptical),

aswellaslongitudinal(temporalphase)andtransverse

coherence,tunability,polarizationcontrol,andstability

areallneeded.

NGLSwillmeettheseneeds.

NGLSisamultiple-beamX-raylaser.Itutilizesahigh-

current(upto1mA)superconductingelectronaccelera-

tor (nominally 1.8 GeV energy) to produce a train of

(1attosecond=10-18second),theatomictimescale

determinedbytheratioofelectronorbitsizetoelec-

tronvelocity.

• Coherent scattering and imaging:Thisthemecap-

turestheabilityoftheNGLStorevealstructureand

dynamicsatthenanoscale,througheithercoherent

X-rayscatteringordiffractiveimaging.Thehighrep-

etitionrateandhighaveragepowerofNGLSwillnot

onlyallowimagingofthestructureofsystemswith

long-rangeorderorhomogeneoussamples,butits

highpulserepetitionrate,whencoupledwithhigh-

speedreadoutdetectorsandadvancedcomputational

techniques,opensthepossibilityofacquiringand

processingbillionsofimages,inordertounderstand

heterogeneousand/orfluctuatingmicroscopicsys-

tems (e.g., evolving nanoscale catalytic particles

underfunctionalconditionsorchangingproteincon-

formationsintheirnativeenvironment).

InSection3,wedescribethescientificdriversfora

nextgenerationlightsource.Wedetailaprospectiveset

ofninescientificchallengesforwhichNGLSwillsingu-

larlyaddresscriticalknowledgegaps:

• Photosynthesis,tounderstandallofthestepsofnat-

ural photosynthetic processes, and to guide the

designofartificialdevicesforconvertingsolarenergy

tofuel

• Fundamental charge dynamics,todevelopanewlan-

guagetoaccuratelydescribeandpredictchargeand

energytransferinmolecularsystems

• Advanced combustion science,tounderstandspatial-

ly,chemically,andtemporallydependentphenomena

inawidevarietyofburningfuels,inordertooptimize

combustionefficiencyandtovalidatecomputational

modelsofcombustion

• Improved catalysis,toenhanceefficiencyandselec-

tivitybyinvestigatingin-situprocessesoffunctioning

catalyticsystemsonmultipletimeandlengthscales

• Nanoscale materials nucleation, to observe and

controlthekineticsofnano-materialformationand

self-assembly

• Dynamical nanoscale heterogeneity in materials,to

understandspontaneousfluctuationsspanningmulti-

pletimeandlengthscales,theevolutionofnanoscale

morphology,andtheirrelationshiptotheproperties

andfunctionalityofcomplexmaterials

• Quantum materials,todirectlyprobethenatureof

correlatedelectronsystems

4


InSection7webrieflydescribehow the facility is

designedtobeupgradable:expandingcapacitybyadd-

ingadditional simultaneouslyoperating free-electron

lasers(FELs),andexpandingcapabilitybyextendingthe

energyrangetobothlower(100eV)andhigher(10keV)

photonenergies.

Section8providesaproposedNGLSmanagement

structure,cost,andtimeline.

IntheAppendicesweprovide:(1)ashortdescription

ofthepotentialforperturbationofsamplesbytheX-ray

pulses,andtherationaleforlimitingthenumberofpho-

tonsperpulse;and(2)alistofrelevantworkshopsheld

atLawrenceBerkeleyNationalLaboratory(LBNL).

electronbunches(at1MHzandultimatelysignificantly

higher repetitionrates),whicharesequentially fed to

multipleundulators,whichinturndeliverindependent,

simultaneousX-raylaserbeamsintoend-stationinstru-

mentsformultipleusers.

Eachexperimentalend-stationinstrumentattheNGLS

facilitywillreceiveabeamofX-raypulseswithhighrepeti-

tionrate(typically100kHzormore).Initially,theX-raypho-

tonenergyrangewillextendfrom280eVto1200eV,and

thepulsedurationfrom250asto250fs,withpulseshav-

ingbetween108and1012photons.Harmonicsoftheundu-

latoroutputwillproducephotonenergiesextendingto

3keVandabove,albeitwithfewerphotonsperpulse.

2 Overview of revolutionary X-ray science tools at NGLS

Thescienceprogramat theNextGenerationLight

SourcewillbebasedonX-raymeasurementtoolswith

spatial, temporal, and energy resolution that are far

beyondwhatcanbeachievedwithpresentsources.Most

importantly,thisnewsciencewillexploitentirelynew

X-raymeasurementcapabilitiesandapproachesthatare

qualitativelydifferentfromanythingavailablefromcur-

rentX-raysources,orfromanyotherX-raysourceinthe

foreseeablefuture.

TheNextGenerationLightSourcewillrevolutionize

X-ray science by providing unprecedented coherent

power(upto~100W)inacontinuoustunabletrain(ulti-

matelyupto100MHz)ofultrafast(femtosecondorless)

pulses.Muchaspassivemode-lockingofthecontinuous-

wavelaserinthe20thcenturyusheredintheeraofnon-

linear optical spectroscopy and ultrafast science, a

versatile X-ray laser facility combining high average

power,highrepetitionrate,andtunableultrafastpulses

willusherinaneweraofX-rayscienceforthe21stcentury.

Followingisabriefintroductionofthenewscientific

toolsenabledbysuchanX-raylaser.Section3presents

examplesofthescientificimperativesforanextgeneration

lightsource,andillustrateshowthenewmeasurement

capabilitiesofNGLSwillenablenewareasofscience.

Section4discussesthesekeycapabilitiesindetail,and

providessomecomparisonwithexistingapproaches.

2.1 Multi-dimensionalX-raySpectroscopy

Multi-dimensional X-ray spec-

troscopyreferstoabroadclassof

measurementcapabilities incorporating time-ordered

sequencesofX-raypulsestogenerateasignalthatisa

functionofmultipletimedelaysand/orphotonenergies.

ThesearenonlinearX-raytechniques,andinsomecases

coherentwave-mixing,inwhichX-raypulsesareusedas

bothapump,topreparespecificnear-equilibriumstates

ofmatter,andasaprobeoftheseevolvingstates.These

newtoolsrelyonsimultaneouscombinationsof:high

peakpower,highaveragepower(highrepetitionrate),

spatialcoherence,temporalcoherence,andtunability.

IntheX-rayregion,thetremendouspromiseofmulti-

dimensionalspectroscopyliesinthecapabilitytofollow

coherentchargeflowandenergyrelaxationonfunda-

mental (attosecond to femtosecond) timescaleswith

accesstothefullrangeofvalencestates(unrestrictedby

dipoleselectionrules).Importantly,theelementsensitivi-

typrovidedbyX-rays(tunedtocore-levelabsorptions)

willenableusforthefirsttimetofollowchargeandener-

gyflowbetweenconstituentatomsinmaterials.These

essentialcapabilitiesarenotattainableusinginfraredor

visiblelaserpulses,andwillprovidecriticalinsighttocor-

relatedelectronsystems,andmolecularcomplexeswith

strongcouplingbetweenelectronicandnucleardynamics.

Theanalogoustechniqueofnuclearmagneticreso-

nance(NMR)illustratesthetremendouspotentialimpact

ofmulti-dimensionalX-rayspectroscopy.NMRincorpo-

ratessequencesofradio-frequencypulsestogeneratea

two-dimensional signal-map that is a function of the

Fouriertransformofthetimeintervalsbetweendifferent

pulsepairs.NMRsignal-mapsarefingerprintsofspecific

chemicalstructures,andtheirrelativepositions,withina

molecular complex.The scientific significance is evi-

dencedbythe1991NobelPrizeinChemistrywhichwas

6

2 . OVERVIEW OF REVOLUTIONARY X-RAY SCIENCE TOOLS AT NGLSULTRAFAST DYNAMICS

interest,~meVformanyimportantscienceapplications.

Inaddition tosacrificingphotons that lieoutside the

bandwidthofinterest,therelativelypoorefficiencyof

X-rayopticsresultsinadditionallossofphotonswithin

thebandwidthofinterest.Incontrast,thebandwidthgen-

eratedbyanX-raylasercanbedirectlycontrolledbythe

seedingprocess.NGLSwillultimatelybecapableofgen-

eratingpulsesupto500fsdurationFWHMwithaband-

widthof~10meV(neartheFouriertransformlimit).The

averagefluxavailableinthisbandwidthwillbemany

orders of magnitude beyond any present or planned

source,andwilldrivedramaticadvancesinhigh-resolu-

tionX-rayspectroscopy.

2.2 UltrafastDynamics

NGLSwillprovideimportantnew

capabilitiesforinvestigatingfun-

damentaldynamicsofchargeandenergyflowinmatter:

on the attosecond and few femtosecond time scales

(characteristic of electron correlations and coherent

charge-transferprocesses),andonthe10-100femtosec-

ondtimescale(characteristicofatomicmotionandvibra-

tionalmodes).Dynamicstudieswillbeindispensiblefor

separatingcoupledphenomenainthetimedomain,such

ascollectiveelectronicexcitationsinmaterials,andcou-

pledelectronicandnuclearmotioninreactingmolecules.

While theattosecond frontierhasbeenopenedby

high-orderlaserharmonicsourcesatthe10-100µWaver-

agepowerlevelsandkHzrepetitionrates,NGLSwillenable

X-raypump/X-rayprobeattosecondresearchat100kHz

rateswithinitialtunabilityfrom280eVto1.2keV,and

averagepowerof~1mW.Upgradepathsarealreadyidenti-

fiedtoreachWattlevelaveragepowerinpulsesofafewfs

duration,withspectralrangeextendingtothehardX-rays.

TheflexibledesignofNGLScanreadilyincorporatenew

developmentsinseedlaserstoenhancetheX-raylaser

performance.Thecombinationoflaserseedingandtiming

stabilityprovidedbyacontinuous-wavesuperconducting

RFlinacwillallowforsynchronizationtoexternallasersources

atthefewfemtosecondlevelforsampleexcitationwith

ultrafastpulsesintheUV,visible,near-IR,andTHzregions.

awardedtoR.Ernstforhisdevelopmentofmulti-dimen-

sionalNMR.Inadramaticadvance,vibrationalmultidi-

mensional spectroscopy was demonstrated nearly a

decadeago,usingsequencesofultrafastinfraredlaser

pulses.Theinfraredsignal-mapsprovideafingerprintof

thecouplingbetweendifferentvibrationalmodesina

molecule,therebyrevealingnewinsighttothemolecular

structure and its evolution on the femtosecond time

scale.Thedevelopmentofelectronicmultidimensional

spectroscopynowprovidesanapproachtoexploitultra-

fastvisiblepulsestomapthedynamiccouplingbetween

electronicstates.Overthepastseveralyears,thistech-

nique has become invaluable for following quantum

coherencesandcharge relaxationbetweenelectronic

statesinsystemsrangingfromchlorophyll(responsible

forlightharvestinginphotosynthesis)toexcitonicstates

insemiconductors.

Multi-dimensionalX-rayspectroscopyandnonlinear

X-raysciencewillbehallmarksofNGLSastheyrequire

capabilitiesthatarenotavailablefromanyotherX-ray

source.Highpeak-powerX-raypulsesarejustoneofsev-

eralessentialrequirements.Equallyimportantistheabil-

ity to control the degree of X-ray nonlinearity while

resolvingsmallsignalswithhighfidelity.Highrepetition

rate isabsolutelyessential toachieve this inorder to

avoiddisruptingtheelectronicstates(orothersample

attributes) that are being investigated.An important

benchmarktorecognizeisthatthescientificimpactof

multi-dimensional laser techniqueswas realizedonly

after thedevelopmentofmulti-kHzandMHzultrafast

laser sources.These laserscombinedbothhighpeak

powerandhighaveragepowertoenableextremelysen-

sitivemeasurementsofcontrollednear-equilibriuminter-

actionsoflaserpulsesequenceswithmatter.

InadditiontononlinearX-rayspectroscopy,high-reso-

lutionspectroscopywillalsobetransformedbythecapa-

bilities of NGLS. A fundamental limit of present

synchrotronsources(andSASEFELs)forhigh-resolution

spectroscopyistheirlackoflongitudinal(temporalphase)

coherence.ThegeneratedX-raysareinherentlybroad-

band(typicallyseveral10’sofeVat1keV),*andasacon-

sequence, high-resolution measurements must use

monochromatorsinordertofilteroutthebandwidthof

*Forsynchrotronsources,thefractionalbandwidthinthecentralconefromanundulatorscalesasΔλ/λ~1/NuwhereNuisthenumberofundulatorperi-ods.Presentcapabilities(typicallyNu~100,or1%fractionalbandwidth)aremanyordersofmagnitudebeyondthemeVresolutionofscientificinterest,andarefundamentallylimitedbye-beamemittance(whicheventuallydegradesthecoherentsuperpositionofradiationoverthelengthoftheundulator)andpracticallylimitedbythemaximumundulatorlengthsinastoragering(~10m).

7

2 . OVERVIEW OF REVOLUTIONARY X-RAY SCIENCE TOOLS AT NGLSCOHERENT SCATTERING AND IMAGING

X-ray cinematic imagingusestomographictechniques

withultrafastpulsesandhighrepetitionratestocreate3D

tomographic movies of fluid dynamics coupled with

chemistry.NGLSwillapplyX-raycinematicimagingto

understandcombustiondynamicsand reactive flows

withanunprecedentedcombinationofvolumetricreal-

timeprobingandchemicalspeciation.

Coherent X-ray scattering(X-rayphotoncorrelation

spectroscopy)islaserspeckleintheX-rayregime.Whilevis-

iblelaserspeckle,ordynamiclightscattering,probesdif-

fusionaldynamicsinsoftmatter(e.g.polymers,solution

suspensions,glasses)onthemicronscale,coherentX-ray

scatteringprobesspatialcorrelationsanddynamichetero-

geneityonthenanometerscale,withthechemicalsensi-

tivityandmagneticcontrastmechanismsprovidedby

tunablesoftX-rays.NGLSwillapplycoherentX-rayscat-

tering(at100kHzrepetitionrates,withultrafastpulses)to

understanddynamicnanoscaleheterogeneityinmaterials,

including:(1)transientnanoscalefluctuationsofchargeand

orbitalorderingphenomenainsolids;(2)vortexdynamics

inhighTcsuperconductors;and(3)protein-proteininter-

actionsinphysiological conditions.

Coherent diffractive imagingusesiterativephase-recon-

structionalgorithmstoinvertcoherentX-rayscatteringpat-

ternsandtherebyreconstructthree-dimensionalimagesof

objectsatthenanoscale,avoidingtheresolutionlimitations

imposedbyX-rayoptics.NGLSwillapplycoherentdiffrac-

tiveimaging(at100kHzrepetitionrates,withultrafastpuls-

es) to capture and image: (1) the earliest events in

nanoparticlenucleationandsynthesis;(2)ultrafaststruc-

turalchangesinsupramolecularcatalystsin operation;and

(3)heterogeneousbiologicalsystemsrangingfrommultiple

conformationsanddynamicsofbio-molecules,tomolecu-

larmachines,towholecells—all in native environments.

Thesecapabilitiesfordynamicstudiesrepresentadra-

maticadvanceoverexistingandplannedX-rayFELsources.

Finally,thetemporalcoherenceandversatilityofNGLS

willbeexploitedtotailorthepulseduration(timeresolution)

andbandwidth(energyresolution)forspecificexperiments.

2.3 CoherentScatteringandImaging

MHz ultrafast X-ray lasers at

NGLSwillrevolutionizeourabili-

tytoimagethenanoscalestruc-

tureinmatterwithunprecedented

detail.Structuralheterogeneity, rare transientnano-

structures,spontaneousfluctuations,anddynamicevo-

lutionatthenanoscalewillberevealedusingadvanced

coherentX-rayscatteringanddiffractionmicroscopy

techniquesthatcannotbeachievedwithpresentX-ray

sources.Importantly,thecapabilitiesofNGLSwilllever-

agestate-of-the-artcomputationalapproachesandhigh-

speeddetectorsnowundergoingrapiddevelopment.

X-ray scattering and imaging represent powerful

scientifictoolsthathavebeendevelopedoverthepast

century, based primarily on incoherent (or partially

coherent)Xraysources.Importantly,synchrotronsourc-

esprovidelimitedspatialcoherence,enablingtheemer-

gence of coherent X-ray scattering and diffractive

imagingtechniques.Thesetechniqueswillconvergeand

reachtheirfullpotentialwithcoherentX-raysources.

Spatial and temporal (longitudinal) coherence at

MHzrepetitionrateswillenableNGLSX-raylasersto

revolutionizethecharacterizationofnanoscalestructure

anddynamics.

NGLS – science drivers3

thataccomplishtheconversionofcarbondioxideand

watertocarbohydratesinasingleintegratedsystem.This

motivatesthedevelopmentofartificialphotosynthetic

systemsforthesingularpurposeofgeneratingadesired

fuelonalargescale.

Whileconsiderableprogresshasbeenmadetoward

thisgoal,andsomeartificialsolarfuelsystemsareeffi-

cient,4theycontainrarematerials,arenotdurable,orrely

onsyntheticprocessesthatarenotscalable.5Conversely,

partialorcompletesystemsmadeofabundantmaterials

areinefficientandoftennotrobust.Bridgingthescientific

3.1 Photosynthesis

Research on Solar Fuel Generating Systems at the NGLS

Therisingdemandforenergy,thediminishingsupply

ofoil andnaturalgas, and theenvironmental conse-

quencesfromtheuseoffossilfuels,allhighlighttheneed

forrenewableenergysources.Ofthemanyalternatives,

solarenergyisbyfarthemostabundantandinherently

cleanenergysource.1,2,3Thegoalofgeneratingsolar

fuelsbydirectconversionoflightenergytofuelmole-

culesisinspiredbynature’sphotosyntheticorganisms

Detailed understanding of the processes that comprise photosynthesis — the set of reactions that use solar energy to convert water and carbon dioxide into organic compounds and oxygen — will have both fundamental and applied importance. Fundamental studies can be traced back hundreds of years, with modern investiga-tions including the Nobel prize winning work of Melvin Calvin on critical carbon pathways, and the recent work of Graham Fleming on the quantum coherence underlying photosynthetic reactions. Today, artificial photosyn-thetic systems carry the promise of sustainable energy by producing fuels from sunlight.

The unique capabilities of the NLGS will provide a far greater understanding of natural photosynthetic reactions. Ultrafast pulses will reveal the chemical dynamics that occur on time-scales ranging from the fastest quantum mechanical transfer of electronic charge across molecules, to the relatively gradual regulation of reactions asso-ciated with changing solar flux. Wavelength-tunable, high-repetition-rate X-ray pulses will for the first time allow interrogation of specific photosynthetic molecules and charge states in a time-ordered and non-perturbing manner. These capabilities will also help identify the optimal pathways for efficient conversion of sunlight to fuels in arti-ficial photosystems.

10

3 . SCIENCE DRIVERSPHOTOSYNTHESIS

3.1.1 Natural Photosynthesis — Critical Knowledge Gaps

3 .1 .1 .1 Scientific Gaps

Themostcriticalreactionduringphotosynthesisisthe

photo-inducedoxidationofwater(Figure2).Wateroxida-

tioninnaturalphotosynthesisconsistsoffouroxidation

stepsdrivenbythesuccessiveabsorptionoffourpho-

tonsbythePhotosystemII(PSII)reactioncenter(Figure

3).2Thetimescalesofelectrontransportfromonepig-

menttoanotherspanaboutnineordersofmagnitude.

Afterexcitationoftheantennasystemthelightenergyis

transferredtotheprimarydonorChlorophyllcomplex

P680,locatedinthereactioncenterofPSII,andthensub-

sequentlytootheracceptorpigments.TheoxidizedP680

returnstothegroundstatebyreceivinganelectronthatis

extractedfromtheoxygen-evolvingcomplex(OEC)by

thewateroxidationreaction.Namely,PSIIcombinesone

photochemical reaction at the P680 and four electron

redoxchemistryat theOECtocompleteonecatalytic

cycle.Throughthiscyclicprocess,thecentralMn4Caclus-

terstoresfouroxidizingequivalents,whichareusedto

extractfourprotonsandfourelectronsfromtwowater

gapsfordevelopingviablesolarfuelgeneratorsisafor-

midableyethighlypromisingtaskthatwillbedramati-

cally accelerated by the spectroscopic capabilities of

NGLSX-raylasers.Essentialcapabilitiesincludetimeres-

olution,chemicalspecificityandbonding,andhighrepe-

titionratetoproberaretransientstates,intheiroperating

environment,withoutdisruptingthem.

Naturehasdevisedaremarkablydiversesetofpath-

waystoconvertsolarphotonsintochemicalfuelsthrough

acomplexmechanismoperatingintheleafofaplant.

Simplyspeaking,thephotosyntheticreactioncenterina

plantleafcaptureslight,createsandtransportselectrons

andpositivechargesthat,withthehelpofanaturally

occurringcatalyst,oxidizeswaterandproducesoxygen

moleculesandhydrogenions.Inlaterstagesofthepro-

cess,electronsareusedtoreducecarbondioxideandfix

carbonbycombiningcarbonwithhydrogenionsinthe

formationofasugarthatcanlaterbeconvertedtoalcohol

fuel.Whilethenaturalsystemdealswithacomplexsetof

tasksthatgoeswellbeyondproducingsugarmolecules,

artificialphotosynthesisfocusesonthesingletaskofgen-

eratingafuelfromwater,carbondioxideandsunlight.

CapabilitiesoftheNGLSX-raylaserswillbecrucialto

advanceourunderstandingofkeyenergytransfer,charge

transportandchemicalprocessesofnaturalphotosyn-

thesis,andtodevelopfundamentalprinciplesfortarget-

eddesignofartificialsystemsthatareefficient,durable,

andmanufacturablefromearth-abundantmaterialsusing

scalableprocesses.

Photocathodematerial

Photoanodematerial

Surface-boundcatalyst for O2evolution

Surface-boundcatalyst for H2evolution

2H2O

O2 + 4H+

4H+

2H2

4e–

4e–

H+ permeablemembrane

H+

H+

H+

H+

H+

H+

Sunlight

Carbon fixingreactions

NADP+

non-cyclice- transport

ADP + Pi ATP

CF1

CF0

C14 = 50 – 100 rpm

C14

Lhca

3Lh

ca2

Lhca

4Lh

ca1

A

KH

B

D EFd

JIOG

QKAQKBFx

PC

PC

L C

osep

H+ H+

H+ H+

3H+

H+

½PQH2

PQ-pool

GQ

QH22Fe-2S

A

N F

A

FeCytbH

Qi

Qo

CytbL

M NLQ

cyclic e-

transport

IVcytb6

Stroma

b

Lheb1+2+3Lheb4Lheb6

Psb29(biogenesis)Psb29

extrinsic

Lheb5

ThylakoidMembran

(5nm)

Lumen

16 nm11 nm

Y YD

SC J K Z N X H

B

DD2

AD1 E F

WY MLTc

Cn2+?

Cn2+

Chla

ISPb

Mn MnMn Mn(on D1)Psb27

(repair)P

O

Q

RTn

P680

Phe

QA

cytƒ Fe

e-

e- e-

e-

e-

e-

e-

e-Car

RieskeISP

1/2 H2O 1/4 O2+C (CP43)(CP47) B

a

dF6Fe FB

A0 A0

FA

P700

Figure1Left: natural photosynthetic system. Right: conceptual design of an artificial photosynthetic system (Figure courtesy of N. Lewis).

2 H2O 2 H2 + O2

4H + + 4 e– 2 H2

2 O 2- O2 + 4 e–

Figure2Steps in photo-induced oxidation of water.

11


es themforasufficientperiodnecessary forsplitting

water,therefore,remainsasacentralquestion.

3 .1 .1 .3 Capabilities Lacking

For a complete under-

standingofthephotocata-

lyt ic react ions, i t is

necessary tostudycritical

stepssuchasthetransient

S4state,and thechemical

dynamics that govern the

directionalityofthecatalytic

reaction cycle. A detailed

molecularpicturerequires

time-resolved measure-

mentsunderambientcon-

ditions.However,forbiologicalcatalystsliketheOECthat

usuallyfunctioninadiluteaqueousenvironment,this

hasbeenachallengeduetohighsusceptibilitytoradia-

tiondamageevenatcryogenictemperatures.9Forthis

areaofscience,NGLSX-raylaserswillbridgeseveralsig-

nificantcapabilitygapsofmodernsynchrotrons,namely:

(1)anabilitytoprobetheseprocessesonthefundamental

timescalesandatambientconditionswherecatalytic

reactions,bondformation,andchargetransferprocesses

occur;(2)acapabilityfortunabletwo-colorX-raypump,

X-rayprobe,andmultidimensionalX-rayspectroscopy

techniques for following the flow of valence charges

betweendifferentatomicsites;(3)provisionoftherequi-

siteaveragebrightness(unachievablefromsynchrotron

sources)thatwilldirectlyrevealthechargecorrelations

molecules,catalyzingtheformationofoxygen,andeven-

tuallyreleasingO2atarateofnearly500molecules/sec.

The electrons on the acceptors are available for CO2

reduction,orforthereductionofH+toH2.

Despitemanydecadesofstudy,essentialcomponents

ofthewateroxidationprocessremainpoorlyunderstood.

Acompleteunderstandingofthefundamentalelectron

dynamicsthatcontrolthiscomplexreactionisagrand

challengeofscience.

3 .1 .1 .2 Current Understanding from Synchrotron-Based

Experiments

ThedetailedchemistryoftheOEChasemergedslowly,

but critical design aspects remain to be elucidated.

Synchrotron-basedexperimentssuchasX-raydiffrac-

tion6,7 and X-ray absorption methods have revealed

insightabouttheproteinscaffoldandtheoverallgeome-

tryoftheMn4Cacluster.8X-rayemissionandabsorption

spectroscopyhavealsoprovidedinformationaboutthe

electronicstructureofsomecryo-trappedintermediate

states(S0toS3).TheMn4Cacomplexplaysakeyrole

owingtotheversatiled-electronorbitalsandtheirmanip-

ulationvia ligand fields, fromwhichemerge: charge-

transferstates,acapabilitytochangeoxidationstateat

relativelysmallenergycost,andahighlevelofcatalytic

activity.Whileitiswidelyacceptedthatallofthemanga-

neseionsareinhighoxidationstatesandbridgedbyoxy-

gen,theexactoxidationstateofeachmanganeseatom,

thenatureofthebondingwithoxygen,theirevolution,

andtheroleofcalciumremainlargelyunknown.Howthe

OECaccumulatesfouroxidizingequivalentsandstabiliz-

Light absorptionand

charge separation

2H2O

Pheo

O2+4H+

OEC

QA

YZ

QB

hν

20-300 ns

~ms

ps

<400 ps

OOEC

P680*

P680

O2

2H2O

S0

Water oxidation

S4

S3

e–, 2H+

e–, H+

e–, H+

e–

S1

S2

Tyr 161

Asp 170His 190

Gln 165

Glu 189Mn

Mn Mn

Mn

Ca

Glu 333

His 332

His 337

CP43

Glu 354

Ala 344c-term

Asp 342

Figure3 Right: The absorption of photons by the PS II reaction cen-ter P680 and the subsequent elec-tron transfer steps that trigger the water oxidation chemistry in the OEC. Left Sequential states, Si, of the Mn4Ca cluster following indi-vidual photon absorption (hν), where i=0-4 indicates the number of oxidizing equivalents stored in the cluster. Inset: One of the pro-posed structures of Mn4Ca cluster.1

UV-visible-THz pump, X-ray probe

Time-resolved XAS, XES, XANES, EXAFS

Time-resolved ambient-pressure XPS

Native environments

Sample replacement between pulses

12

3 . �SCIENCE�DRIVERSPHOTOSYNTHESIS

ates and products interact-

ing with a solid catalyst

surface.13 These techniques

open up monitoring of het-

erogeneous photocatalytic

processes by probing bond-

ing interactions of water,

carbon dioxide, or reaction

intermediates on the cata-

lyst surface. Concurrent

monitoring of transition

metal L-edge absorption and

ligand K-edge spectra of

surface metal centers, using

grazing incidence to enhance surface sensitivity, provides

complementary electronic structure information on the

participating catalytic centers. However, present sources

lack the critical capability to follow these processes on

their natural time scales. Measurements of photosynthetic

systems on time scales faster than ~100 ms are beyond

our reach due to the limited flux and unfavorable time

structure of storage rings.

Time resolved X-ray absorption spectroscopy

(TR-XAS) has been used for understanding the dynamic

structural changes of ligand environments upon excita-

tion of organometallic light absorbers. For example,

Della-Longa et al., have shown an expansion of the por-

phyrin ring of a nickel porphyrin chromophore in the

excited state upon absorption of light, along with detailed

information on electronic structural changes.14 Khalil et

that are thought to play a key role in important catalytic

processes; (4) requisite pulse spacing to allow for sample

replacement, preparation (pump), and probe — on each

pulse, and in native (liquid) environments.

3.1.2� �Artificial�Photosynthesis�—��Critical�Knowledge�Gaps

3.1.2.1 ScientificGaps

Breakthroughs in present thermodynamic and quantum

efficiency limits of artificial photosynthetic systems will

require new understanding of: (1) adequately matched

redox potentials of light absorbers, charge separators,

and catalysts, which are essential prerequisites for con-

verting a maximum fraction of the solar photon energy to

chemical energy of the fuel (thermodynamic efficiency);

(2) efficient and durable contacts for directed charge

transport between components — typically either mole-

cule-solid or solid-solid interfaces (high quantum effi-

ciencies require fast directed charge transport in order to

compete with undesired pathways); (3) robust catalysts

for water oxidation, or proton or carbon dioxide reduc-

tion that operate at sufficiently fast rates for the catalysis

to keep up with the photon flux at high solar intensity;

(4) efficient coupling of the fuel-generating and water-

oxidation half reactions across a proton permeable, prod-

uct impermeable membrane that affords separation of

fuel molecules from evolving oxygen. While some these

scientific barriers are overcome by using components

made of non-scalable materials such as noble metals, an

overarching challenge is to bridge these scientific gaps

with components made of abundant, robust materials.

3.1.2.2 CurrentExperimentalCapabilities

andLimitations

Various X-ray spectroscopy techniques are now being

applied to understand the time-averaged geometric and

electronic structure of artificial photosynthetic systems.

For example, cobalt EXAFS measurements revealed the

atomic structure of a recently discovered cobalt contain-

ing electrocatalytic film for water oxidation10,11 (Figure 4).

EXAFS spectroscopy of multiple metal edges have been

applied to investigate structural relationships of polynu-

clear light absorbers — catalyst assemblies in nanopo-

rous silica supports.12 In situ X-ray emission spectroscopy

of oxygen and carbon K-edges provides atom-specific

details on the electronic structure of reactants, intermedi-

00 2

ExperimentFit

4

R (Å)

6 8

5

10

15Co–O

Co

Co Co

O

Figure4��Co-Pi�EXAFS�spectrum11.

Two-color�X-ray�probe

High-resolution�RIXS

X-ray�pump,�X-ray�probe

Stimulated�X-ray�Raman�(CXRS)�–�wave�mixing

Core-hole�correlation�–�wave�mixing

see�Section�4.3

13


3 .1 .2 .3 Capabilities Lacking

Acriticalcapabilityfordevelopingefficientartificial

photosyntheticsystemsmadeofviablematerialsisto

observeandunderstandthesequenceofelementarypro-

cessesfromabsorptionofsolarphotonstothereleaseof

fuelandoxygenmolecules.Thereiscompellingevidence

thatenergeticsandstructuralaspectsoflightabsorbers,

chargeseparatinginterfaces,catalyticcomponentsand

linkagesbetweenhalf reactionsaremutuallyaffected

throughoutthesequenceofenergytransfer,chargetrans-

portandcatalyticprocesses.Therefore,thedesignofeffi-

cient solar fuel generators depends on the ability to

understand the electronic properties and structural

changesofactivesites,ontheirnaturaltimescales,under

operatingconditions,andacrossthecompletesystem.

Current synchrotron-based X-ray spectroscopies

describedabovedonotprovidethecapabilitytofollow

changesinelectronicpropertiesandstructureonrelevant

timescales,andwithrequiredsensitivity.Additionally,

currentX-raysourcesarepractically limited tosingle

probewavelengths,whichpreventsmonitoringofpro-

cessesatmorethanonemetalcenteror ligand,orof

morethanonechemicalspeciesatatime.Importantly,no

X-raysourceexiststhathasatimeresolutionoffemto-

seconds,sufficientpulseenergy,andtherequiredhigh

pulserepetitionratetomakesimultaneousmonitoringof

multipleabsorptionedgesfeasible.

al.,havedetectedultrafastbondlengthchangesinorga-

noiron-basedlightabsorbers(spin-crossovercomplex)

duringtheinitialphototriggeredevents.15Veryrecently,

TR-XAShasbeenappliedforthefirsttimetoL-edges,

identifyingdetailedchangesinbondingconfigurationof

thehybridizedmetal-ligandorbitalsinthetransienthigh-

spincrossovercomplex(Figure5).16Thesestudiesare

substantiallylimited(bypresentsourcecapabilities)to

coarsetimeresolution,andsimplemolecularsystems

withrelativelylargetransientsignals.Thesimultaneous

monitoringofnuclearandelectronicmovementsduring

photon-inducedenergyandchargetransferprocesses,

howeverlimited,demonstratethetremendouspotential

ofultrafastX-rayspectroscopyforobtainingdynamic

informationwithhighspatialandtemporalresolutionin

thevicinityofspecificelements.

BeyondTR-XAS,ResonantInelasticX-rayScattering

(RIXS)isanincisiveX-raytoolforprobingchargetrans-

fer,andotherlow-energyexcitationssuchas d-dtransi-

tionsandprotonenergytransfer.RIXS,incombination

withXAS,hasdemonstratedthemetaltoligandcharge

transferoccurringbetweenaConanoparticlecatalystand

surfactantligandsonitssurface.17Forexample,Figure6

showsthechargetransferpeakinCo3O4nanoclusters

grown in silica nanopores18 that act as efficient and

robustcatalystsforwateroxidation.

–1.5 –1.0 –5 0 5Energy (eV)

35% reductionof CT in nanos

CT

dd

SBA (Nanostructured Co3O4 in nanoporous silica)

Co3O4

Inte

nsity

(a. u

.)

Co3O4

Co Co 2H2O

O2+ 2H+

O

SiSi

O OOO

Si SiO O O

Figure6 RIXS spectra of Co3O4 microcrystalline powder and nano-structured Co3O4 in nanoporous silica (SBA15).

Abso

rban

ce/m

OD

40

0

80

704 708

A

Ground State (Exp.)

[Fe(tren)(py)3]2+

712 716 720 724Energy /eV

1.7 eV

L3

L2

Low Spin

High Spin

90ps delay

Multiplet Theory

Multiplet Theory

Figure5 Photoinduced changes in Fe(II) L-edge spectra.16

14


Secondly,thequantumefficiency(productevolution

perphotonenergyabsorbed)ofagivenprocess—beit

electrontransferbetweencatalystandlightabsorberor

theelectrontransferstakingplaceonthecatalyticsurface

—isdeterminedbynotonlythoseintermediatesthat

leadtoproductevolution,butisfurtherinfluencedbyall

thecompetingpathways.Sincetherewillbeavarietyof

competingpathways,thesignalduetoeachwillbesmall.

TransientRIXSisanexperimentthatcannotbedoneat

currentX-raysourcesandwillbeidealforidentifyingrel-

evantchargetransfersbetweenboundreactantsandthe

catalyticsurfaceorbetweenthelightabsorberandcatalyst.

Innaturalphotosynthesis, themanipulationof the

ligandfieldsoftheMnd-electronorbitalsintheMn4Ca

complexmodulatesthecharge-transferstatesandthe

chargedensityofthemetal/ligands,whicharecriticalfor

catalyticactivity.Thesameideaisapplicabletothecata-

lysts(water-splitting,hydrogenproduction,orCO2reduc-

tion) that are embedded in the artificial systems. In

addition,directionalityofthereaction,whichiscontrolled

bythemetal-ligand,catalyst-linker,andabsorber-linker

interfaces,willalsoperturbthefunctionofthecatalysts.

Suchelectronic structural changescanbestudiedby

3.1.3 NGLS: New Capabilities for Research in Natural and Artificial Photosynthesis

Thesub-femtosecondpulseduration,highpulseenergy,

and100kHz–1MHzpulse repetition frequencyof the

NGLSwillopenuptime-resolvedX-rayabsorption,emis-

sionandRIXSexperimentsonnaturalandartificialpho-

tosynthetic systems that will lead to a new level of

mechanisticunderstandingbeyondthereachofexisting

experimental tools. Inparticular, simultaneousmulti-

wavelength,time-resolvedX-rayprobingofmetalcen-

tersandcoordinationenvironmentsoflightabsorbers,

catalystsandinterfacesacrosscompletephotosynthetic

assembliesuponexcitationwithultrafastlightpulseswill

revealtheinterplayofenergy,chargemovement,and

chemical transformationsunderoperatingconditions.

Thehighspatialresolutioncombinedwithtime-resolved

capability from ultrafast to milliseconds will allow

3Dmappingofchargeflowacrossthecomplexheteroge-

neousstructuresofasolar fuelsystem.Theresulting

understandingofhowdynamicandreactiveeventsaffect

energeticsandelectronic structureofallpartsof the

assembly,underreactionconditions,willprovideinsight

forimprovingsolartofuelefficiencythatiscurrentlynot

available.

Outlined below are examples of experimental

approachesopenedupbytheNGLSforunderstanding

bothnaturalandartificialphotosyntheticsystems.

3 .1 .3 .1 Photon Demanding Experiments

Time-resolvedX-rayabsorptionexperimentsatsyn-

chrotronsourceshaveverylimitedcapabilitytofollow

theelectronicandstructuralconfigurationofphoto-excit-

edmoleculesandbulkphasetransitions(owingtolimits

intimeresolutionandaveragefluxavailable).Inaddition,

criticalforthedesignofhighefficiencyartificialphoto-

syntheticsystemsismonitoringtheevolutionofinsitu

photo-driven catalysts (similarly limited by available

averageflux).Experimentsevenatmstimescalesarea

significant challenge, and key experiments requiring

ultrafasttimeresolutionareimpossible.Theseinclude

transientdelocalizationor localizationoforbitals,and

evolutionofnewbondingconfigurationsthatprecede

anddirecttheformationofthosecatalyticintermediate

states.Allrequirehighaveragephotonfluxandhightime

resolutionsincethefasterintermediatescannotbeaccu-

mulatedovertime.

Transition metalK-edge

Transition metalL-edge

M M

Continuum

Valencelevel LUMO

HOMOLUMOHOMO

3p 3p

2p 2p

1s

Abso

rptio

n

Emis

sion

1s

Continuum

Abso

rptio

n

Emis

sion

Figure7 Left: The energy level diagrams showing the hard-X-ray Kß emission lines that probe the charge density and electronic structure of the ligands of Mn. Right: The soft X-ray emission ener-gy level diagram that are sensitive to d-d transitions and charge transfer states that are probed by soft X-ray emission and RIXS.

15


FEL’sfallshortoftheserequirementsbymanyordersof

magnitude.

3 .1 .3 .2 Multi-color Time-Resolved Experiments

TheNGLSopensupthepossibilityofsimultaneous

measurementswithX-raysoftwoormoredifferentcol-

ors.Thiscapabilityhasaprofoundinfluenceonourability

tounderstandinteractionsbetweendifferentelements

and/orspatiallyseparatedunits.

AfundamentalmysteryoftheOECinnaturalphoto-

synthesisisthedistributionofchargeassociatedwiththe

fouroxidizingequivalents.Howarethechargesdistribut-

edanddoescoherentcorrelationamongthechargespro-

videforenhancedstability?Inartificialphotosynthetic

systems, in which multi-metals are involved as light

absorbersandoxidationandreductioncatalysts,probing

differentmetalsitessimultaneouslyovermanydecades

oftimeprovidesapowerfultoolforstudyinghoweach

activecomponentissynchronizedwithothers.Element

specificity of X-ray spectroscopy with time-resolved

detectionwillbeessential foraddressingtheseques-

tions.Ahigh-repetition-rateX-raylaserwillprovidepow-

erfulprobesincludingRIXS(spontaneousX-rayRaman

scattering)andmoreadvancedmulti-colorapproaches

suchasstimulatedX-rayRaman(CXRS)andX-raywave

mixing(asdescribedinSection4.3).

BothhardandsoftX-rayRIXSspectroscopyprovide

detailedelectronicstructuralchangesatthecatalyticsites

orlightabsorbersbysimultaneouslyprobingtwoX-ray

photonfrequencies,i.e.incoming(absorption)andscat-

tered(emission)photons(seeFigure7).StimulatedX-ray

Ramanspectroscopy(Section4.3)furtheraddstime-sen-

sitivity toRIXS,andprovides informationofvalence-

excited-statedynamicsbyfollowingthechargeflowfrom

oneatomicsitetoanother.Intheartificialsystem,for

example,a localizedvalence-excitedstateat the light

absorberatomiscreatedbyapumppulsetunedtoaspe-

cificcore-leveltransition(impulsiveX-rayRamanexcita-

tion,seeFigure74),andtheevolutionofsuchastateis

followedbyacontrolleddelaytimewhenasecondprobe

pulseinteractswithacatalyticsite.Thus,onecanfollow

thechangeofsiteAinresponsetothechangeofsiteB,

anddetectcoherentcouplingbetweenthem.Thecapabil-

ities of NGLS will enable such multi-color multi-

dimensional pump-probe experiments in the light

absorber—catalystpair,ordifferentelementsiteswithin

thecatalyst.ThisapproachisanalogoustoNMRtech-

time-resolvedKβemission(Figure7, left)orsoftX-ray

emission/RIXSspectroscopies(Figure7,right).However,

these much more photon-demanding experiments

require sensitivity to detect weak signals that probe

valencetocoretransitions,d-d transitions,andcharge-

transferstateswhicharelessprobable(lessintense),yet

highlysensitivetothechemistry.Thesewillonlybepos-

sibleattheNGLSwithhighrepetitionrate(100kHzto

1MHz),moderatefluxperpulsetoavoiddisruptingthe

statesbeingprobed,andtimestructuretoallowforsam-

plereplacement/recoverybetweenpulses.

Time-resolvedhardX-ray

XES studies of ligand-to-

metaltransitionsareessen-

tial for understanding the

water oxidation chemistry,

as these measurements

directlyprobetheligandsof

the metal (Kβ). However,

such transitions are much

weaker(~10timesforKβ1,3

andKβ’,~500timesforKβ2,5

andKβ”)thantheKαemis-

sion signals making them

impossible without NGLS

capabilities.

Time-resolvedsoftX-ray

absorption/emissionstudies

arealsokeytounderstanding

thecomplexchemistryofthe

catalytic function as they

directlyprobethetransient

metal electronic states

throughtransitionsfromthespin-orbitsplitmetal-2plevels.

Furthermore,softX-rayRIXSallowsdirectmeasurementof

chargetransferprocessesthatarenotpossiblebyanyother

knownmethod.Additionally,itisnearlyimpossibleto

collectsoftX-rayspectroscopydatafrombiologicalcata-

lystsatsynchrotronfacilitiesunderambientcondition

withinthetimescaleofradiationdamage.X-raysource

requirementsinclude:(1)tunabilityacrossthemetalL-

andligandK-edges;(2)temporalresolutionof~50fsor

better;(3)averagesourcefluxupto1015ph/s/(0.1%BW);

and(4)timingstructuretoenablerapidsamplereplace-

mentbetweenmeasurements(e.g.flowingliquidjets).

TheserequirementsarewellmatchedtoNGLS,while

present3rdgenerationsynchrotronsandsoft/hardX-ray


Two-color X-ray probe

Time-resolved XAS, XES, XANES, EXAFS

Native environments


High-resolution RIXS

X-ray pump, X-ray probe

Stimulated X-ray Raman (CXRS) – wave mixing

Core-hole correlation – wave mixing

see Section 4.3

16


towardshighefficiencysystemsisexpectedtotakemany

decadesintheabsenceofmajornewtools(forcompari-

son,considerthe65yearsofresearchonsolarphotovol-

taicmaterialsforsunlighttoelectricityconversionsince

theirfirstappearance).OneanticipatedimpactofNGLSis

theshorteningofprogresstowardshighefficiencyartifi-

cialphotosyntheticsystemsbydecades.Thisisinaddi-

tiontoanticipatedmajorfundamentalbreakthroughsin

thefieldofexcitation,chargetransport,andchemical

transformationincomplexheterogeneoussystems.

Beamlines for Photosynthesis Research

Visible-pump,X-ray-probespectroscopyexperiments

onnaturalandartificialphotosyntheticsystemswillrely

primarily on the seeded NGLS beamlines 1 and 2 as

describedinSection5(Table2).Theseexperimentswill

useone-color(andinsomecasestwo-color)Xrayprobes

tofollowvalencechargedynamicsviaXASandXESat

transition-metalL-edgesandligand(e.g.O,N)K-edgesin

thesoftX-rayrange.EXAFSprobesoflocalstructural

dynamicswillrelyonhardX-raysatthe3rdand5thhar-

monicstoprobetransition-metalK-edges(andbeyondfor

hardX-rayXES).Multi-dimensionalspectroscopyexperi-

mentswillrequirethetwo-colorsub-femtosecondcapa-

bilitiesofbeamline2.SoftX-rayRIXSexperimentswill

relyonthehighenergyresolution(andhighaverageflux)

of NGLS beamline 1 in long-pulse seeded operation

(<50meVresolutionwithoutamonochromator,andhigher

resolutionwithamonochromatoratsomelossofflux).

Manyoftheseexperimentswilluseflowingsamples

(orsamplereplacementorrastering)inordertoprovide

forphysiologicalconditionsandtopreparethesameini-

tialstateforeachprobepulse.Thissetsalimitof~100kHz

ontheusablerepetitionrate.Amaximumfluenceper

pulse in themJ/cm2 range is anticipated, inorder to

insurethattheX-rayprobedoesnotdisrupttheelectronic

statesbeingmeasured(seeAppendix1).

niques which resolve chemical-specific nuclear spin

coherences.MultidimensionalX-raytechniques(Section

4)willenable2Denergymappingofelement-specific

valencechargecoherencesforthefirst time.Withthe

short-pulsecapabilitiesofNGLS,wewillbeabletofollow

theevolutionofsuchcoherencesonthesub-femtosec-

ondtimescale.

3 .1 .3 .3 Ambient, Real Time Measurements

UltrafastX-raylasersoperatingwithuniformlyspaced

pulsesat10μsintervals(100kHz)willenableanimpor-

tantnewexperimentalapproachfortime-resolvedX-ray

spectroscopymeasurements,particularlyforthenatural

photosyntheticsystem.Inthisscheme,thesampleisnot

frozen,butremainsatambienttemperatureinsolution,

consistentwiththenaturalenvironmentinwhichphoto-

synthesisoccurs,andthesampleisreplaced(e.g.via

flowing jet)oneachpulse.Thus,eachcombinationof

laser-pumppulse(s)initiates,fromafreshsample,the

formationofaparticularstate(S1,S2,..Sn)storinganum-

berofoxidizingequivalents,andanultrafastX-rayprobe

pulseinterrogatestheformationandevolutionofthat

statewithunprecedenteddetail.Suchanapproachisnot

practicalatpresentsynchrotronsourcesowingtothe

ultrashort(~2ns)intervalbetweenpulsesthatisusedin

ordertoachievethehighestpossibleaverageflux.

Impact of NGLS on Solar Fuel Generator Development

Thelevelofmechanisticunderstandingofcomplete

integratedsolarfuelsystemsunderreactionconditions

thatwillemergefromresearchenabledbyNGLSX-ray

laserswilldramaticallyacceleratethedevelopmentof

highly-efficientsolarfuelgeneratingsystems.Withfirst

prototype systems performing at modest efficiencies

expectedtobedevelopedinthenextfewyears,progress

17


Time-resolved XES Experiments — Photosynthesis

Thefollowingestimatesillustratethescientificneedforahigh-repetition-rateultrafastXraylaserforphotosyn-

thesisresearch:Required integrated flux on the sample: ~1017 photons for 100 time points

ph/pulse (usable) Rep . rate [Hz]Time to do experiment

Time resolution

StorageRing 105[3] 105[1] 100 days 100ps

PulsedFEL 108[2] 102 100 days ~fs

NGLS 108[2] 105[1] 3 hrs ~fs

[1] Rate limit:~105Hz,determinedbysamplereplacementrestrictions

-physiologicalconditions—liquidjets

-storagerings(multibunch)presentlylimitedto

cryogenicallytrappedstates

[2] Fluence limit:~1mJ/cm2toavoiddisruptionofelectronicproperties;100meVBW

(e.g.1keV,108ph/pulse,50μmfocalspot⇒1mJ/cm2)

[3] Bandwidth limit:100meVBWand~10xlossesfrommonochromatoroptics

Nominal Storage Ring Source:

Flux ~5x1015ph/s/0.1%BW@1keV(withoutmonochromatorlosses)

Rep.rate 5x108Hz

Pulseduration 100ps

Nominal Storage Ring Source with Bunch Tilting:

Flux ~6x1012ph/s/0.1%BW@1keV(~106ph/pulse/0.1%BW)(withoutmonochromatorlosses)

Rep.rate 6x106Hz

Pulseduration ~1ps

18


References:

1. Yano, J., et al., Where Water Is Oxidized to Dioxygen: Structure of the

Photosynthetic Mn4Ca Cluster. Science, 2006. 314(5800): p. 821-825.

2. Kok, B., Forbush, B., and McGloin, M., Photochem. Photobiol., 1970. 11: p.

457-476.

3. Lewis, N.S. and D.G. Nocera, Powering the planet: Chemical challenges

in solar energy utilization. Proceedings of the National Academy of

Sciences of the United States of America, 2006. 103(43): p. 15729-15735.

4. Khaselev, O. and J.A. Turner, A Monolithic Photovoltaic-

Photoelectrochemical Device for Hydrogen Production via Water

Splitting. Science, 1998. 280(5362): p. 425-427.

5. Office of Science Workshop Report: Basic Research Needs for Solar

Utilization. 2005, U.S. Dept. of Energy.

6. Ferreira, K.N., et al., Architecture of the photosynthetic oxygen-evolving

center. Science, 2004. 303(5665): p. 1831-1838.

7. Loll, B., et al., Towards complete cofactor arrangement in the 3.0 angstrom

resolution structure of photosystem II. Nature, 2005. 438(7070): p. 1040-1044.

8. Yano, J. and V.K. Yachandra, Where Water Is Oxidized to Dioxygen:

Structure of the Photosynthetic Mn4Ca Cluster from X-ray Spectroscopy.

Inorganic Chemistry, 2008. 47(6): p. 1711-1726.

9. Yano, J., et al., X-ray damage to the Mn4Ca complex in single crystals of

photosystem II: A case study for metalloprotein crystallography.

Proceedings of the National Academy of Sciences of the United States

of America, 2005. 102(34): p. 12047-12052.

10. Risch, M., et al., Cobalt-Oxo Core of a Water-Oxidizing Catalyst Film.

Journal of the American Chemical Society, 2009.131(20): p. 6936-6937.

11. Kanan, M.W., et al., Structure and Valency of a Cobalt-Phosphate Water

Oxidation Catalyst Determined by in Situ X-ray Spectroscopy. Journal of

the American Chemical Society, 2010. 132(39): p. 13692-13701.

12. Weare, W.W., et al., Visible Light-Induced Electron Transfer from Di-μ-

oxo-Bridged Dinuclear Mn Complexes to Cr Centers in Silica Nanopores.

Journal of the American Chemical Society, 2008.130(34): p. 11355-11363.

13. Nilsson, A. and L.G.M. Pettersson, Chemical bonding on surfaces probed

by X-ray emission spectroscopy and density functional theory. Surface

Science Reports, 2004. 55(2-5): p. 49-167.

14. Della-Longa, S., et al., Direct Deconvolution of Two-State Pump-Probe

X-ray Absorption Spectra and the Structural Changes in a 100 ps

Transient of Ni(II)-tetramesitylporphyrin. Inorganic Chemistry, 2009.

48(9): p. 3934-3942.

15. Khalil, M., et al., Picosecond X-ray absorption spectroscopy of a photo-

induced iron(II) spin crossover reaction in solution. Journal of Physical

Chemistry A, 2006. 110(1): p. 38-44.

16. Huse, N., et al., Photo-Induced Spin-State Conversion in Solvated

Transition Metal Complexes Probed via Time-Resolved Soft X-ray

Spectroscopy. Journal of the American Chemical Society, 2010. 132(19):

p. 6809-6816.

17. Liu, H., et al., Nano Lett., 2007. 7: p. 1919-1922.

18. Jiao, F. and H. Frei, Nanostructured Cobalt Oxide Clusters in Mesoporous

Silica as Efficient Oxygen-Evolving Catalysts. Angewandte Chemie, 2009.

121(10): p. 1873-1876.

19

3 . SCIENCE DRIVERSFUNDAMENTAL ENERGY AND CHARGE DYNAMICS

areseparable from thoseof thenuclei inamolecule

based on their disparate time responses (Born-

Oppenheimerapproximation).Thisassumptionbecomes

increasinglyinvalidandmisleadingwhenattemptingto

understandphotochemicalprocessesinlargemolecules.

Thus,asecondandcrucialstepinpresenttheoretical

descriptionsistorepairthisassumptionwhenitbreaks

downnearspecificpointsinthecourseofaphotochemical

reaction.Theseareconicalintersections(seamsofinter-

sectionbetweenthepotentialenergysurfacesofdifferent

electronicstatesofamolecule)thatattempttodescribe

viaquantumchemistrytheregionswhereelectronicand

nucleardynamicsare closely coupled (and theBorn-

Oppenheimerapproximationisnotapplicable).

Importantly,theconversionofenergyfromlightinto

chemicalenergyincomplexmoleculesoftenproceeds

throughmultiplebreakdownsoftheBorn-Oppenheimer

approximation,withprofound influenceonthecourseof

thereactionpathway.Presently,however,weareunable

toobservethisfundamentalphenomenoninunambigu-

ousdetail—eveninsimplemolecules.Toadvanceour

fundamentalunderstandingofphotochemistryandsolar

Theflowofenergyandelectricchargeinmolecules

arecentraltobothnaturalandman-mademolecularsys-

temsthatconvertsunlightintofuelsordirectlyintoelec-

tricity.Understandingandcontrollingtheseprocesses

remainsafundamentalsciencechallenge,inlargepart

becausewelacktherequisitetoolstoprobethesepro-

cesses—simultaneouslyattheatomiclevelandonnatural

timescales.NGLSwillprovidequalitativelynewprobes

ofenergyandchargeflowandhowtheyworkinsimple

andcomplexmolecularsystems.

How is Electronic Energy from the Absorption of Visible or Ultraviolet Light Converted into Chemical Energy in Molecular Systems?

Energyfromtheabsorptionofvisiblelightbyelec-

tronsinamoleculeisconvertedtochemicalenergyvia

couplingtonuclearmotionandbondingbetweenatoms.

Today,muchofourunderstandingofthiscentralprocess

insolarenergyconversionandphotochemistryisbased

onaninitialassumption:namelythattheelectrondynamics

When visible or ultraviolet light interacts with a molecule, the energy is initially absorbed by the electrons that are also responsible for molecular bonding, and then couples to motion of the atoms. Unless this excess energy is rapidly channeled into a coordinated rearrangement of chemical bonds and/or migration of electrical charge, it is quickly converted into heat and lost. For example, a key feature of light harvesting complexes which produce chemical fuels or electrical current, is that they are designed to rapidly channel the energy of electronic motion into specific and useful molecular pathways, by breaking and forming particular chemical bonds, or moving elec-trical charge to specific molecular locations.

The fundamental mechanisms of energy and charge migration in molecular systems are still only partially under-stood, and modeled only by limiting approximations. Direct observation of the intramolecular machinery remains difficult because measurements must be made on the ultrafast time scales of electron motion, bond breaking and formation, and subtle nuclear motion. NGLS will provide ultrafast pulses of extreme ultraviolet and X-ray radiation at high repetition rates that will allow us to probe the details of these fundamental processes of energy and charge migration on the relevant time-scales of attoseconds to femtoseconds. These experiments will bridge a critical gap in our understanding, and will facilitate bottom-up molecular design principles for the development of new energy producing systems.

3.2 FundamentalEnergyandChargeDynamics

20


energysurfaces.Wecurrentlyhaveonlyarudimentary

understandingofthiscentralprocess,eveninsmallmol-

ecules,andverylittleabilitytoobserveitonanatomic

scaleinmoleculesofanysize.NGLSwillenableanarrayof

experimentstofollowchargemigrationinbothlargeand

smallmoleculeswithatomicresolutionbytargetingspe-

cificatomic“reporter”sitesforX-raydynamicalinvesti-

gationsonfemtosecondandevenattosecondtime-scales.

3.2.1 NGLS: Probing and Visualizing Coupled Electronic and Nuclear Dynamics in Molecules — Motion Through Conical Intersections

3 .2 .1 .1 The Conversion of Electronic Energy to

Chemical Energy

Biologicalsystemsandlightharvestingcomplexesare

drivenintoelectronicallyexcitednon-stationarystatesby

theabsorptionofvisibleorUVphotonsorthroughcharge

exchange with neighboring molecular systems.The

resulting coherent superposition of excited states (a

quantumwavepacket)rapidlyevolvesundertheinflu-

enceofthecoupledelectronicandnucleardegreesof

freedom.Inthiswaytheenergyofelectronicexcitationis

converted into nuclear motion and chemical energy.

Manytheoreticalandexperimentalstudies2-6haveshown

thatconicalintersectionscanprovidethemechanismfor

extremelyfastchemicalprocesses,e.g.photo-dissocia-

tion,photo-isomerization,andinternalconversiontothe

electronicgroundstate.Time-dependentquantumwave-

packetcalculationshaveestablishedthatradiationless

transitionsviaconicalintersectionsbetweenelectronic

statescantakeplaceonatimescaleof10fsorless.Itis

nowwidelyacceptedthatconicalintersectionsareomni-

presentinpolyatomicmolecules,andarefundamentally

important for understanding reaction mechanisms in

photochemistryandphotobiology.7-10

3 .2 .1 .2 Prior State-of-the-Art

Femtosecond time-resolved methods have been

appliedtochemicalreactionsrangingincomplexityfrom

bond-breakingindiatomicmoleculestodynamicsinlarg-

erorganicandbiologicalmolecules, andhave led to

breakthroughsinourunderstandingoffundamentalpro-

cesses.11,12Asachemicalreactioninitiatedbyapump

pulseevolvestowardproducts,oneexpectsthatboththe

electronicandnuclearcomponentsunderobservation

fuelproductionweneedanexperimentalcapabilitythat

willprobethecentralmechanismofenergyconversionin

molecules.Thecombinationofhighaveragepowerand

ultrafast pulses provided by NGLS X-ray lasers will

enableustoprobeindetailthecoupledelectronicand

nuclearmotionthatareincreasinglyacknowledgedto

determinethemolecularpathwaysinlight-drivenreac-

tions.This new class of experiments will allow us to

developnewprinciplesthatwillguidethedesignofmol-

eculesthatwillbeessentialcomponentsofrenewable

energytechnologiesrangingfromsolarfuelproduction

toutilizationandstorage.

How Does the Combination of Nuclear Dynamics and Electron Dynamics Cause Charge Migration in Large Molecules?

Chargemigrationinmoleculesisnotjustthemove-

mentofelectrons.1Thenucleimustalsomoveinorderto

directandlocalizechargeatanewlocation.Thisisakey

stepintheoperationoflightharvestingmolecularsys-

tems,whethernaturalorartificial.Theroleofenergetic

barriersandthedegreeoftheirreversibilityiscentralto

theprocessofpassingchargefromonestructuretothe

next.Electronicsuperpositionstatescaninitiatecharge

flowevenonatime-scaleofafewfemtoseconds,which

mustbeinvestigatedtounderstandtheroleofthesubtle

nuclearmotionsthatguidesystemsfromonestateto

anotherthroughimportantconicalintersectionsofpotential

t = 0 t = 4 fs

t = 8 fs

Figure8Simulation of charge migration in a molecule with nuclei fixed. A localized electron hole is created, migrates to the opposite end of the molecule but returns on the femtosecond time scale in the absence of simultaneous nuclear motion. (Courtesy L. Cederbaum32)

21


system, very little progress has been made from the

experimentalside,mainlyduetothelimitationsofexist-

ingtoolsandphotonsources.NGLSX-raylaserswith

MHzrepetitionrateandshortpulseswillchangethat.The

excitationwillbeperformedwithaUVphotonandthe

probewillbeperformedwithadelayed,ultrashortX-ray

pulsetunedjustabovethecarbonK-edge.

Hereonecanuseareactionmicroscope(COLTRIMS)

typeofdetector toperform“rare-event” coincidence

measurements and provide complete momentum-

resolvedmeasurementsateachtime-stepbyresolving

themomentaofthephotoelectron,Augerelectron,and

two positively charged ionic fragments, as shown in

willchange,andinthecaseofnon-adiabaticcouplings

thesetwoareentangled.Variousexperimentalpump-

probetechniqueshavebeenusedinthepast,buteach

givesaccesstospecificaspectsofthecomplexnon-adia-

baticreactiondynamicswhileremainingmoreorless

blindtoothers.Amongtheexperimentalmethodsused

nowaretransientabsorption,time-resolvedionproduc-

tion, time-resolved photoelectron spectroscopy, and

morerecently,time-resolvedmolecularframephotoelec-

tronangulardistribution.13-20Thislattermethodallowsa

directmonitoringoftheevolvingexcitedstateelectronic

configurationsduringthechemicalreaction.

3 .2 .1 .3 An Entirely New Capability at NGLS

Inordertoprobesimultaneouslytheelectronicaswell

asthenuclearpartofthecoupledsystem,onehasto

measurealloftheaboveelementsincoincidencebyper-

formingakinematicallycompleteexperimentat each

time step.Akinematicallycompleteexperimentisonein

whichthemomentaofallthecomponentsofasystem

beingpumpedaremeasuredsimultaneously(typically

viaprobe-pulseionization,followedbyTOFspectroscopy

ofthechargedfragments).Kinematicallycompleteexper-

imentsatthirdgenerationlightsourcesaresuccessfully

andwidelyusedtostudystationaryground-statemole-

cules using reaction microscopes.21-24The NGLS will

enabletheextensionoftheseexperimentstoevolving

excitedstatedynamics.Itwilladdthecriticaldimension

oftimeonthefemtosecondandsubfemtosecondscales.

3 .2 .1 .4 . An Example Experiment

Asanexample,theπ→π*transitioninethyleneserves

asaprototypicalsystem.Ethylenehasattractedanenor-

mousamountofattentionfrombothexperimentalists

and theorists for its highly non-Born-Oppenheimer

behavior.25-28Varioustheoreticalmethodspredictthat

afterπ→π*excitation,themoleculeexperiencesanultra-

fastdecaybacktothegroundstatethroughtwoconical

intersections,oneoccurring

ata twisted-pyramidalized

structureshowninFigure9

andtheothernearanethyli-

deneconfiguration(HCCH3)

whereoneofthehydrogens

has migrated across the

double bond. Despite the

apparent simplicityof this

0

2

4

6

PyramidalizationTwist

S1

S0

Ener

gy /

eV

8

10

12

Figure9 Illustration of a conical intersection that provides control for directing chemical reaction pathways. (Courtesy Todd Martinez)

H

Ethylene

Fermtosecond delayUV pump

Photoelectron Auger electron

IonIon

X-ray probe

C

H

C

H H

Ethylidene

Figure10Illustration of coupled electronic and nuclear motion in ethylene. Isomerization dynamics are initiated with a UV excitation pulse (left), and the molecular structure is probed a variable fem-tosecond time delays using an X-ray pulse to ionize the molecule.


Two-color, X-ray probe (sub-fs)

Time-resolved Reaction Microscope

22


willdetect incoincidencetwophotoelectronsemitted

fromthetwodifferentmetalcenters.Thephotoelectron

energyfromeachcenterisexquisitelysensitivetothe

valenceelectrondensityaroundthecenter.Anillustration

measurementofthecoincidentphotoelectronenergiesis

showninFigure12.The2Dcoincidencemapswillpro-

vide,ateachtimestep,afingerprintofthestateofthe

valencechargedensityateachmetalcenter,thusprobing

thechargeflowasthechargeoscillatesbackandforth

betweenthesites.Thisisaverypowerfulcapabilitythat

will take the traditional time-resolved photoelectron

spectroscopy(TRPES) techniquetomulti-dimensional

spectroscopy,inthiscasetwo-dimensionaltime-resolved

photoelectronspectroscopy2D-TRPES.Tofurtherinsure

thatthetwophotoelectronsareemittedfromasingle

molecule,theionicfragmentsaredetectedincoincidence

withtheelectronsviaatime-of-flighttechnique.

Figure10.Thedetectionoftheheavierfragmentswillpro-

videamolecularframeofreferenceanddeterminethe

energythatischanneledintonuclearmotion.Thecom-

pletetransientenergymapofthefourparticlesprovides

critical informationonthestronglycoupledelectronic

andnucleardynamicsatandaroundtheconicalintersec-

tion.ThephotoelectronangulardistributionsandAuger

electronangulardistributionsinthemolecularframeare

exquisitelysensitiveto theelectronicstatesandtheir

symmetriesaswellasthepositionofthenucleiatthe

timeoftheprobe.29-32Thiscoincidentdetectionofallthe

fragmentsasafunctionofdelaytimefromtheUVexcita-

tion,combinedwiththeoreticalwork,willgiveaunique

insightthatwillallowustounderstandthecoupledelec-

tronicandnucleardynamics,andelectroncorrelationin

thevicinityofconicalintersections.

3.2.2 Direct Probe of Charge Flow in Molecules: Multi-Color X-ray Pump-Probe Experiments

3 .2 .2 .1 A New Capability at NGLS

MultipleX-raypulsescanserveasprobesofcharge

migrationincomplexmolecules.Asimpleexampleinvolves

the electron dynamics in dimeric metalloporphyrins.

Suchmetalloporphyrinsaremostlyineithertheend-on

(linear)ortheside-on(cofacial)configuration.Whilecofa-

cialdimersrepresentthesimplestmodelsystemforbio-

logicallightharvestingcomplexes,end-ondimersareof

interestduetotheirpotentialapplicationasmolecular

wires. It has recently been postulated that quantum

coherencemightplayanimportantroleinthetransferof

excitationenergyinbiologicallightharvestingcomplexes

withhighquantumefficiencies.33Byemployingmetallo-

porphyrin dimers with dissimilar metal centers, e.g.,

Fe(II)-Ni(II),itwillbepossibletotracktheelectronmotion

alongtheaxisofthedimerbyprobingthecoreleveltran-

sitionsassociatedwitheachmetalcenter.Theelemental

specificityofX-rayprobingmakesitpossibletodeter-

mineonwhichsubunittheelectrondensityislocalizedas

afunctionoftime.

NGLSwillprovideamuchmorepowerfultoolbeyond

existingcapabilitiesbyenablingsimultaneousprobing

withtwoX-raysattwodifferentsites.EachX-rayistuned

toafeweVabovethecorrespondingedgeofoneofthe

metals,asshowninFigure11.Insuchanexperimentwe

UV pump at t = 0

eFe

eNi

Fe

N N

N N

Ni

N N

N N

X-ray probe at t = T/2eNi eNi

eFe eFe

X-ray probe at t = T

Figure11 Dimeric metalloporphyrin complex. Ultrafast optical excitation (e.g. of the charge-transfer band) initiates inter- and intra-molecular charge dynamics that can be probed with element specificity via time-resolved photoelectron spectroscopy.

Figure12 Illustration of 2D coincidence maps of photoelectron spectra. Charge oscillations between Ni and Fe centers will lead to characteristic shifts of the respective photoelectron spectra that will be out of phase by a half oscillation period.

23


evolve,forexampleduringchargetransferprocesses,or

duringreactionsthatchangethemoleculargeometry.

Core-holecorrelationspectroscopyprobesthecorrela-

tionsbetweencore-excitedvalencestatesattwodistinct

atomic sites in a molecule.34 Multidimensional X-ray

RamanspectroscopyexploitsonestimulatedX-rayRaman

processtocreateavalenceexcitation(localizedatone

atomicsite),andasecondstimulatedRamanprocessto

probethemigrationofthatexcitationtodifferentatomic

sites(withoutanycoreexcitations).35Multidimensional

X-rayspectroscopywillprovidecriticalinsightintocorre-

latedelectronsystemsandmolecularcomplexeswith

strongcouplingbetweenelectronicandnucleardynamics.

Beamlines for Investigating Charge and Energy Flow in Molecules

CoincidenceexperimentsandReactionMicroscope

experimentsofchargeandenergyflowinmoleculeswill

relyonvisible/VUV-pumpandsoftX-ray-probeathigh

repetitionratestocapturerarecoincidenceevents.Some

experimentswillrelyontheseededNGLSbeamlineas

describedinSection5(Table2).Themostdemanding

two-colorcore-hole-correlationmulti-dimensionalspec-

troscopyexperimentswillrelyontwo-colorsub-femto-

secondcapabilitiesofbeamline2.

3.2.3 Imaging Energy Flow in Large Molecules Using Multi-Dimensional X-Ray Spectroscopy

A new kind of X-ray spectroscopy that can be devel-

oped only at NGLS: MultidimensionalX-rayspectroscopy

(asdescribedinsection2.3.3)incorporatestime-ordered

sequencesofcoherentX-raypulsestogenerateasignal

thatisafunctionofmultipletimedelaysand/orphoton

energies.Theresulting2Dsignalmapsfollowcoherent

charge flow and energy relaxation between specific

atomic sites. Figure 13

shows a schematic four-

wavemixinggeometry(as

istypicallyemployedinthe

visibleregime).Inthecase

of the dimeric porphyrin

complexes as described

above, with X-ray pulses

tunedtotheNandNi(orN

andFe)absorptionedges,

off-diagonalfeaturesinthe

core-levelcorrelationspec-

troscopymapsshowthedegreeofcorrelationbetween

valence charges associated with N and with Ni (Fe).

Furthermore, suchmaps revealhow thesecorrelations

N-1s Ni-2p(Fe-2p)

Valence coupling

Time

Sample

t1 t2 t3

k1

k1

k2

k2

k3

k3

k4

k4

kS

Figure13 Schematic multi-dimension-al spectroscopy in dimeric porphyrin complexes. X-ray pulse sequences tuned to the N-1s and Ni-2p (or alter-natively Fe-2p) probe correlations between N-2p and Ni(Fe)-3d levels. Alternatively, X-ray pulses tuned to the Fe and Ni 2p-3d transitions may probe d-d transitions, correlations, and charge flow between the Ni and Fe sites.

Attosecond spectroscopy



see Section 4.3

24


6. Bixon, M. and J. Jortner, Intramolecular radiationless transitions. J.

Chem. Phys., 1968. 48: p. 715-726.

7. Satzger, H., et al., Primary processes underlying the photostability of iso-

lated DNA basis: Adenine. PNAS, 2006. 103: p. 10196.

8. D. Polli, et al., Conical intersection dynamics of the primary photoisomer-

ization event in vision. Nature, 2010. 467: p. 440.

9. C.E. Crespo-Hernandez, et al., Ultrafast excited-state dynamics in nucle-

ic acids. Chem. Rev., 2004. 104: p. 1977-2019.

10. Schultz, T., et al., Efficient deactivation of a model base pair via excited-

state hydrogen transfer. Science, 2004. 306: p. 1765-1768.

11. Zewail, A., Femtochemistry: past, present , and future. Pure and Applied

Chemistry, 2000.72: p. 2219.

12. Zewail, A., Atomic-scale dynamics of the chemical bond. J. Phys. Chem.

A, 2000. 104: p. 5660.

References:

1. Lünnemann, S., A.I. Kuleff, and L.S. Cederbaum, Ultrafast charge migra-

tion in 2-phenylethyl-N,N-dimethylamine Chemical Physics Letters, 2008.

450: p. 232-235.

2. Yarkony, D.R., Diabolical conical intersections. Rev. Mod. Phys., 1996. 68:

p. 985-1013.

3. Baer, M. and B.G. D., eds. The Role of Degenerate States in Chemistry.

Advances in Chemical Physics. Vol. 124. 2002, J. Wiley & Sons: Hoboken

Publishing Company: Hackensack, NJ.

4. Domcke, W., D.R. Yarkony, and H. Koppel, eds. Conical Intersections:

Electronic Structure, Dynamics, and Spectroscopy. Advanced Series in

Physical Chemistry. Vol. 15. 2004, World Scientific Publishing Company:

Hackensack, NJ.

5. Levine, B.G. and T.J. Martinez, Isomerization through conical intersec-

tions. Annu. Rev. Phys. Chem. , 2007. 58: p. 613-634.

Time-resolved Reaction Microscope Experiments

Thefollowingestimatesillustratethescientificneedforahigh-repetition-rateultrafastXraylaserforfundamental

researchonchargeandenergyflowinmolecularcomplexesusingreactionmicroscopetechniques:

Required integrated flux on the sample: ~1019 photons for 100 time points*


Time resolution


PulsedFEL 1010[2] 102 100 days ~fs

NGLS 109[2] 106 3 hrs ~fs

[1] Rate limit:~107Hz,determinedbyTOFenergyanalyzer.

Coincidencemeasurementsrequire<<1event/pulse,andtypically10-5oftheeventssatisfythecoincidencecriteria.

Thesampleconcentrationandflux/pulsearebalancedtosatisfythecriteriaof<<1event/pulse

[2] Temporal requirement:Fewfspulseduration,low-chargemode

[3] Bandwidth limit: 0.1%BWand~10xlossesfrommonochromatoroptics



Rep.rate 5x108Hz

Pulseduration 100ps



Rep.rate 6x106Hz

Pulseduration ~1ps

*Forcomparison,atypical(non-timeresolved)COLTRIMSexperimentataStorageRingrequiresdaysofsignalaccumulationat~1MHzrepetitionrate,withafluxof~1012ph/seconthesample,or~1017photonsintotal(foreffectivelyonetimepoint).

25


13. Stolow, A., A.E. Bragg, and D.M. Neumark, Femtosecond time-resolved

photoelectron spectroscopy. Chem. Rev., 2004. 104: p. 1719.

14. O. Gessner, et al., Femtosecond multidimensional imaging of a molecular

dissociation. Science, 2006. 311: p. 219-222.

15. Liu, S.Y., et al., Time-resolved photoelectron imaging using femtosecond

laser and VUV free-electron laser. Phys. Rev. A, 2010. 81: p. 031403(R).

16. Suzuki, T., Femtosecond time-resolved photoelectron imaging. Ann. Rev.

Phys. Chem., 2006. 57: p. 555.

17. Stolow, A. and J.G. Underwood, Time-resolved photoelectron spectros-

copy of non-adiabatic dynamics in polyatomic molecules. Advances in

Chemical Physics, 2008. 139: p. 497.

18. Reid, K.L., Photoelectron angular distributions. Annu. Rev. Phys. Chem.,

2003. 54: p. 397.

19. Bisgaard, C.Z., O.J. Clarkin, and G.R. Wu, Time-resolved molecular frame

dynamics of Fixed-in-space CS2 molecules. Science, 2009. 323: p. 1464.

20. Wollenhaupt, M., Engel.V., and T. Baumert, Femtosecond laser photo-

electron spectroscopy on atoms and small molecules: prototype studies

in quantum control. Annu. Rev. Phys. Chem., 2005. 56: p. 25.

21. Doerner, R., et al., Cold target recoil ion momentum spectroscopy: a

‘momentum microscope’ to view atomic collision dynamics. Phys. Rep.,

2000. 330: p. 95.

22. Ullrich, J., et al., Recoil-ion and electron momentum spectroscopy: reac-

tion-microscope. Rep. Prog. Phys., 2003. 66: p. 1463.

23. T. Jahnke, et al., Multicoincidence studies of photo and Auger electrons

from fixed-in-space molecules using the COLTRIMS technique. J.

Electron Spectr. and related Phenomena, 2004. 141: p. 229.

24. M. S. Schöffler, et al., Ultrafast probing of core hole localization in N2.

Science, 2008. 320: p. 920.

25. Kosma, K., et al., Ultrafast dynamics and coherent oscillations in ethylene

and ethylene-D4 excited at 162 nm. J. Phys. Chem. A 2008. 112: p. 7514.

26. Stert, V., et al., Femtosecond time-resolved dynamics of the electronically

excited ethylene molecule. Chem. Phys. Lett. , 2004. 388: p. 144.

27. Ben-Nun, M., J. Quenneville, and T.J. Martinez, Ab-initio Multiple

Spawning: Photochemistry from first principles quantum Molecular

Dynamics. J. Phys. Chem. A, 2000. 104: p. 5161.

28. Tilborg, J.v., et al., Femtosecond isomerization dynamics in the ethylene

cation measured in an EUV-pump NIR-probe configuration. J. Phys. B:

Atom. Mol. Opt. Phys. , 2009. 42: p. 081002.

29. T. Osipov, et al., Carbon K-shell photoionization of fixed-in-space C2H4.

Phys. Rev. A, 2010. 81: p. 033429.

30. Landers, A., et al., Photoelectron diffraction mapping: molecules illumi-

nated from within. Phys. Rev. Lett., 2001. 87: p. 013002.

31. A. Rudenko, et al., Exploring few-photon, few-electron reactions at

FLASH: from ion yield and momentum measurements to time-resolved

and kinematically complete experiments. J. Phys. B: Atom. Mol. Opt.

Phys., 2010. 43: p. 194004.

32. Krasniqi, F., et al., Imaging molecules from within: Ultrafast angstrom

scale structure determination of molecules via photoelectron holography

using free-electron lasers. Phys. Rev. A, 2010.81: p. 033411.

33. Liddel, P.A., et al., Photoinduced electron transfer in a symmetrical dipor-

phyrin-fullerene triad. Phys. Chem. Chem. Phys., 2004. 6: p. 5509.

34. Schweigert, I.V. and S. Mukamel, Coherent Ultrafast Core-Hole

Correlation Spectroscopy: X-Ray Analogues of Multidimensional NMR.

Physical Review Letters, 2007. 99(16): p. 163001.

35. Tanaka, S. and S. Mukamel, Coherent X-ray Raman spectroscopy: A non-

linear local probe for electronic excitations. Phys Rev. Lett., 2002. 89(4): p.

043001.

26

3 . SCIENCE DRIVERSADVANCED COMBUSTION SCIENCE

3.3 AdvancedCombustionScience

Combustionisnowandwillremainformanydecades

themostimportantmeansofenergyutilizationonearth.

Theenormousbenefitsofmoderncombustiontechnolo-

gies(e.g.reliableelectricity,rapidtransportation,heating

andcoolingetc.),areaccompaniedbynegativeconse-

quences,suchasthehealtheffectsofcombustionparticu-

lates,photochemicalsmog,andanthropogenicclimate

change.Itisincreasinglyimportanttoutilizecombustion

withgreaterefficiencyandfewerharmfulimpactsonthe

planetanditsinhabitants.Forexample,inthetransporta-

tionsector,newenergysourcessuchasbiomass-derived

fuelsofferanopportunitytooptimizethefuelstreamfor

newhighlyefficientengines,andtodevelopnovelfuels

that will help reduce greenhouse gas emissions and

enhancenationalenergysecurity.Climatechangecon-

cernscreateanurgentneedforthesesolutions,reflected

inthegoalfor80%greenhousegasemissionreductions

intheU.S.by2050.Becauseoftheinherentadvantagesof

liquidhydrocarbons,e.g.intransportabilityandenergy

density,itislikelythattheywillcontinuetobeusedas

energycarriersviafutureenergytechnologiessuchas

solarfuels.Accordingly,thereisanincreasingneedfor

predictivemodelsofenginecombustionthatareaccurate

fromthescaleofmoleculesandelectronsthroughthe

macroscopicscaleofenginecylinders.

ThegroundbreakingcapabilitiesofNGLSwillbeinstru-

mentaltocreatethesciencebaseforpredictivecombus-

tionmodels.Withthepromiseofnewdetailedexperiments,

andtheastonishingadvancesincomputation,aconcerted

effortwillbeabletodelivertrulyrigorousscience-based

modelsofcombustion,withgenuinepredictivepowereven

forunexploredfuelsandcombustionstrategies.Validating

suchmodelswillentailground-breakingexperimentalprobes

of combustionchemistryandphysics,manyofwhich

couldberealizedwiththehighrepetitionrate,highaver-

agepower,andcoherentradiationpromisedbyNGLS.

The extraction of useful energy from combustion — the liberation of heat by combining a fuel and an oxidizer in a rapid chemical reaction — provides the foundation for the transportation age. The internal combustion engine has dramatically altered the landscape for the movement of goods and people over the past century, and will continue to dominate transportation for decades. Air travel will depend on combustion for the foreseeable future. While the efficiency of combustion is continually improving, developing a fundamental basis for predict-ing performance in novel engine designs, with the concurrent emergence of dramatically different energy carri-ers such as biomass based fuels, will engage research for many decades. Validation of new high-level computational models of combustion and engine design will require ground-breaking experimental techniques to probe — at high pressure and temperature — the physical and chemical processes of combustion, from the injec-tion of liquid fuel sprays, through turbulent reacting flows, to pollutant formation.

NGLS will provide an arsenal of X-ray tools to image and quantify the dynamics of rapid combustion processes under realistic engine conditions. This includes the visualization of concentration gradients and turbulent flows, with chemical specificity, for the first time. The high repetition rate of NGLS will provide time-resolved tomo-graphic reconstructions of fuel spray breakup and soot particle formation in a truly real-time (high frame rate) movie of the physics governing the ignition and completion of the combustion cycle. With the available high pulse energy, X-ray coherent anti-Stokes Raman spectroscopy will reveal chemical speciation of the heteroge-neous mixture during the combustion cycle.

NGLS X-ray lasers will provide the advanced experimental tools for validation of predictive models that will per-mit high performance, high efficiency, and minimum environmental impacts to be designed directly into engines, a goal that is urgently needed to address national and global challenges in energy security and climate change.

27


discoveriesusinglaserdiagnostics.Itisclearthatmost

emergingenginetechnologieswillusedirectinjectionof

liquidfuelintothecombustionchamber.Itisimperative

tounderstandtheentirespray,howitbreaksup,mixes,

vaporizesandburns,inordertodevelopfullypredictive

modelsforenginecombustion.Althoughtheprocessof

fuel/airmixturepreparationviaanatomizingspraycon-

trolscombustionefficiencyandemissionsformationin

everytypeofdirect-injectedengine,itispoorlyunder-

stood,evenattheleveloffundamentalfluidmechanics.

Furthermore,manypotentiallyimportantcouplingsofthe

sprayfluidmechanicsandchemicalenvironmenthave

simplynotbeenexplored.Inordertomakefurtherprog-

ress,significantleapsarerequired.

Asasimpleexample,itisspeculatedthatundernor-

maldieselengineoperatingconditions,asmallfuel-rich

flameisstabilizedjustdownstreamoftheliquidjet,lead-

ingtosootformationatthetipoftheflame.2However,in

thisregion,thefuel/airratioandtheexistenceofaflame

haveneverbeenmeasured.Furthermore,undervery

highlevelsofexhaustgasrecirculation(EGR,usedto

lowerNOproduction)thisregionofthesprayappearsto

becompletelychanged,especiallyasitevolvesdown-

stream.3Again,thefuel/airratioisunknown,asisthe

chemical effect of EGR entrainment, or the differing

effectsof fuel-boundor freeoxygen.Exploring these

issuesdemandsinformationabouttheinteriorofsprays,

withchemicaldetailneverbeforeavailable.Finally,the

performanceofinjection,asformanyaspectsofengine

Challenges and Opportunities in Combustion Science

Becausecombustionreliesonacomplexinterrelation-

shipofchemistryandturbulentfluidmechanics,itspro-

cessesexhibitinhomogeneitiesandcorrelationsacrossa

widerangeoflengthandtimescales.Themostpowerful

classofexperimentalmethodsforturbulentcombustion

isdynamic imaging,thatis,resolvingthespatialandtem-

poraldistributionsinacombustionprocess.Furthermore,

becausethecouplingofchemicalandfluid-dynamical

phenomenaliesattheheartofcombustion’scomplexity,

chemically-specific imagingiskeyforcombustioninves-

tigations.NGLSX-raylaserswillimagethefundamental

chemistryandphysicsthatgoverntheentireprocessof

combustion,fromfuelspraystogas-phasecombustion

to particulate formation and evolution.The following

presentsthreecriticalareasincombustionscience(fuel

sprays,turbulentreactingflows,andparticulateformation

andoxidation)wherethesebreakthroughcapabilitiescan

addressproblemsthatremaindifficultorimpossibleto

solvewithothertechniques.Finally,wediscusshowthe

characteristicsofNGLSwillprovideunprecedentednew

experimentalcapabilitiestoaddressthesechallenges.

Chemistry and Physics of Fuel Sprays

Combustionenginesareundergoingradicalchanges,

broughtonlargelybynewscientificandtechnological

A B C D E

Figure14 A diesel spray in a chamber with a surrogate wall (lhs) to investigate wall interactions (lhs) and compare to a free jet (rhs), images cap-tures with a high speed camera: a) early in the injection, b) late in the injection, c) OH chemi-luminescence, d) Temperature via soot black body fits, e) relative soot levels [via emission (κ) determinations]. (From Eismark et al.1)

28


Ramanscatteringare low.

Here,thehighpower,coher-

ence, and narrow band-

widthofNGLSX-raylasers

willenablecoherentstimu-

lated Raman spectrosco-

pies7 that canbringmany

orders of magnitude

increaseinefficiencyoverspontaneousRaman.X-ray

coherentanti-StokesRamanspectroscopy(XCARS),for

example,alongaline-imaginggeometry8asshownin

Figure15,isapossiblewaytospatiallyresolvethechang-

esinchemicalbondingacrossthereactingspray,evenfor

highly scattering environments.The changes in the

molecularK-edgespectraaremappedontothefrequency

offsetoftheXCARSsignal.Thetemporalresolutionwill

bedeterminedbytheamountofaveragingrequired(as

dictatedbytheavailableaveragepower),butcoherent

X-rayRamanspectroscopyopensauniquewindowinto

the interiorchemistryofa fuelspray.Becauseall the

X-raywavelengthscanbechosentotransmitthroughthe

spray,andbecausetheXCARSsignalarisesonlyfromthe

areawhere thebeamscross, chemical information is

obtainedatchosenlocationswithinevenadensespray.

3.3.2 Cinematic Imaging of Reacting Flows

Thestochasticnatureofturbulentreactingflowsisa

criticalfactorincombustordesign,andunderstanding

turbulence-chemistryinteractionsiskeytodeveloping

robustandefficientcombustionstrategies.Despitesig-

nificant advances in 1D and 2D imaging of turbulent

flames,manyimportantobservablesarestillnotaccessi-

blewithcurrentimagingtechniques.Byitsnature,the

spatio-temporalstructureofturbulenceisnotrepeatable,

andthereforecomparisonsbetweennumericalsimula-

combustion,isoftengovernedbystochasticprocesses

andstatisticallyunlikelyadverseevents,suchaswallor

pistonwetting,makingtemporalandspatialresolution

important.

TheexperimentdepictedinFigure14illustratesthe

differencesbetweenchemicalenvironmentswhencom-

paringafreejettoawallinteraction.Inthecombusting

spraythechemicalbondingenvironmentofcarbon,for

example, moves: from liquid fuel to vaporized fuel;

throughthecombustionprocess;tocarbonmonoxide,

carbondioxide,ortoparticulatecarbon(soot).Correlation

ofthesechemicaltransformationswiththephysicsofthe

spray will be a tremendous breakthrough that will

becomepossiblewithNGLS.

Probingthemasstransport,differentialevaporation,

andoxidativechemistryoffuelflowsatthehighpres-

sures (~30 atm) and temperatures (~1000K) that are

directly relevant tohigh-efficiency/low-pollutionnext

generation engine designs is extremely challenging.

Lightscatteringinadensespraycanconfoundoptical

methods,requiringstrategiessuchasultrafastballistic

imaging4,5andX-rayabsorption6toprobethecritical

sprayformationregion,wheretheliquidcoreinitially

breaksup.Infact,evenwiththecurrentstateoftheart,

spatialandtemporalimagingofthefluidstructuresisat

bestincomplete.Imagingtheinteriorofthespraywith

chemicalspecificityisessentiallyimpossibleatpresent.

TherevolutionarycapabilitiesofNGLSpresentavery

promisingroutetomeetthesechallenges.Thepotential

fortime-resolvedchemically-specifictomographyoftur-

bulentgas-phasereactingflowsisdiscussedbelow(and

inSection4.2).Atthedensitiesofanevaporatinghigh-

momentumfueljet,transmissionimagingnearthecar-

bonoroxygenK-edgesissimplyimpossiblebecauseof

thesmallpenetrationdepths.Nevertheless,forfunda-

mental fluiddynamics,single-pulsedirectabsorption

tomographyinthe2-3keVregion(NGLSharmonics)will

permit unprecedented resolution of transient three-

dimensionalliquidstructureswithseparationofdroplet

andliquidcorefeatures.

ThetrulytransformationalaspectsofNGLS,however,

areintheabilitytochemicallyresolvespatio-temporal

structures.Thecouplingofthechemicalnatureofmulti-

phasereactingflowswiththefluidmechanicsislargely

unexplored.Inprinciple,X-rayRamanscatteringcanpro-

videsimilarchemicalinformationtonear-edgeabsorp-

tionspectroscopy,butcrosssectionsforspontaneous

Fuel spray(reactive flow)

Figure15 X-ray CARS diagram for chemically specific line imaging.

XCARS or Stimulated X-ray Raman (CXRS) – wave mixing

see Section 4.3

29


andwillrequirecinematic imagingexperiments(high

framerateimaging)tomakeacontinuous“movie”of

scalarandvectorflameproperties.Amongthepressing

needsforexperimentalmeasurementsare4Dmeasure-

mentsofspecies,temperature,mixturefraction(fueldis-

tribution), and scalar dissipation (rate of molecular

mixing) spanning the full range of turbulence length

scales.Thesemeasurementswillprovidenewinsights

into important phenomena, such as localized flame

extinctionandignition.HighrepetitionrateX-raylasers

offernewandrevolutionaryopportunitiestomakesuch

measurements.

Thefollowingsectionsdescribeafewofthepossible

experimental imagingprobesthatwillprovidecritical

insightintoturbulence-chemistryinteractionsandfacili-

tatethedevelopmentandvalidationofnext-generation

turbulentcombustionmodels.Accuratemodelsofthe

coupling between fluid dynamics and chemistry are

neededtoadvancethepredictivecapabilitiesofhigh-

fidelity,large-eddysimulations(LES)anddirectnumeri-

calsimulations(DNS).

Theimagingtoolsforspatio-temporalinterrogationof

turbulentflamesincludesingle-pulseX-rayfluorescence

imagingandsingle-pulsetomographyusingeitherfluo-

rescenceordirectabsorption.Thenarrowbandwidth

radiation(<100meV)ofNGLSwillbecriticalinproviding

chemicalspeciation,andphasecoherentimagingdemod-

ulation(cameraswithpixel-by-pixellock-indetection)will

increase sensitivity. Furthermore, the combinationof

highrepetitionrateandhighpulseenergyiscriticaland

notavailablefromsynchrotronsoranyenvisionedtable-

toplasersource.Finally,X-rayimagingwillbecombined

with(synchronized)conventionallaserdiagnosticsthat

indicateregionsoflocalflameextinctionorvelocityfield

measurements.

Time-resolvedX-rayfluorescenceimagingwillusethe

NGLSbeams(formattedintoalasersheet)forexcitation

atthelow-energysideofthecarbonK-edgenear284eV.10

Red-shifted fluorescence (265–283 eV) arising from

valenceelectronsfillingthe1scoreholewillbeimaged

perpendiculartothelasersheet.11Thefluorescencewill

notbesubstantiallyabsorbedbytheflameduetoitsred-

shift.Absorptioncrosssectionsof~10-18cm2perCatom,

fluorescencequantumyieldsof~3x10-3,and90%quan-

tumefficiencyimagingdetectorswillprovidesufficient

signaltonoiseforsingle-pulseimaging.Cinematic2D

imagesacquiredevery10µs(utilizingthe100kHzNGLS

tionsandexperimentsarecurrentlyperformedonasta-

tistical basis using conditionally averaged quantities

(mixturefraction,localstrainrate,dissipationrate,spatial

scales,etc.).Therelevantcorrelationsbetweenspecies

concentrations and flow conditions are incompletely

characterizedduetoalackoftemporalresolutionandthe

absenceofinformationonthethirdspatialdimension.

Theultimategoalofexperimentalreactingflowsisto

measurecomplete4Dmovies(space+time)ofthefluid

dynamics while simultaneously resolving molecular

identitiesandconcentrations.Achievingthisgoalwill

permittheuseofexperimentally-derivedinitialboundary

conditionsfornumericalflamesimulations.Thisapproach

willallowadirectcomparisonofthemodelwithexperi-

ment,andwillbearevolutionaryleapforwardinunder-

standingthecouplingofchemistryandfluiddynamics

thatisattheheartofcombustioninrealdevices.

AlthoughVUVradiationfromsynchrotronshasmade

major advances in our understanding of combustion

chemistry(e.g.,thediscoveryofenols,anewclassof

combustionintermediate9),theproposedexperiments

onturbulence-chemistryinteractionsrequirethecapabili-

tiesofahigh-repetition-rateX-raylaser.Direct4Dcom-

parisonwithcombustionmodelsarenotyetpossible,

TunableX-rayFEL

Laser-sheetformingoptics

High-repetition rate camera

Figure16 Planar X-ray fluorescence imaging of a turbulent flame.

30


otherspecies).Bycontrast,

in X-ray fluorescence, the

fluorescencelifetimeisgov-

erned by the dominant

Auger electron emission

lifetime(~10fs).16Thislife-

timeisthreetofourorders

of magnitude shorter than

thetimebetweenmolecular

collisionsinanatmospheric

pressure flame; therefore

theX-rayimagewillbeunaf-

fectedbycollisionalquench-

ing, greatly simplifying

quantitativeinterpretation.

Powerfulmulti-dimensionalvariantsonthistechnique

(asdescribedinmoredetailinSection4.3)willprovide

unprecedenteddetailforinvestigatingturbulentreacting

flows.Four-dimensional(space+time)imagingispossi-

blebyscanningthelasersheettomultipledifferentposi-

tionswithinthedepthoffieldofasingleimagingdetector

(seeSection4.2).Inthiswayafullspatialandtemporal

mapcanbeacquiredwithsufficienttimeresolutiontofol-

lowtheevolutionofflamestructures(usingNGLSrepeti-

tionratesupto1MHz).Chemicalspeciationcouldbe

achievedbytuningtheexcitationtonearedgefeatures

characteristicofimportantfunctionalgroups,providing

criticalinformationontherelativeconcentrationsofe.g.

aromatic,aliphatic,carbonyl,oretherfunctionalities.The

excitationlightmayalsobetunedtotheredsideofthe

oxygenedge(543eV)orthenitrogenedge(410eV)to

probe the distributions of species containing these

atoms.

Ansecondapproachto4Dchemicallyresolvedimag-

ingissinglepulsetomography—obtainingprojectionsof

theflametoreconstructitsspatialstructure,withsuffi-

cientframeratetofollowitstemporalstructure.Weenvi-

siontwotomographicapproachesappliedatthecarbon

K-edge,basedonfluorescenceandabsorption,eachwith

itsownstrengthsandchallenges.

Fluorescenceexcitationtomographyisavariantofpla-

nar fluorescence imaging described above. However,

insteadofformingathinX-raylasersheet,thebeamis

expandedtooverlapasignificantvolumeoftheflame.

Fluorescence is imaged at multiple viewing angles

(~20–100 views) using an array of imaging detectors.

Becausefluorescenceexcitationisalinearprocess,the

repetitionrate)willbefastenoughtocorrelatestructures

fromoneframetothenext.Spectrallyintegratedfluores-

cence(265–283eV)willenableimagingofthetotalcar-

bon atom distribution, which when coupled with

temperaturemeasurements(fromRayleighscattering)

willprovidespatio-temporalmapsofmixturefraction.

Mixturefraction,ξ,isacentralquantityinthetheory

andmodelingofturbulentnon-premixedandpartially

premixedcombustion.Thestateofmixingbetweenthe

fuelandoxidizerstreamsisquantifiedbymixturefrac-

tion,andtherateofmolecularmixingisgivenbythesca-

lar dissipation rate, χ = 2D(∇ξ∙∇ξ), where D is the

mixture-averagedmassdiffusivity.Multi-dimensional

mixture fractionmeasurementsarerequired todeter-

minethescalardissipationrate.Opticallaser-basedmea-

surementsofmixturefractioninflamesareachallenge

becauseofspatialvariationsintemperatureandchemical

composition. Single-point and line measurements of

mixturefractionarefeasibleusingsimultaneousRaman/

Rayleigh/CO-LIFtomeasureallthemajorspeciesconcen-

trationsandtemperature.12However,thisapproachis

not practical for multi-dimensional measurements of

mixturefraction.

State-of-the-artmethodsfor2Dmixturefractionimag-

ing13-15aresingle-pulsemeasurementsthatdonotpro-

videinsightintothetemporalevolutionofthe3Dflame

structure.This structuregovernsphenomenasuchas

localizedextinctionandre-ignition. Inaddition, these

techniquesrelyonmeasurementsofmultiplereactive

speciestoconstructaconservedscalar(aquantitythatis

neitherconsumednorproduced,e.g.mixturefraction).

X-rayfluorescenceimagingofcarbonwillenablespa-

tiallyandtemporallyresolvedmeasurementsofallcar-

bon-containing species in the flame. Because X-ray

fluorescenceislargelyanatomicprocess,thismeasure-

mentwilltracktheredistributionofcarbonatomsthat

originatedfromthefuel,independentofchemicalreac-

tionsthatredistributethemintheflame.Theconcentra-

tion of carbon atoms is a true conserved scalar that

cannotbedirectlymeasuredwithoptical techniques.

Spatiallyresolvedcarbonconcentrationswillbecoupled

withRayleighscatteringtodetermineacarbon-based

mixturefraction.

Quantitativevisible/UVfluorescenceimagingiscom-

plicatedbyspatiallydependentfluorescencecollisional

quenchingrates(i.e.,convertingsignaltoconcentration

requiresconcentrationmapsandquenchingratesforall

Time-resolved X-ray fluorescence

Time-resolved XAS

Real-time cinematic imaging

Time-resolved tomography

see Section 4.2

31


sitizedsolarcells,19andasacatalystsupportindirect

methanolandotherfuelcells.20Asamajorcontributorto

anthropogenicaerosols, soot isdetrimental toatmo-

spheric visibility,21-24 global climate25,26 and human

health.Moreover,after-treatmentcontrolofparticulates

addssubstantial costanddecreases theefficiencyof

vehicles,makingreductionofin-cylindersootproduction

ahighlyimportanttechnicalareaforenginemanufacturers.

Thetheoryofsootformationhasbeenreviewedperi-

odically22,23 and many contemporary problems have

beendiscussedinarecentworkshop.27Yetbecauseofthe

intricatenatureofsootformation,nottomentionthelarge

number of tightly coupled elementary processes

involved,themechanismandkineticsofsootproduction

remainspoorlyunderstood.Althoughthereisincreasing

consensusonchemicalpathwaysfortheformationofthe

firstandsecondaromaticrings,currenttechniquesare

notwellsuitedtostudythedetailed,time-resolvedchem-

istryof latergrowthoroxidationphases.This lackof

quantitativeorevenqualitativeunderstandingposes

severechallengestofutureenginedesignandoptimiza-

signallevelswillbethesameasinplanarimaging.The

extraexpenseofmultiplecameraswillenablefullspatial

and temporal reconstruction of the flame structure.

Chemical specificity would be achieved as described

aboveforplanarfluorescenceimaging.

Analternativeapproachtofluorescenceisspatially

andchemicallyresolvedabsorptiontomographyatthe

carbonK-edge.Thisapproachpromisesbettersignal-to-

noisethanfluorescence(duetothelowquantumyieldof

fluorescence),butrequiresasophisticateddesigntocre-

ateanangulararrayofexcitationbeams.The3Dspatial

structureof the flamecanbe tomographically recon-

structedfromthesetofprojectedabsorptionimagesat

variousangles.Thissingle-pulsetomographywilldeliver

thedesired4Dcinematicimagingatthe100kHzrepeti-

tionrateofNGLS.Chemicalfunctionalgroupspeciation

couldbeachievedby tuning theexciting radiation to

appropriatenear-edgefeatures,asdescribedabove.This

single-pulsetomographicapproachwouldbearevolu-

tionarycapabilitynotonlyforreactingflows,butalsofor

imagingmanyirreproducibleobjects,andreliesonthe

characteristicsofNGLSradiation(simultaneouslyhigh

repetitionrateandpulseenergy).

AkeyadvantageofalltheseX-raybasedtechniques,

compared to similar approaches in the UV-IR-optical

regime,isthatsignalsfromthesecoreelectronspectros-

copieswillnotbeweakenedbydiminishingBoltzmann

quantumlevelfractionsathighertemperatures,norby

unfavorable Franck-Condon factors.The new multi-

dimensionalimagingcapabilitieswillbeinstrumentalin

makingprogressinunderstandingignitionandextinction

phenomena.Understandingthedynamicsofthesetran-

sientprocesses is increasingly importantbecausethe

peakefficiencyinmanyadvancedcombustionsystemsis

obtained near their stability limits, where the risk of

extinctionandexcesspollutantformationisalsogreater.

3.3.3 Uncovering the Chemistry and Kinetics of Particle Formation and Oxidation

Today’sscientificinterestincombustionparticlefor-

mationchemistryhasbeendirectedtowardsbothmiti-

gatingairpollutionproblemsthatinvariablyresultfrom

theuseoffossilfuels,andtowardsoptimizingproperties

oftechnologicallyusefulnanoparticlesproducedbycom-

bustion.17Recentapplicationsincludetheuseofsootor

carbonnanoparticlesasacathodecatalyst,18indye-sen-

Fuel + Oxidizer

CO, H2, CO2, H20, C2H2

Figure17 A conceptual model of soot formation. (Adapted from Reference 30)

32


thechemical composition, structureandmorphology

evolvesasafunctionofitsreactionhistory.Forexample,

recentstudiesshowthatthenascentsootparticlescan

haveanaromatic-core/aliphaticshellstructureandthe

ratioofaliphatic-to-aromaticconstituentscanvarywidely

asafunctionofthelocalflameconditionandparticlehis-

tory,30,31 as seen in Figure 18. Currently, we lack the

appropriatetoolsforinsituprobingoftheparticlechemi-

calcomposition,morphology,andreactivity.Thesecom-

plexitieshaveledtothecurrentsituationthatneitherthe

reactants nor the products are defined in the kinetic

descriptionofthesootingprocess.Withoutrevolutionary

newexperimentalcapabilitiesthatenableafundamental

approachanalogous to thoseemployed ingas-phase

reactionkinetics,themodelofsootformationwillremain

phenomenologicalratherthanrigorousandpredictive.

NGLSX-raylaserswillenablenewimagingtechniques

todirectlyinterrogatethetime-dependentelectronicsur-

facestructure,chemicalcomposition,sizeandmorphology

duringtheevolutionfrompolycyclicaromatichydrocar-

bons(PAHs)toparticulate,includingthecriticalandpoor-

lycharacterizedparticlenucleation,massandsizegrowth

andoxidationprocessesofnascentparticles.Particlesat

differentstagesofgrowthcanbeinterrogatedinsituin

flamesorreduced-dimensionalreactors,orheldassingle

freely-suspendedparticles.Byemployingthehighphoton

fluxcontainedwithinasinglepulsetoprobetheevolu-

tionofproducts,theheterogeneousreactionkineticsmay

beexaminedatafundamentallevel.Time-resolvedfluc-

tuationSAXS(small-angleX-rayscattering)andrelated

approaches(describedinsection3.5and4.1)willbepow-

erfulnewtoolsforunderstandingparticulateformation

andgrowthduringcombustion.

tion.Sootformspersistentlyinmanytypesofcombus-

tionengines,andthisproblemcanbesolvedonlywhena

quantitativeanddetailedknowledgeaboutparticlenucle-

ation, mass/size growth, aggregation and oxidation

becomesavailable.

Forexample,ourcurrentlevelofkineticsmodelingfor

sootoxidation28remainsrootedinasemi-empiricalglob-

alkineticsapproachusuallycalledthe“Nagle/Strickland-

Constable”expression.29 It isbecomingmorewidely

appreciatedthatsootoxidationmaybeevenmoreimpor-

tantthanformation,andyetwehaveonlyaroughglobal

expressionatourdisposal.Aswithsootformation,the

sootoxidationproblemishighlycomplex.Sootagesover

time,andatthestagewhereoxidationbecomesimpor-

tantinanengine,fueliscondensingonthesurfaceaspar-

ticlesaggregate.Tobreakthisproblemintoelementary

steps one must first characterize the complexity and

natureoftheproblem.

The overall process of

soot formation, from pre-

cursor formation, particle

inception,andfinallytosoot

mass/size growth (see,

Figure17)occurs typically

overafewmilliseconds.At

theendofthereaction,soot

particles typically contain

106Catoms.Therearetwo

fundamentalchallengesthatlimitourunderstandingof

particleformationkinetics.First,theaveragerateofaddi-

tionofCatomstotheprecursormoleculeis~1atom/ns.

Followingthisrapidprocessrequiresexceptionaltempo-

ralandspatialresolution.Second,duringparticlegrowth,

C3; H = 0.7cm

20 nm( i ) ( ii )

z (n

m)

x (nm)

a

h

420

Distance from Burner Surface, H (cm)

8

6

4

2

0 0.4 0.5 ? 0.6 0.7 0.8 0.9 10

( iii )[Alfphatic C-H][Aromatic C-H]

Equivalent Diameter, Dv?AFM (nm)

Aspe

ct ra

tio, h

/a

0.4

0.3

0.2

0.1

0.00 5 10 15 20 25 30

0.51.0

??

??

?

?

?

m/z0 200 400 600 800 1000

78 78 78 78 78

78 78 78 78

74 74?

?

Figure18 Panel (i) TEM micrograph, panel (ii) AFM images and aspect ratio, and panel (iii) micro-FTIR characterization of aliphatic-to-aro-matic C–H ratios, and thermal-desorption chemical ionization mass spectrometry of nascent soot collected from a 16.3% C2H4–23.7% O2–Ar flame33.

Particulate formation:

Fluctuation X-ray Scattering

Giga-shot diffractive imaging

see Sections 3.5 and 4.1

33


suchasoxidesofsilicon,titanium,andiron.Beyondpro-

vidingatestforpredictivecombustionmodelsaimedat

minimizingoreliminatingsootproductioninpractical

combustiondevices,theyofferaplatformforoptimizing

combustionsynthesisofnovelmaterials.

3.3.4 NGLS Impact — Combustion Science

ThefullycoherentX-rayradiationfromNGLS,deliv-

eredwithhighaveragepower,highrepetitionrate,mod-

eratepeakpower,anduser-selectabletime/bandwidth

characteristicsarethekeyfeaturesofthislightsource

thatdifferentiateitfromcurrentlightsources,andwill

enableexplorationofnewfrontiersincombustionsci-

ence.Theneedforreal-timecinematicimagingofnon-

repeatablephenomena,suchasdescribedhere,cannot

bemetwitheithercurrentX-rayFEL’s(<100Hzrepetition

rate),norwithsynchrotrons(manyMHzrepetitionrate,

butlowpulseenergy).Specifically,NGLSwillforthefirst

timeenableawiderangeofnon-linearcoherentspec-

troscopiesinthesoftandhardX-rayregimeswhilesimul-

taneouslyenablingtime-resolved cinematicimaging.

First,thecoherentnatureofFELradiationwillallowthe

implementationofmanynon-linearspectroscopictech-

niques(e.g.,softX-raycoherentanti-StokesRamanspec-

troscopy,XCARS,andsoftX-rayholography)previously

developedintheopticalregime.Thesecoherenttech-

niqueswillprovideauniquecombinationofspatialimaging

withchemicalspeciesidentificationthatwillbeunaffected

bytheBoltzmannandFranck-Condonlimitationsofoptical

spectroscopyprobes.Theabilitytotradetemporalpulse

widthforspectralpulsewidthwillbeespeciallypowerful

withnon-linearspectroscopies.Forexample,a0.2fspulse

hasafullbandwidthof~9eV,allowingcoherentwave-

packetexcitationofalargeportionofthenear-edgeX-ray

absorptionsthatdistinguishbondingenvironmentsof

carbon,nitrogen,andoxygen.Conversely,longerpulses

(withnarrowerbandwidth)canbeusedtoselectivelyexcite

resonances of chemical subsets within a chemically

diversesample.Thisbandwidthflexibilitymaybecombined

intheXCARSexperiment,coherentlydrivingmanynear-

edgeexcitationswithbroadbandx-rayradiationinthepump

andStokespulses,andspectrallyresolvingeachofthese

chemicalsubsetsinthedispersedsignalbyutilizinganar-

rowbandprobe.Theuniversalityofcore-levelspectroscopy

providesimportantadvantagesovermolecule-specific

Previousexperimentalevidencesuggestssootnucle-

ationoccursthroughtheclusteringofPAHs.Figure19

depicts a possible reaction sequence as predicted in

molecular dynamics (MD) calculations byVioli and

coworkers.32Criticalquestionsincludetheroleofaro-

maticπradicalsintheclusteringprocess,33andtheroles

ofenergytransferandvibrationalrelaxationinstabilizing

thecluster.34-37Core-levelNEXAFSspectroscopyand

harderX-rayRamananaloguesofferthepossibilityto

simultaneouslyprobethechemicalbondingenvironment

andthespatialstructureofsootgrowth.Inparticularthe

radicaland/orbiradicalsitesthatarepostulatedtobethe

keyreactivesitesformolecularweightgrowthoroxida-

tionofnascentsoot33havespectralfingerprintsinthe

carbonK-edgeregion.

Kineticexperimentsonsingleparticlesmaybeenvi-

sionedusingafreelysuspendedparticleextracteddirectly

from a flame and immobilized by optical tweezers.

NanoscaleX-raycinematicimagingoftheparticlecan

providetime-resolveddataonitsstructure,morphology,

composition,and reactionkinetics.Tobridge thegap

betweenfundamentalkineticstudiesonnascentsootand

sootformationinflames,simultaneoustemporallyand

spatially resolved X-ray CARS and Mie scattering in

steadyorunsteadyflamescouldprobethedynamicsof

particleformation.

Understandingtheexceptionallyrichchemistryofcar-

bonhasyieldedseveralNobelprizes,mostrecently(2010)

toGeimandNovoselovforgraphene,aswellasthatto

Smalley,Curl,andKrotoin1996forfullerenes.Theinves-

tigationofmolecularweightgrowthinflamesfollowsin

thetraditionofthischemistry,butthescopeofcombus-

tionparticulategrowthiswiderthancarbon.Studiessuch

asthoseenvisionedabovewillprovideabetterunder-

standingofparticlegrowthandoxidationmechanisms

andkineticsfortechnologicallyimportantnanomaterials

Figure19PAH clustering/particle nucleation as predicted by molecular dynamics (A. Violi, University of Michigan).

34


chromatorsonbeamlines1or3),whilenon-linearX-ray

mixingexperimentswill require thehighestpossible

peakpowerandshortestpulsesavailableonbeamline2.

Combustionparticulate formationexperimentswill

relyon“diffractanddestroy”methodsusingthe3rdand

5thharmonicswiththehighestfluxperpulseontheseed-

edNGLSbeamline1at100kHzrepetitionrates,andon

theunseededSASEbeamline3,atMHzrepetitionrates

(ashigh-speeddetectorsallow)asdescribedinSections

5and6.6.Choiceofwavelengthwillbedeterminedby

balancingthescatteringefficiencyandtherequiredreso-

lutionforparticularsamples.

References:

1. J. Eismark, et al., Role of formation and transortation of hydroxyl radicals

for enhanced late soot oxidation in a low emissions heavy-duty diesel

engine, in THIESEL 2010 Conference-on Thermo- and Fluid Dynamic

Processes in Diesel Engines. 2010: Valencia, Spain.

2. J.E. Dec, A Conceptual Model of DI Diesel Combustion Based on Laser-

Sheet Imaging, in SAE Technical Paper 970873. 1997. p. DOI:

10.4271/970873.

3. Musculus, M. and L. Pickett, Chapter 33: In-cylinder spray, mixing, com-

bustion, and pollutant-formation processes in conventional and low-

temperature-combustion diesel engines, in Advanced Direct Injection

Combustion Engine Technologies and Development: Volume : Diesel

Engines, H. Zhao, Editor. 2009, Woodhead Publishing Ltd.: Cambridge, UK.

4. Schmidt, J.B., et al., Ultrafast time-gated ballistic-photon imaging and

shadowgraphy in optically dense rocket sprays. Appl Optics, 2009. 48: p.

B137-B44.

5. Linne, M.A., et al., Ballistic imaging of liquid breakup processes in dense

sprays. Proc. Combust. Inst., 2009.32: p. 2147-61.

6. Wang Y, et al., Ultrafast X-ray study of dense-liquid-jet flow dynamics

using structure-tracking velocimetry. Nature Phys, 2008. 4: p. 305-309.

7. Tanaka, S. and S. Mukamel, Coherent x-ray raman spectroscopy: A non-

linear local probe for electronic excitations. Phys Rev Lett 2002. 89.

8. Kliewer, C., Quantitative one-dimensional imaging using picosecond

dual-broadband pure-rotational CARS. submitted to Applied Optics.

https://share.sandia.gov/crf/crfnews.php?id=307.

9. Taatjes, C.A., et al., Enols are common intermediates in hydrocarbon oxi-

dation. Science, 2005. 308: p. 1887-1889.

10. R. Manne and J. Chem, Physics 1970. 52.

11. Ayoolan, B.O., et al., Spatially resolved heat release rate measurements

in turbulent premixed flames. Combust Flame, 2006. 144: p. 1-16.

12. R. S. Barlow and J.H. Frank, Effects of turbulence on species mass frac-

tions in methane/air jet flames. Proc. Combust. Inst, 1998. 27: p. 1087-1095.

13. J. H. Frank, et al., Mixture fraction imaging in turbulent nonpremixed

hydrocarbon flames. Proc. Combust. Inst, 1994. 25: p. 1159-1166.

opticalspectroscopicmethodsinprobingglobalcombus-

tionphenomena,asdiscussedintheprevioussections.

Second,thehighrepetitionrateofNGLSwillallowcin-

ematicimagingontimescales(~10μsto1μs)thatare

fundamentallyimportantincombustionenvironments.

These time-scalesare fastenoughtoallowframe-by-

framecorrelationoffluiddynamicsandchemistry,which

isnotpossibleusingexistinglightsourcesortable-top

sources.Third,themoderatepeakpowerswillbesuffi-

cientlyintensetodrivenon-linearprocesses,butnotso

intenseastodestroythetargets.Fourth,the3rdand5th

harmonicsoftheFELwillprovidehigh-powernarrow-

bandwidthX-raysatshorterwavelengths.Thegreater

penetrationdepthsofthesephotonswillenablenon-lin-

ear X-ray spectroscopy38 to probe combustion with

chemicalandspatialresolutioneveninsidehighlyscat-

teringorabsorbingenvironmentssuchasdensesprays.

Finally,thehighaveragepowerandbrightnessofNGLS

willallowtheinterrogationofsamplesatmuchhigher

(i.e.,directlyrelevant)pressuresthanwasheretoforepos-

sibleatsynchrotronfacilities.Siliconnitridewindowsand

differentialpumpingwillbeusedtointerfacetheFELto

thehightemperaturesandpressuresofthesamples.

Newexperimentsincombustionsciencemadepossi-

blebyNGLSpromisetobeamajorleapforwardinthe

developmentofpredictivecombustionmodels.These

models, when validated against high fidelity experi-

ments,willbeinvaluableindesigningbothnewfuelsand

combustorsoveramuchlargerparameterspacethanis

possiblewithcurrentengineeringapproaches.Science-

basedpredictivecombustionmodelingwillbenefitsoci-

etythroughpositiveimpactsonenergy,humanhealth,

environmentalhealth,nationalsecurity,andeconomic

competitiveness.

Beamlines for Advanced Combustion Science

Combustionimagingandchemicalspeciationexperi-

mentswillrelyontheun-seededNGLSbeamline3,pro-

vidingthehighestaveragefluxandrepetitionrate,andon

theseededbeamline1providingthehighestresolution

(<50meVwithoutamonochromator).SoftX-raychemical

speciationwillbedoneinthepre-edgeregionofcarbon

(280eV),withsomeexperimentsexploitinghigher-ener-

gyphotonsatthe3rdharmonic(upto3.6keV)inorderto

balanceabsorptionandpenetrationdepth.Someexperi-

mentswillrequirehigherenergyresolution(withmono-

35


28. Vishwanathan, G. and R.D. Reitz, Development of a Practical Soot

Modeling Approach and Its Application to Low-Temperature Diesel

Combustion. Combust. Sci. Tech., 2010. 182(8): p. 1050 - 1082.

29. Nagle, J. and R.F. Strickland-Constable, Oxidation of carbon between

1000-2000°C, in Proceedings of the 5th Carbon Conference. 1962:

Pergamon, Oxford. p. 154-164.

30. Cain, J.P., et al., Micro-FTIR study of soot chemical composition - evi-

dence of aliphatic hydrocarbons on nascent soot surfaces. Phys. Chem.

Chem. Phys., 2010. 12: p. 5206-5218.

31. Cain, J.P., et al., Evidence of aliphatics in nascent soot particles formed

in premixed ethylene flames. Prog. Energy Combust. Sci., 2010. 33: p. doi:

10.1016/j.proci.2010.06.164.

32. Chung, S.-H. and A. Violi, Peri-condensed aromatics with aliphatic

chains as key intermediates for the nucleation of aromatic hydrocar-

bons. Proceedings of the Combustion Institute, 2010. 33: p. in press.

33. Wang, H., Formation of nascent soot and other condensed-phase mate-

rials in flames. Proceedings of the Combustion Institute, 2010: p. in press

(doi:10.1016/j.proci.2010.09.009).

34. Violi, A., A.F. Sarofim, and G.A. Voth, Kinetic Monte Carlo molecular

dynamics approach to model soot inception. Combustion Science and

Technology, 2004. 176: p. 991-1005.

35. Violi A and Venkatnathan A., Combustion-generated nanoparticles pro-

duced in a benzene flame: A multiscale approach. Journal of Chemical

Physics, 2006. 125.

36. Schuetz, C.A. and M. Frenklach, Nucleation of soot: Molecular dynamics

simulations of pyrene dimerization. Proceedings of the Combustion

Institute, 2002. 29: p. 2307-2314.

37. Wong, D., et al., Molecular dynamics simulations of PAH dimerization, in

Combustion Generated Fine Carbonaceous Particles, H. Bockhorn, et al.,

Editors. 2009, KIT Scientific Publishing: Karlsruhe. p. 247-57.

38. Harbola, U. and S. Mukamel, Coherent stimulated x-ray raman spectros-

copy: Attosecond extension of resonant inelastic x-ray raman scatter-

ing. Physical Review B, 2009. 79.

14. M. B. Long, et al., A technique for mixture fraction imaging in turbulent

nonpremixed flames. Ber. Bunsenges. Phys. Chem., 1993. 97: p. 1555-

1559.

15. J. Fielding, et al., Polarized/depolarized rayleigh scattering for determin-

ing fuel concentrations in flames. Proc. Combust. Inst., 2003. 29: p. 2703-

2709.

16. A. J. Seen and F. P. Larkins, Ab initio studies of molecular x-ray-emission

processes - Ethanol. Phys.B-At. Mol. Opt. Phys., 1992. 25: p. 4811-4822.

17. Strobel, R. and S.E. Pratsinis, Flame aerosol synthesis of smart nano-

structured materials. J. Mater. Chem., 2007. 17: p. 4743-56.

18. Kay A. and G. M., Low cost photovoltaic modules based on dye sensi-

tized nanocrystalline titanium dioxide and carbon powder. Solar Energy

Materials and Solar Cells, 1996. 44: p. 99-117.

19. Oregan B. and Gratzel M., A low-cost, high-efficiency solar-cell based

on dye-sensitized colloidal TiO2 films. Nature, 1991. 353: p. 737-40.

20. Bashyam R. and Zelenay P., A class of non-precious metal composite

catalysts for fuel cells. Nature, 2006. 443: p. 63-66.

21. Horvath, H., Atmospheric Light-Absorption — A Review. Atmospheric

Environment Part A-General Topics, 1993. 27: p. 293-317.

22. Kennedy, I.M., Models of soot formation and oxidation. Prog. Energy and

Combust. Sci., 1997. 23: p. 95-132.

23. Richter, H. and J.B. Howard, Formation of polycyclic aromatic hydrocar-

bons and their growth to soot — a review of chemical reaction path-

ways. Prog. Energy Combust. Sci., 2000. 26: p. 565-608.

24. McEnally, C.S., et al., Studies of aromatic hydrocarbon formation mecha-

nisms in flames: Progress towards closing the fuel gap. Prog. Energy

Combust. Sci., 2006. 32: p. 247-94.

25. Hansen, J. and L. Nazarenko, Soot climate forcing via snow and ice albe-

dos. PNAS, 2004. 101: p. 423-8.

26. Service, R.F., Climate change — Study fingers soot as a major player in

global warming. Science, 2008. 319: p. 1745.

27. Bockhorn, H., et al., eds. Combustion Generated Fine Carbonaceous

Particles. 2009, KIT Scientific Publishing: Karlsruhe.

36

OVERVIEW OF REVOLUTIONARY X-RAY SCIENCE TOOLS AT NGLS

X-raysathighrepetitionrate,withtimescalesofattosec-

ondstohundredsoffemtoseconds,comparabletoelec-

tronicandnuclearmotion inmolecules,andthehigh

brightnessofNGLSbeamsallowingspectro-nanoscopy

downtosub10nmspatialresolutionopenspreviously

unthinkablepossibilitiesfordiscoveringhowspecificele-

mentaryreactionstepsproceedonheterogeneouscata-

lysts.Suchanunderstandingwillfacilitatethediscovery

ofnovelreactionpathwaystomakemoreselective(green

chemistry)andmoreefficient(energysaving)catalysts.

3.4.1 Heterogeneous Catalysts in Action — Microscopy and Dynamics

It iswellknownthatcatalystsareneitherstaticnor

homogeneousentitiesduringoperation.Theenergetic

processesofphononandelectronexchangesoccurringat

their surfaces, and the formation of chemical bonds

betweenthereactantmoleculesandthecatalystcanleadto

restructuringofthecatalyst.Knowingtheatomicandelec-

3.4 Catalysis

Catalyticreactionsareofvitalimportanceinvirtually

allareasofenergygeneration.Examplesareindustrial

processingoffossilfuels;reductionofharmfulemissions

frompowerplantsandcars;large-scaleproductionof

chemicals;retrievalofstoredenergyaselectricalenergy

orheat;andalternativeenergyuseandconversion(fuel

cells, artificialphotosynthesis).Theworld’s increased

needforenergyinthecomingdecadescanbesustained

only ifnew,moreefficientandcleanerprocesses for

energygenerationandtransformationarefound.This

requiresthedesignofnewclassesofcatalysts,basedon

inexpensiveandabundantmaterials,withtheversatility

toprocessnewsourcesofbiofuelsandcatalyzeexisting

reactionsmoreefficientlyandecologically.Twokeyper-

formance goals are increased output (i.e. yield) and

decreasedwasteproduction(i.e.selectivity).NGLSwill

enable unique spectro-nanoscopy experiments with

unprecedentedspatialandtemporalresolutionthatwill

provide transformational knowledge to help achieve

thesetwogoals.Theabilitytodeliverultrafastpulsesof

Catalysis is critical to nearly all energy production and utilization cycles. The process whereby a chemical activa-tion barrier is lowered to permit a normally unfavorable chemical reaction to occur on a rapid time scale, catalysis has revolutionized humankind. The Haber-Bosch catalytic process in the early 1900s addressed the fixation of nitrogen, leading to the production of fertilizer, and resulting in the award of two Nobel prizes. The automobile catalytic converter is a remarkable success story, producing an effectively clean engine exhaust, using complex catalysts as the driving force to eliminate carbon monoxide and nitrogen oxides.

NGLS will provide the capability to follow the changes in structure of catalytic sites during the processes of catalytic conversion. Just as gas phase dynamics studies with lasers have pioneered a wealth of understanding about transition states, reaction pathways, and energetics, NGLS will unfold these stories using X-Ray spec-troscopies for the first time on dynamic catalytic sites. NGLS provides the ability, through coherent imaging, to visualize the individual locations and movements of a complex set of metal atoms, set in precise configuations by a supramolecular framework. In situ electrochemical and photochemical processes will be analyzed by pump-probe X-ray absorption spectroscopy, and time-resolved ambient-pressure photoemission spectroscopy, made possible by the high pulse repetition rates, short pulses, and high fluxes of NGLS.

Today, novel catalytic processes are paramount to successful water splitting reactions using sunlight and, indeed, for the internal processing in every living cell. A comprehensive understanding of artificial enzyme catalysis will provide visionary tools for the production and utilization of energy in the future.

37

3 . SCIENCE DRIVERSCATALYSIS

To elucidate the differences between synchrotron

investigations and those of NGLS, consider a recent

example.AttheALSresearchershavedemonstratedthat

acatalystchangesitsstructureinresponsetochangesin

reactantcompositionduetothechemistryoccurringatits

surface.IntheexampleshowninFigure20,thecore-shell

structure of bimetallic nanoparticle Rh-Pd catalysts

changesdramaticallyinresponsetotheadsorptionand

associatedreactionsatthesurface.Underoxidationcon-

ditions,involvingforexampleNO,CO,andO2,Rhiscon-

centrated in the shell in oxidized form, while Pd is

concentratedinthecoreregion.Thedistributionreverses

uponreducingconditions.Whileunique,thisinforma-

tion,obtainedbytakingadvantageofthedifferentmean

freepathsofphotoelectronsoftheRhandPdduringin

situXPS,providesonlytime-averagedinformationover

anaveragedspatialdistribution.Thecombinationoftem-

poralresolution,spatialresolution,andchemicalspecific-

ity,availablefromNGLSX-raylasers,willrevealhowand

whythesechangesoccurbyprovidingdetailedinforma-

tiononatomicpositionsandbondinggeometriesofeach

atominthecatalystandadsorbates.Thisnewknowledge

willenablethedevelopmentofcatalyststhatcanadapt

theirstructuretooptimizereactivityandselectivity.

Inanotherexample,bimetalliccatalystsconsistingof

PtnanoparticlespromotedwithSn,Ga,orInexhibitvery

highactivityandselectivityforthedehydrogenationof

lightalkanes(C2toC4)intoH2andalkenes.Bothproducts

ofthisreactionarevaluable,sinceH2isalwaysrequired

forheteroatom(S,N,O)removalfromenergyfeedstocks,

andlightalkenescanbeconvertedtoproductsthatcan

beblendedintogasolineanddiesel.RecentstaticXAS

studieshaveshownthatthepromotingelementsmigrate

tronicstructureofspatially-resolvedcatalyticsurfaces

andinterfaces,duringthereactionanditstimeevolution,

isfundamentalforpredictivecatalystdesign.Unfortunately

todayitisnotpossibletofollowthisstructuralevolutionon

relevant timescales: fs topseccharacteristicofatomic

motion,ornstomscharacteristicofdiffusionandmateri-

alsevolution.Determininghowthisrestructuring,occur-

ring over picoseconds or longer, is connected to the

thermalandelectronicprocessesofthereactionsthatit

precedes,isfundamentalforunderstandinghowthecata-

lystoperates.Althoughsomeattemptstoobtainsuch

informationhavebeenperformedwithultrafasttrans-

missionelectronmicroscopyandelectrondiffraction,

NGLSwillprovideuniqueandtrulyrevolutionarywaysto

visualize dynamic catalytic processes. In particular,

advancedcoherentimagingtechniquessuchascoherent

diffractiveimagingandptychography(discussedinSections

4.1and4.2),willexploitthehighcoherenceandintensity

ofNGLSX-raylaserstoprovidechemically-specificimaging

atthenanoscale.Evenifthehighintensityislikelytodestroy

thecatalystsandtheadsorbedmolecules,theaccumulation

ofrepeatedscatteringpatternsofnewandnearlyidentical

particlescanbedeconvolvedtorestore,throughmodern

imageprocessingmethods,therealspacenanostructure.

Fromtheexperimentalstandpoint,itisimportantto

developinstrumentationthatmakespossiblethestudyof

catalystsintheirworkingenvironment,i.e.,underhigh

pressureofreactantsandathightemperature.Thisisnot

trivialforelectronspectroscopyexperiments;thephoto-

electron mean-free path in gas environments is very

short.Forexamplea500eVelectronwilltravelapproxi-

mately3mmthrough1mbarofO2beforeacollisionwith

agasphasemoleculecausesittolosetheinformationit

carries(kineticenergy,direction).ScientistsattheALS

pioneeredtheuseofdifferentially-pumpedspectrome-

terstoaccomplishthis.1,2Thetechnique,calledambient

pressurephotoelectronspectroscopy(APPES)3,4isnow

beingimplementedinmanysynchrotronfacilitiesaround

theworld.APPEShasledtomanyimportantdiscoveries

incatalysis,5-8waterandenvironmentalscience,9,10fuel

cells,11,12andotherenergyrelatedscience.Togetherwith

X-rayabsorptionspectroscopy(XAS),evenliquidenvi-

ronmentsandelectrochemistryexperimentsarenow

possibletostudyatsynchrotronsources.1However,syn-

chrotron-basedstudiesarenotcapableofobtainingthe

time-evolvingstructuresthatwillberequiredinfuture

generationsofcatalyticscience.

Atom

ic fr

actio

n

Rh0.5Pd0.5

Rhodium

Palladium

Reactants

Shell

Rh Kαl

18 19

PdPd

Rh

Rh ex-situ TEM

20 21 22 23 24 2

Pd Kαl Core-Shell

Core

NO

1.00.90.80.70.60.50.40.30.20.1

0 NONO+CO NO+CO O2

Figure20In-situ XPS (Pgas = mbar) shows core-shell atom exchange in Rh-Pd catalyst nanoparticles induced by oxidation and reduction reactions of NO, CO, H2, and O2. (From Reference 13)

38


environment,non-covalentinteractionscanplayacritical

roleinstabilizingtransitionstates.Suchenvironments

offer thepossibilityof (1)precise controlbya three-

dimensionalframeworkinwhichsubstratesinteractwith

oneanotherandwiththehost;and(2)precisecontrolof

theratesatwhichthereactingmoleculesenterandleave

thecatalyticcenter.

Natureexploitsthissupramolecularprincipletodesign

biologicalcatalyticsystemssuchasenzymesandribo-

zymes.Heresupramolecularframeworksarecriticalfea-

turesoftheactivesitesinwhichbiologicalreactionsoccur

withexquisiteselectivityandhighlyenhancedreaction

rates,comparedtothoseexhibitedbyanalogousprocess-

esthattakeplaceinthegasphase,onsurfaces,orinsolu-

tion.However,evenbiologicalchemistshavehistorically

focusedprimarilyonthefeaturesoftheactivesites,and

areonlynowbeginningtounderstandtherolethatthe

supramolecularframeworkplaysincontrollingcatalysis.

Asoneexample,recentinsightssuggestthat“conforma-

tionalgating”playsasignificantroleinenzymecatalysis

(seeforexample,Reference14).Todate,thehomogenous

catalysisfieldhasfocusedlittleattentionondesigningabi-

ologicalsystemsthattakeadvantageofsupramolecular

controltoproducecatalyststhatdemonstratethelevelof

chemoselectivityandrateenhancementtypicallyattribut-

edtoenzymes.However,recentworkbyRaymondand

Bergmanhavedemonstratedsupramolecularcatalysts

(Figure21)thatapproachenzyme-likerateaccelerationsof

overamillionfold.15Amongmanypotentialapplications,

onemayexploitsuchassembliestoencapsulatemetal

complexescapableofcatalyzinghighlyselectivecarbon-

hydrogenbondactivationreactions.

NGLSX-raylaserswillbeessentialtothedevelop-

ment of this new field of supramolecule catalysis by

enablingcompletecharacterizationofthesupramolecu-

larassembliesinwaysthatarenotachievablewithpres-

ent sources. In particular, time-resolved X-ray

spectroscopy techniques

(XANES and EXAFS) will

probe the local bonding

geometry, coordination,

andbond-distancesofcata-

lystswhiletheyareoperat-

ing in solution, and with

temporal resolutions that

canfollowthefundamental

chemistry and turn-over

inandoutofthePtnanopar-

ticles during the overall

reaction, and during cata-

lystreprocessingtoremove

accumulated carbon.The

NGLSwillprovideaunique

opportunity to observe

theseprocessesasafunc-

tionofbothtimeandspace

via time-resolve ambient-

pressure photoemission,

XAS,andXES.Ofparticular

interest are the real-time

dynamicsandchemistryof

carbon deposition on the

nanoparticle surface and

the migration of these

depositsontothesupport-

ingmaterial.Understandinghowpromotingelements

affecttheinitiationandgrowthofcarbonaceousdeposits

andhowtolimittheseprocesseswillprovideessential

informationforthedesignofsuperiordehydrogenation

catalysts.

3.4.2 Homogeneous Catalysis

Bothheterogeneousandhomogenouscatalysishave

focusedhistoricallyon reactions controlledby short-

rangeinteractions,usuallybetweenasmallnumberof

moderate-sizedmolecules.Whilethisapproachinhomo-

geneouscatalysishasledtothedevelopmentandunder-

standing of many interesting and useful processes,

especiallyintheareaofasymmetriccarbon-carbon,car-

bon-hydrogenandcarbon-heteroatombondformation,

animportantfuturedirectionincatalysisliesinthedevel-

opmentofnewreactionscontrolledbylonger-range,or

“supramolecular,”interactions.

Homogeneous “supramolecular” catalysis differs

fromheterogeneoussurface-catalyzedreactionsinthat

thelatterhastraditionallyfocusedon“externalspace,”in

whichreactingmoleculesbindtoextendedsurfaces,and

thosemoleculesexchangerapidlybetweenthebound

stateandanotherphasesuchasaliquidsolutionorgas

phase.Incontrast,supramolecularchemistrytargetsthe

studyofan“internalmolecularspace”,wherereactive

substratesenteraconfinedenvironmentsurroundedon

allsidesbythesupramolecularframework.Insuchan


Time-resolved XAS, XES, XANES

Time-resolved ambient-pressure XPS (APPES)

Spectro-nanoscopy (<10 nm)

Coherent diffractive imaging

Ptychography

see Section 4.2

Fluctuation X-ray scattering


see Section 4.1

39


(Figure22),but only on long time scales(typically100’sof

msorlonger).Significantadvanceswillbemadewith

NGLS,whereitwillbepossibletoprobethedynamicsof

oxidationprocessesonultrafasttimescalesandtopene-

trateindepthandachievespatialparametersfarbeyond

thelimitedexperimentspossibleatpresentX-raysources.

Figure22showsaschematicsetupandfirstresultsofa

recentinsituelectrochemicalstudyoftheformationof

copperoxides (CuO, cupricoxideandCu2O, cuprous

oxide)asafunctionoftheelectrodepotential.1Similar

studieswillbeinvaluabletoolsinstudiesofrenewable

energysciences,suchasLi+-basedbatteries.

Spectroscopicstudiesofsolidfuelscellshavealsobeen

performedinsituforthefirsttimeattheALSusingambient

pressureXPS(APPES).TheexampleinFigure22showsthe

simultaneousmeasurementofsurfacepotentialandcerium

oxidationstateacrosstheelectrodegapbetweenPtand

CeOinthepresenceofwatervaporandH2.Pump-probe

spectroscopicexperiments,withsub10nmspatialresolu-

tion,wheretheelectrontransfertoH2OtoformO-andHare

followedasafunctionoftime,willbeseminalindeter-

miningelementaryreactionsteps.Pumpexcitationcanbe

accomplishedviaUVpulseirradiationoftheanodeand

cathode,ordirectexcitationofthemolecules.Onecanalso

envisiontheuseofultrafastswitchingdevicestogether

withthehighpulseenergyX-rayprobesofNGLStofirst

injectelectronsintotheelectrodematerials,thenfollow

withprobeanalysis(XPS,XAS)ofthechargestructureof

themoleculeorsurfacespecies,includingtheconduction

bandintheelectrolyte.Furthermore,thehighbrightness

ofNGLSwillallowthestudyofcharge-transferprocesses

withtheuseofin-situRIXS,andwillrevealthedynamics

ofiontransportusingstimulatedphotondesorption.

3.4.4 Pump-Probe Catalysis Studies at NGLS

Pump-probetechniquesatNGLSwillmakeitpossible

toinvestigatethemostfundamentalstepsincatalysis

thatspantherangefromattosecondstopicoseconds.

Experimentsthatwillbecomepossibleincludethecom-

binationofspectroscopyandmicroscopy,withsub-10nm

spatialresolution(spectro-nanoscopy),followingtheini-

tialexcitationof:

•electronicstatesviacharge-transferexcitationsand

photo-ionizationprocessesonfemtosecondandsub-

femtosecondtimescales(photochemistry,electro-

chemistry,electronictransitionsinredoxcycles)

ratesinrealtime—fromfstoms.Inaddition,NGLSX-ray

laserswillenablequalitativelynewapproachesforimag-

ingspontaneousdynamicsofheterogeneousensembles

ofmacromoleculesinsolution,asdescribedinSection

4.1.Thisnewapproachisbasedoncollectingbillionsof

coherentdiffractiveimagestoresolvetheconformational

dynamicsofsupramolecularcatalystsandprovidenew

insighttoprocessessuchasconformationalgating.The

experimentalapproachcloselyfollowsthatdescribedin

Section4.1forimagingheterogeneousensemblesofpro-

teinconformations.Thenewunderstandingofsupramo-

lecularcatalysisfromNGLSexperimentswillenablethe

developmentoffundamentaloperatingprinciplesthat

willdramaticallyacceleratethepresentiterativeprocess

oftargetedcatalysisdesign.

3.4.3 Fuel Cells, Electrochemical Reactions, Batteries

Understandingandcontrollingelectrochemicalreac-

tionsisparticularlyimportantinmanytechnologiesand

processes,rangingfrombatteriestopotassiumchannels

incellmembranes.SoftX-rayspectroscopyhasbeen

extensivelyemployedforex-situinvestigationsofelec-

trochemicallyactivematerials,butitsuseasanin-situ

probehas laggedbehindhardX-rayexperiments for

technicalreasons.TheALSandothershaverecentlydem-

onstratedthefeasibilityandpowerofusingsoftX-ray

XPS,XAS,XES,andRIXStodeterminethestructuraland

chemical changes of electrochemical systems during

operation. Scientist have explored evaporated metal

electrodes during cyclic voltammetric experiments

= Galll. 1

NH O

O O

O O

OHN

12-

Figure21 The water-soluble, self-assembled, tetrahedral assem-bly shown to catalyze the Nazarov cyclization of 1,3-pentadienols with extremely high levels of efficiency. Left: blue lines represent bisbidentate ligands. Right: space-filling model.

40


3 .4 .4 .1 Use of THz Pump Pulses to Excite Nuclear

Motion (Heat, Vibrations)

Nuclearmotions,inthetimescaleoffstops,canbe

excitedbyTHzpulses.Thiswilltriggerthesubsequent

reorganizationof theelectronicwavefunctions in the

HOMOandLUMOlevelsofthemoleculeofinterest.The

abilitytodeterminethiselectronicstructurewillallowfor

anexplorationoftransitionstatesandreactionpathways

inprocessestriggeredbyheating.Unlikeheating,how-

ever,whichdistributesenergytoanumberofvibration

modesaccordingtotheBoltzmandistribution,THzexcita-

tioncanexcitespecificvibrationalmodes.Followingare

examplesofreactionsthatcanbenefitfromthistypeof

experiment.

COisafundamentalingredientintheFischer-Tropsch

reactiontomakegasolinefuelsfromcoal.Itiscurrently

undergoingastrongrevival,withcountrieslikeNorway

investingheavilyintheprocess.Themostfavoredmecha-

nisminvolvesCOdissociationtoCandO,whichcanbe

mediatedbyhydrogen.16ExcitingtheC-Ostretchmode

• specificmolecularvibrationalmodesviacoherent

mid-infrared andTHz excitation on 0.1-1 ps time

scales

• transientthermally-drivenreactionsviaTHzexcita-

tiononpstimescalesandlonger

NGLSexperimentsatthehighesttemporalresolution

willrelyonrepetitiveprocesses,wherethesystemcanbe

preparedrepeatedlyforeachpulse.Forexample,reac-

tionsonnanoparticlescouldbeperformedusingagas-

phase jet of particles (or a liquid droplet injector as

described in Section 4.1.3) crossing the NGLS X-ray

beam.Reactantsarepre-absorbedoneachnanoparticle,

forexamplebycrossingajetofparticleswiththereactant

moleculesofinterest.Excitedstatedynamicsandtransi-

tion-statespeciesarerevealedviaXPS,XASorRIXSfol-

lowinginitialexcitationbyaTHz,UV,orX-rayphoton

pulse,orpulsede-beam.

0.0Electrochemically active region

O2–

H2O H2

YSZAu

Au

580 nme–Pt

–1.0 –0.5 0Distance (mm)

0.5

PtYSZ

X-rays

CeO2-x

CeO2-x

0

2

4

6

8

0.5

1.0

Surf

ace

pote

ntia

l (V)

CE

WEPEEK

Support tubeSi3N4

Fluorescenceout

X-raysin

–1.0x10–5

–0.5 –0.4 –0.3Potential (V)

Photon energy (eV)In

tens

ity (a

. u.)

Cu2+Cu+

–0.2 –0.1 0.0 0.1

920 930 940 950 960 970

–5.0x10–6

0

5.0x10–6

RE

–0.9 V—Reduction300 nm Cu in 2 mM NaHCO3

Cu2+ Cu/Cu+

0 V—Oxidation300 nm Cu in 2 mM NaHCO3

300 nm Cu in thin film in air

Cu foil

Cu L2,3-edge XAS

In situ electrochemical XAS

Figure22 Left: schematic electrochemical cell assembly for in situ XAS-ray absorption spectroscopy studies, cyclic voltammogram of a Cu thin film working electrode in NaHCO3 solution, XAS of Cu L2,3-edge after reduction1. Right: Ambeint pressure X-ray photoemission spec-troscopy (APPES) measurements of surface potential and oxidatation state of solid-oxide fuel cell electrodes in operation.

41


compositions,andnewreactionconditions.AftertheTHz

pulse,X-raypulseswillthenbeusedtoprobetheelec-

tronicstructureoftheexcitedmolecule,viaXPS,XESand

XAStofollow,withattosecondsorfemtosecondssteps,

andtheirevolutionoverseveralpicoseconds.

3 .4 .4 .2 Use of UV Pulses and/or Electrochemical Charge

Transfer (Pump) to Excite Electronic Levels

Hereweenvisionexcitingmolecularorbitalsandelec-

troniclevelsdirectly,ratherthanatomicvibrations.Inthis

experimentwecaneitherinitiallypopulateemptyLUMO

orbitalsandconductionbands,orcreateholesinHOMOs

orvalencebands.Asequenceoffemtosecondandatto-

secondtimedelayedprobeX-raypulseswillfollowthe

wavefunctionevolutionviaXPS,XAS,XESorRIXSover

timeperiodsextendingtopicoseconds.Inthefollowing

wedescribeexamplesofprocessesandreactionsinvolv-

ingelectrontransfertomolecularorbitals.

Inthephotoelectrochemicalsplittingofwatertohydro-

genandoxygen,photonsfirstinteractwithoneormore

lightabsorbingelementstocreateelectron-holepairs.The

holesdiffusetooneendoftheabsorberwheretheyinteract

withacatalystthatpromotestheoxidationofwatertoO2

andliberatestwoprotons.Theprotonsthendiffusethrough

apolymericmembraneandreactwiththephoton-gener-

atedelectronsatasecondcatalysttoformH2.Thedynamics

ofelectron-holetransport,andofelectronandholeinter-

actionwiththecatalystsateachendofthelightabsorber

arepoorlyunderstoodbecauseofthedifficultyinmea-

suringthesephenomena.Similarly,thedynamicswith

whichchargedspeciesinteractwithmoleculesandanions

orcationspresentnearthecatalystsurfacearelargely

unknown.NGLSX-raylaserswillbeusedtoprobethese

processesviapump-probeexperimentswherefewfem-

tosecondtoseveralhundredfemtosecondlightpulses

(fromsynchronizedvisiblelasers)areusedtogenerate

electron-holepairs.Thedynamicsoftheinteractionofthese

chargedspecieswithcatalystsandmoleculescanthenbe

followedbyXASandXPS.Time-resolvedXES,RIXS,and

RamanspectroscopyusingUVstimulationshouldalso

enable the observation of molecular transformations

occurringinthepresenceofaliquidelectrolyte.These

studieswillcontributeinformationcriticaltounderstanding

thelifetimesofchargedspeciesproducedatthesurfaceof

electrocatalysts,whichmaybemetals,metaloxides,ormetal

complexes.Theinfluenceofchangesinthecomposition

andstructureofelectrocatalystswillalsobeinvestigated

mightbefollowedbydissociation.Otherchannelsthat

canbeselectivelyexcited includeCO-metalsubstrate

frustratedmodes(translation,rotation,bending).NGLS

experimentswillmakeitpossibletofollowthewavefunc-

tionofthemolecularorbitalsinresponsetotheexcitation

ofsuchvibrations,andthushelpdiscoverwhichparticu-

larmodeleadstospecificintermediatesorproducts.For

example, theexcitationofCObendingmodes (when

adsorbedonacatalyst)mightchangethegeometryofthe

moleculesothatelectronsfromthecatalystsubstrate

mightjumpintoanti-bondingorbitalsofthemoleculesor

conversely,electronsfromtheHOMObondingorbitals

mightmovetothecatalyst.Commoncatalystsinclude1st

rowtransitionmetals(Fe,Ni,Co)andalloysofthosewith

Cu,Pt,etc.

Methanoloxidationisamuchstudiedchemicalreaction

toproducepartiallyoxidizedproductslikeformaldehyde,

whileavoidingtheundesirablebutthermodynamically

favoredtotaloxidationtoCO2andwater.CatalystslikePt,

Au,Ag,Cu,andtheiralloysareusedtoaccomplishthisin

anefficientandhighlyselectivemode.17,18Theexcitation

ofC-Hbondsinboundmethoxidesbyheat(viaselected

THzpulsesthatcanselectivelyexcitedifferentvibration

modesdirectlyinthemoleculeorinthecatalystmetal

atoms),willleadtotherearrangementoftheH,CandO

atomstoformintermediates.Capturingtheevolutionof

theorbitalwavefunctionsfollowingtheexcitationwill

provideuniqueinformationonthemechanismofthevar-

iouspossiblereactionpathways,whichwillthenmakeit

possibletostudyandselectdifferentcatalystmaterialsor

CH3OH + O2

Methanol oxidataion on metal alloy clusters

(1) CH3OH + ½O2 → CH2O+ H2O

(2) CH3OH + 3/2O2 → CO2+ 2H2O

hv

hv

e-

CO2

296

C 1s XPS

CH2O CH

3OH

292 288 284Binding energy (eV)

Figure23 Experimental schematic of pump-probe studies of metha-nol oxidation on metal alloy clusters.

42


References:

1. Jiang, P., et al., In Situ Soft X-ray Absorption Spectroscopy Investigation

of Electrochemical Corrosion of Copper in Aqueous NaHCO3 Solution. E.

Chem. Com., 2010. 12: p. 820.

2. Ogletree, D.F., et al., A differentially pumped electrostatic lens system for

photoemission studies in the millibar range. Rev. Sci. Instr., 2002. 73: p. 3872.

3. Salmeron, M. and R. Schlögl, Ambient pressure photoelectron spectros-

copy: A new tool for surface science and nanotechnology. Surf. Sci.

Rep., 2008. 63: p. 169.

4. Bluhm, H., et al., In situ x-ray photoelectron spectroscopy studies of gas/

solid interfaces at near-ambient conditions. MRS Bull., 2007. 32: p. 1022.

5. Ketteler, G., et al., In situ Spectroscopic Study of the Oxidation and

Reduction of Pd (111). J. Am. Chem. Soc., 2005. 127: p. 18269.

6. Tao, F. and e. al., Reaction Driven Restructuring of Rh-Pd and Pt-Pd Core

Shell Nanoparticles. Science, 2008. 322: p. 932.

7. Tao, F. and e. al., Break-Up of Stepped Platinum Catalyst Surfaces by

High CO Coverage. Science, 2010. 327: p. 850.

8. Teschner, D. and e. al., The Roles of Subsurface Carbon and Hydrogen in

Palladium-Catalyzed Alkyne Hydrogenation. Science, 2008. 320: p. 86.

9. Ghosal, S. and e. al., Electron Spectroscopy of Aqueous Solution

Interfaces Reveals Surface Enhancement of Halides. Science, 2005. 307:

p. 563.

10. Andersson, K. and e. al., Autocatalytic water dissociation on Cu(110) at

near ambient conditions. J. Am. Chem. Soc., 2008. 130: p. 2793.

11. Zhang, C.J., et al., Measuring fundamental properties in operating solid

oxide electrochemical cells by using in situ X-ray photoelectron spec-

troscopy. Nature Materials, 2010. 9: p. 944.

12. Gabaly, F.E., M.E. Grass, and e. al., Measuring individual overpotentials in

an operating solid-oxide electrochemical cell. Phys. Chem. Chem. Phys.,

2010. 12: p. 12138.

13. Tao, F., et al., Surface Structure and Chemistry of Bimetallic

Nanoparticles under Reaction Conditions. J. Am. Chem. Soc., 2010. 132:

p. 8697-8703.

14. Danyal, K., et al., Conformational Gating of Electron Transfer from the

NItrogenase Fe Protein to MoFe Protein. J. Am. Chem. Soc., 2010. 132: p.

6894-6895.

15. Hastings, C.J., et al., Enzymelike Catalysis of the Nazarov Cyclization by

Supramolecular Encapsulation. Journal of the American Chemical

Society, 2010. 132(20): p. 6938-6940.

16. Ojeda, M., et al., CO Activation Pathways and the Mechanism of the

Fischer Tropsch Synthesis. Journal of Catalysis, 2010.272: p. 287.

17. Lichtenberger, J., D. Lee, and E. Iglesia, Catalytic Oxidation of Methanol

on Pd Metal and Oxide Clusters at Near Ambient Temperature. Phys.

Chem. Chem. Phys., 2007. 9: p. 4902.

18. Liu, H. and E. Iglesia, Selective Oxidation of Methanol and Ethanol on

Supported Ruthenium Oxide Clusters at Low Temperatures. J. of Phys.

Chem. B, 2005. 109: p. 2155.

inthismannerprovidinginvaluableinformationforguiding

theidentificationofcatalystpropertiesforsuchhigheffi-

ciencyprocessesasthesplittingofwatertogenerateH2or

thereductionofCO2toproducemethanolandotherfuels.

AnotherexampleisthesplittingofO2moleculesthat

precedesmostoxidationprocesses.Oneimportantstep

hereistheformationofchargedO2-species.Howdoes

O2-formfromO2andhowdoesitevolvetoOatoms?

Howdoestheprocessdependonthesubstrate(e.g.oxide

filmsinoxidationreactions,metalsinthecatalyticpro-

cessofCOandNOoxidation)?CO2andH2Oaresomeof

manystrategicallyimportantmoleculesoflowmolecular

weight.Theyareofgreatinterestinprocesseslikephoto-

synthesis,CO2conversiontousefulchemicals,andwater

photo-splitting.UVlasersorshortsoftX-raypulsescan

beused topopulatespecificLUMOwithelectronsor

HOMOswithholes,followedbyprobingwithXPS,XAS

orRIXS.

Beamlines for Catalysis Research

Time-resolved spectroscopy experiments (XANES,

EXAFS,XES,APPES)oncatalyticsystemswillrelypri-

marily on the seeded NGLS beamlines 1 and 2 as

describedinSection5(Table2).Theseexperimentswill

useone-color(andinsomecasestwo-color)X-rayprobes

tofollowthelocalchemicalenvironment,bonding,and

coordinationattransition-metalL-edges(andK-edgesof

C,O,Netc.)inthesoftX-rayrange.EXAFSprobesoflocal

structuraldynamicswillrelyonhardX-raysatthe3rdand

5thharmonics toprobetransition-metalK-edges.Soft

X-rayRIXSexperimentswillrelyonthehighenergyreso-

lution(andhighaverageflux)ofNGLSbeamline1 in

long-pulseseededoperation.

Diffractiveimagingexperimentswillrelyon“diffract

anddestroy”methodsusingthe3rdand5thharmonics

withthehighestflux/pulseontheseededNGLSbeamline

1at100kHzrepetitionrates,andontheun-seededSASE

beamline3,atMHzrepetitionrates(ashigh-speeddetectors

allow)asdescribedinSections5and6.6.Choiceofwave-

lengthwillbedeterminedbybalancingthescatteringeffi-

ciencyandtherequiredresolutionforparticularsamples.

Akeycomponentoftheseexperimentswillbeahigh-

speedparticleinjectorsynchronizedtotheCWSCRFlinac

(seeSection4.1).

43

3 . SCIENCE DRIVERSNANOSCALE MATERIALS NUCLEATION

Characterizingandcontrollingmatterfarfromequilib-

rium,andachievingsynthesisofmaterialswithtailored

propertiesthroughcontrolatthemolecularlevelaretwo

oftheoutstandinggrandchallengesinmaterialsscience.

Understanding the processes through which crystals

nucleatefromsolutioniscentraltobothchallenges.The

importanceofnucleationphenomenaiswidespread:they

areharnessedtoformallthebiomineralsrequiredbyour

bodies;theyunderpinenvironmentalandatmospheric

phenomena;andtheyenablethesynthesisoftechnologi-

calmaterials includingnanoparticlesandcatalysts.1,2

However,experimental,theoreticalandsimulationmeth-

odsforrevealingthemolecular-scaleprocessesthatcon-

trolnucleationallfacemajorchallenges.Nucleationisan

inherently stochastic process, requiring interactions

amongmanyatomsormolecules,andtheearlieststable

nucleiarehighlytransientobjects,withalargethermody-

namicdrivingforcefor furthergrowth.Consequently,

singlenucleationeventsarerareandhighlytransient.

Theyarevirtuallyimpossibletocaptureeitherinexperi-

mentorrealisticatomisticsimulations.Thislimitsourabil-

ity to control nucleation processes in technological

settings,andinhibitsourabilitytoevenlearntheprinci-

plesfordirectedcrystalformationfromthebestknown

examples—biomineralizingproteins.3

NGLSofferstheopportunitytoperformanewkindof

scientificexperimentthatisdesignedforstudyingrare

eventssuchasnucleationinhomogeneoussolution.The

highrepetitionratewillallowaverylargenumberofindi-

vidualreactorvolumestobeanalyzedatspecifiedtime-

points followingsamplepreparation.X-rayscattering

from individualnanoscalecrystallites,enabledby the

highpulseintensityandfastdetectorreadout,willprovide

atomisticand/ormorphologicalinformationattheearliest

stages of crystal formation and growth. Detection of

amorphousprecipitateswillbeaccomplishedbyaverag-

ing the X-ray exposures creating powder patterns.

Moreover,eachX-raypulsecanbeprecededbyoptical

Understanding the processes by which crystals nucleate from solution is a fundamental challenge in materials science with far reaching relevance. Nucleation phenomena affecting our everyday life are harnessed to form all the biominerals required by our bodies, underpin environmental and atmospheric phenomena, and enable the synthesis of technological materials including nanoparticles and catalysts. Recent discoveries have revealed that nucleation pathways are far more complex than envisioned in classical theories, which also fail to provide an energetic basis for observed nucleation rates. Development of a new predictive theory requires an understand-ing of these pathways. However, nucleation events and the molecular processes that control phase, composi-tion, morphology, and final materials properties are exceedingly difficult to capture experimentally or through simulation.

NGLS will be a new tool for nucleation studies uniquely able to capture the dynamic aspects of spontaneous and directed crystal formation and growth. It will allow a new kind of scientific experiment to be performed that is ideal for studying rare events such as nucleation. Because NGLS will provide intense X-ray pulses at a high rep-etition rate, single-shot diffraction images can be acquired from large numbers of supersaturated droplets, allowing snapshots of the earliest crystal nuclei to be discriminated. Furthermore, this approach will offer unprecedented insight into the dynamic process of biologically directed mineralization, revealing the protein-inorganic bonding interactions that direct the nucleation of amorphous precursor phases and the crystallization of oriented nanocrystals.

3.5 NanoscaleMaterialsNucleation

44


formationofapre-nucleationassociationofionsprecedesa

(ratelimiting)crystallizationstep.Forcrystalformationin

aqueoussolutionsinparticular,thispathwayislikelya

consequence of the complex interactions between

hydratedionsandsolventmoleculesthatmustoccurfor

anorderedcrystaltoform.Bycombiningsingleandaver-

agedsnap-shotdiffractionpatternsacquiredfromsolu-

tionsasafunctionoftime,NGLSwillenabletwo-step

nucleationpathwaystobeidentifiedandquantified.

3.5.2 Statistical Description of Crystal Nuclei Size Distributions during Nucleation and Growth

Followingtheturbulentmixingoftworeactionsolu-

tions,asupersaturatedsolutionwillbetranslatedacross

anNGLSbeameitherasdiscreteliquiddroplets8orasa

streamthroughawindowlessmicrofluidiccell.9Liquid

dropletscanbepreparedwithsubmicrondiameters(see

Section4.1.3),loweringthebackgroundwaterscattering,

whileflowcellsareanticipatedtopermitanalysisof>10µm

fluidchannelsatsub-microsecondtimescalesfollowing

mixing.Conditionswillbevariedsothat(withinPoisson

statisticslimits)eitherzeroorasinglecrystalnucleusis

presentineachanalyzedsolutionvolume.

AsillustratedinFigure25,wewillacquiresingle-pulse

coherentX-raydiffractionpatternsat3.6keVand100kHz.

EachX-raypatternwillbeindividuallyanalyzedtoidentify

andindexBraggreflectionsfromcrystallitesnucleatedin

solution.10EachBraggreflectionfromasmallcrystallite

ismodulatedbytheshapefactorofthecrystallite,provid-

ingapartialdescriptionofthecrystallitemorphology.11

interrogationpulsesdesignedtoprovideadditionalinfor-

mationonaqueouscomplexespresentinsolutionprior

tocrystalnucleusformation.Theseapproacheswillallow

unprecedentedquantitativetestsofmodelsofcrystalfor-

mationthroughhomogeneousnucleationofinorganic,

organicandproteincrystals.

Inaddition,NGLSoffersnewapproachesforacquiring

molecular-scalestructuralinformationfrommacromole-

culesthatcanbeharnessedtorevealthechemicalinter-

actions at protein-mineral interfaces. Biomineral

formationfrequentlytakesplaceuponahighlyorganized

proteinmatrixthatconfersorientationtotheforming

inorganicphase.Whetherintheformoforganizedfibrils,

polymericsheets,orhighly-orderedlattices,itisbelieved

thattheproteinmatrixdefinesthemolecularcontacts

thatleadtocontrolovercrystallographicorientationand

biastheenergylandscapetowardssite-specificnucle-

ation.However,alackofstructuralprobeswithtimereso-

lutioncommensuratewiththecharacteristictimescaleof

nucleationprocesseshasseverelylimitedinvestigations

ofstructuraldynamicsduringnucleationatanorganic

matrix.

3.5.1 Homogeneous Nucleation

Theestablishedframeworkforunderstandingnucle-

ationisClassical Nucleation Theory (CNT),whichcalcu-

latesthethermodynamicstabilityofformingcrystallites

asasumofcontributionsfrombulkandinterfacialfree

energyterms.4,5Themajorpredictionofthismodelis

thatearlycrystallitesareunstablebelowacriticalsize,

andkineticmodelsofnucleationhavesoughttodeter-

minehowratesofdiffusion,dissolutionandgrowthlead

to thesuccessfulappearanceofstable (andgrowing)

nuclei in saturatedsolutions.1,6While the conceptual

basisforthemodelisappropriateformanysystemsits

shortcomings were appreciated from the beginning.

Becausecentralthermodynamicsquantitiessuchasinter-

facialfreeenergyrelyonmacroscopicapproximations,

andkineticparameterssuchasratesofadsorptionordis-

solutionaregenerallyinaccessibletomeasurement,pre-

dictionsofcrystalnucleationrateshaveerredbyorders

ofmagnitudeeveninthemostidealcases.

Furthermore,ithasbecomeclearrecentlythatthere

arealternativepathwaysforcrystalformationbeyondthe

stepwiseadditionofmonomersfromsolution.1,7Figure

24illustratestheTwo-StepNucleationModel,inwhichthe

Classical Nucleation Model

Growth

(a)

(b) (c)

Two-step Nucleation Model

Figure24Scheme of Classical and Two-Step nucleation path-ways. (a) Formation of crystallites through aggregation of mono-mers; (b) Formation of amorphous prenucleation cluster; (c) crys-tallization. (After Erdimir et al.1)

45


thatyielddescriptionsofallparticlesnucleatedinasys-

tem.6,16Todate,quantitativetestsoftheoreticalpredic-

tionshavebeenbasedoncrystallitenumber17oraverage

crystallitesize.18

Forcrystalsystemsthatformfromsolutionviathe

Two-Steppathway,theamorphousprecursorswillnot

yielddetectableBraggpeaks.However,foragiventime-

pointallsingle-pulsepatternsthatyieldnoBraggpeak

willbeaveragedtoproduceapowderdiffractionpattern

inwhichdiffusescatteringringswillbedetectableabove

background.Although individual-particleanalysiswill

notbepossibleforamorphousprecursors,theapproach

willrevealthekineticsofprecursorformationpriortothe

nucleationofindividualcrystallitesthatwillbeidentified

asdescribedabove.Both thermodynamicandkinetic

contributions to the pathway can be addressed. For

example,becausestudiesofthestabilityofnanophase

materialshaverevealedthestrongcontributionofsur-

facefreeenergyoncrystallitestability19weanticipate

thatthenucleationstepishighlysize-dependentandwe

willobserveaminimumsizeatwhichcrystalsfirstappear.

Moreover, numerous materials can be crystallized in

more than one polymorph, and may undergo phase

transformationsduringgrowth.Suchprocesses,andthe

impactsofconditionsandadditivesonpolymorphselec-

tion,canbestudiedandcomplementedbycurrentand

anticipated methods for molecular simulations using

transitionpathsampling.20-22

Ifthesamplecontainsidenticalobjects,theaccumulation

ofX-raydiffractionpatternsatmanyorientationsallows

directreconstructionofthearbitrarymorphologyofthe

object(thelow-resolutionelectrondensitydistribution).It

wasrecentlyshown,however,thatforcrystallitesthatvary

indimensionbutwhichconsistofidenticalunitcells,the

collectionofBraggspotprofilescanbeanalyzedtoreveal

thedistributionofcrystallitedimensions.12Thisapproach

willprovidesignificantlymoreinformationoncrystallite

dimensionsthancanbeobtainedbyconventionalX-ray

methodssuchastheScherrerequationforpowderX-ray

diffraction,13orbyusingsmall-angleX-rayscattering.

TheX-rayscatteringstudiescanbecomplementedby

priorspectroscopicinterrogationusingopticallaserpuls-

es.Forexample,second-ordernonlinearopticalimaging

ofchiralcrystals(SONICC)hasbeendemonstratedforsub-

wavelengthdetectionofproteincrystalsinturbidcrystal-

lizationsolutions.14Fornon-centrosymmetriccrystals

thatexhibitbulksecondharmonicgeneration(SHG)this

approachwillprovideaneasilydiscriminatedsignalindi-

catingthepresenceofacrystalliteintheanalysisvolume.15

NGLSexperimentswillprovideprobabilitydistribu-

tionfunctionsforcrystallitesizeasafunctionofsolution

chemistryandtime,followingtheformationofasuper-

saturatedsolution.Thiswillbethefirstdirectexperimen-

talobservationofthedistributionofcrystallitespresent

in solution during crystal nucleation and will allow

unprecedentedtestsoftheoriesofnucleationandgrowth

t

Droplet ofsupersaturatedsolution

X-ray scattering

Nanocrystal

visX-ray

[3, 1, 0]

[3, -1, 0]

[3, 0, 0]

[4, 0, 0]

[4, -1, 0]

Optical scattering

Figure25Scheme of NGLS experiment in which size and morphological information on single crystallites are obtained by single-pulse coherent X-ray diffraction. Crystallite size and shape modulates the Bragg peak profiles as illustrated by simulated scattering pattern from 17x17x30 unit cell Photosystem I nanocrystal.11 Depending on experimental requirements, optional visible or infra-red analysis pulses can be designed that precede the X-ray diffraction analysis pulse.

46


complexpathinvolvingclustersofyetadifferentstoichi-

ometryfollowedbyACP,followedbyoctacalciumphos-

phate(OCP)andfinallyHAP.25

Despite thetinydimensionsandhighly-anisotropic

propertiesoftheHAPnanocrystals,thehierarchicalorga-

nizationofthecollagenmonomersintohelices,helices

intofibrils,fibrilsintobundles,andbundlesintomacro-

scopicboneresultsinamaterialwithnearlyisotropic

mechanicalpropertiesandremarkablefracturetough-

ness.Unfortunately,themolecularcontactsthatdirect

thelocationandorientationofthecrystalnucleusandthe

pathwayofphaseandcompositionalevolutionthatleads

tothefinalcrystallineproductremainamystery.

3 .5 .3 .1 Protein-Directed Hydroxyapatite Nucleation at

Collagen Crystal Surfaces

Wewillacquiremultiplesingle-pulsecoherentX-ray

diffractionpatternsofcollagenfibrilsinliquiddropsthat

areeithersupersaturatedrelativetohydroxyapatite(HA)

formationorwhicharephosphate-free.Byfirstaveraging

singleX-rayexposuresofnon-mineralizedcollagenwe

willobtainalow-resolution

electrondensitymapofthe

fibrilsundertheexperimen-

talconditionsagainstwhich

the known amino acid

sequence can be aligned.

Thiswillprovideanatomis-

tic depiction of the nucle-

ationsitesinthefibrilsthat

directHAnucleation.

3.5.3 Biologically-Directed Heterogeneous Nucleation

Livingsystemsprovideexquisiteexamplesofmateri-

alssynthesiswithtailoredpropertiesviamolecular-level

control. Biomineralization, in particular, is a process

throughwhich livingorganismsproducematerials to

solvefunctionalrequirementsbyexertingmolecular-level

controloverinorganiccrystalgrowth.Hereorganicmatri-

cesactastemplatestodirectthenucleationstage.Non-

equilibriumphasesarestabilizedbytheintroductionof

soluble macromolecules that modulate atomic-scale

growthkinetics.Thisenableslivingorganismstoproduce

awidevarietyofcrystallinenanostructureswithfunc-

tionsasdiverseaslightharvesting,magneticsensing,

andmechanicalsupport.3,23

Among the myriad biomineral systems found in

nature,mineralizationofcollagenousproteinsbycalcium

phosphatesisoneofthemostimportant.Itcomprisesthe

skeletalanddentalstructureofhigherorganismsand

presentsanexquisiteexampleofanorganizedprotein

matrix,highlydirectednucleation,andanevolvingmin-

eralnucleus.23,24Attheshortestlengthscale,thecolla-

genmonomerformtriplehelicesthatstackparalleltoone

anotherwithastaggeredgeometry thatcreatesaso-

called“hole zone” (Figure 26). Nanocrystal plates of

hydroxyapatite(HAP,themoststablecalciumphosphate

phase)afew10sofnmacrossand<5nminthickness

form within these hole zones.The formation of HAP

appearstobeprecededbydepositionofamorphouscal-

ciumphosphate(ACP)nanoparticlesofadistinctstoichi-

ometry.Moreover,invitroexperimentssuggestanvery

a b c dHole zone

D ~ 67nm

D ~67nm

250 nm

Figure26Depictions of the Type I Collagen crystals that direct the nucleation of hydroxyapatite at “hole zone” regions between C- and N-terminus protein regions. (a) Atomic force microscopy (AFM) image of aligned collagen fibrils exhibiting the periodic bands (Tao, DeYoreo unpublished). (b) AFM image of single fibril.26 (c) Low-resolution electron density map of collagen helix obtained from single crys-tal diffraction.27 (d) Reconstructed collagen structure revealing amino acid sequence.



see Section 4.1

47


SCRFlinac(seeSection4.1.3),provideforsamplereplace-

mentonapulse-by-pulsebasis.

References:

1. Erdemir, D., A.Y. Lee, and A.S. Myserson, Nucleations of Crystals from

Solution: Classical and Two-Step Models. Accounts of Chemical

Research, 2009. 42(5): p. 621-629.

2. Laaksonen, A., V. Talanquer, and D.W. Oxtoby, Nucleation —

Measurements, theory and atmospheric applications. Annual Review of

Physical Chemistry, 1995. 46: p. 489-524.

3. Dove, P.M., J.J. DeYoreo, and S. Weiner, Biomineralization. Reviews in

Mineralogy and Geochemistry, ed. J.J. Rosso. Vol. 54. 2003, Virginia:

Mineralogical Society of America.

4. Oxtoby, D.W., Homogeneous nuceation — theory and experiment.

Journal of Physics-Condensed Matter, 1992.4(38): p. 7627-7650.

5. Gunton, J.D., Homogeneous Nucleation. Journal of Statistical Physics,

1999. 95(5/6): p. 903-923.

6. Kelton, K.F., A.L. Greer, and C.V. Thompson, Transient nucleation in con-

densed systems. Journal of Chemical Physics, 1983. 79(12): p. 6261-6276.

7. Gebauer, D., A. Völkel, and H. Cölfen, Stable prenucleation calcium car-

bonate clusters. Science, 2008. 322: p. 1819-1822.

8. Weierstall, U., et al., Droplet streams for serial crystallography of pro-

teins. Experiments in Fluids, 2008. 44(5): p. 675-689.

9. Vig, A.L., et al., Windowless microfluidic platform based on capillary

burst valves for high intensity x-ray measurements. Review of Scientific

Instruments, 2009. 80: p. 115114.

10. Leslie, A.G.W., The integration of macromolecular diffraction data. Acta

Crystallographica Section D, 2006. 62: p. 48-57.

11. Kirian, R.A., et al., Structure factor analysis of femtosecond microdiffrac-

tion patterns from protein nanocrystals. Acta Crystallographica Section

A, 2010.inpress.

12. Spence, J.H.C., et al., Ab-initio phasing of femtosecond diffraction from

many nanocrystals. submitted to Physical Review Letters, 2010.

13. Kazanci, M., et al., Complementary information on in vitro conversion of

amorphous (precursor) calcium phosphate to hydroxyapatite from

Raman microspectroscopy and wide-angle X-ray scattering. Calcified

Tissue International, 2006. 79: p. 354-359.

14. Wampler, R.D., et al., Selective Detection of Protein Crystals by Second

Harmonic Microscopy. Journal of the American Chemical Society, 2008.

130(43): p. 14076-14077.

15. Kissick, D.J., et al., Nonlinear Optical Imaging of Integral Membrane

Protein Crystals in Lipidic Mesophases. Analytical Chemistry, 2010. 82(2):

p. 491-497.

16. Noguera, C., et al., Nucleation, growth and ageing scenarios in closed

systems I: A unified mathematical framework for precipitation, conden-

sation and crystallization. Journal of Crystal Growth, 2006. 297: p. 180-186.

Under conditions for which HA nucleation is just

beginning,wewillacquireandaveragemultipleX-ray

exposurestorefineamodeloftheHA—collageninter-

face.Becausethecollagensubstrateimpartsaconsistent

crystallographicorientationto theHAcrystallites it is

expectedthattherewillbeaninterfacialstructurethatis

conservedatallnucleationsites.Fibriltwistingmaylimit

the resolutionofelectrondensitymapsderived from

scatteringdata,sothiswillbelimitedbypreparingthe

shortestcollagenfibers.However,becausethesequence

andstructureof theorganicsubstrate isknown,data

refinementcanincorporatemodelsoftorsionaldisorder.

Thisapproachwillenabletheaccumulationofinorganic

ionsinthegapregionstobevisualizedasabuild-upof

excesselectrondensityandthusidentifywhetherthefor-

mationofadisorderedprenucleationclusterprecedes

crystallization.Themorphologyofbothamorphousand

crystallineHAstructureswillbefollowedwithtime,offering

unprecedentedinsightsintothedynamicprocessofbio-

logically-directed mineralization.The protein-crystal

bondingwillbeestablishedeitherdirectlyfromelectron

densitymaps,orbyaligningcollagen-HAmodelstolower-

resolutiondata.

Thereisnoprecedentforsuchaneffort.Z-contrast

electrontomographyTEMhasrevealedbonemorphology

atfew-nmresolution,butwithnoreconstructionofpro-

teinstructureorchemistry.28Theproposedsingle-pulse

X-raymethod,avoidingtheenormousradiationdamage

associatedwithelectrondiffraction,hasthebestpotential

forobservinghowprotein-mineral interactionsguide

dynamicmineralizingprocessesofenormousmedical

significance.

Beamlines for Nanomaterials Nucleation Research

Materialsnucleationexperimentswillrelyon“diffract

anddestroy”methodsusingthe3rdand5thharmonics

withthehighestflux/pulseontheseededNGLSbeamline

1at100kHzrepetitionrates,andontheun-seededSASE

beamline3,atMHzrepetitionrates(ashigh-speeddetec-

torsallow)asdescribedinSection5and6.6.Choiceof

wavelengthwillbedeterminedbybalancingthescatter-

ingefficiencyandtherequiredresolutionforparticular

samples.Akeycomponentoftheseexperimentswillbea

high-speedliquiddropletinjectorsynchronizedtotheCW

48


23. Mann, S., Biomineralization: Principles and concepts in bioinorganic

materials chemistry. 2001, New York: Oxford University Press.

24. Weiner, S., W. Traub, and H.D. Wagner, Lamellar bone: Structure-

function relations. Journal of Structural Biology, 1999. 126(3): p. 241-255.

25. Habraken, W.J.E.M., et al., The role and composition of calcium phos-

phate prenucleation clusters. submitted, 2010.

26. Cisneros, D.A., et al., Creating ultrathin nanoscopic collagen matrices for

biological and biotechnological applications. Small, 2007. 3(6): p. 956-963.

27. Orgel, J., et al., The in situ supermolecular structure of type I collagen.

Structure, 2001. 9(11): p. 1061-1069.

28. Grandfield, K., et al., Visualizing biointerfaces in three dimensions: elec-

tron tomography of the bone-hydroxyapatite interface. Journal of the

Royal Society Interface, 2010. 7(51): p. 1497-1501.

17. Kelton, K.F. and A.L. Greer, Test of classical nucleation theory in a con-

densed system. Physical Review B, 1988. 38: p. 10089-10092.

18. Viswanatha, R., et al., Growth mechanism of nanocrystals in solution:

ZnO, a case study. Physical Review Letters, 2007. 98(25).

19. McHale, J.M., et al., Surface energies and thermodynamic phase stability in

nanocrystalline aluminas. Science, 1997. 277(5327): p. 788-791.

20. Auer, S. and D. Frenkel, Quantitative prediction of crystal-nucleation

rates for spherical colloids: A computational approach. Annual Review

of Physical Chemistry, 2004. 55: p. 333-361.

21. Desgranges, C. and J. Delhommelle, Molecular simulation of the

Nucleation and Growth of Gold Nanoparticles. Journal of Physical

Chemistry C, 2009. 113: p. 3607-3611.

22. ten Wolde, P.R. and D. Frenkel, Computer simulation study of gas-liquid

nucleation in a Lennard-Jones system. Journal of Chemical Physics,

1998. 109(22): p. 9901-9918.

49

3 . SCIENCE DRIVERSDYNAMIC NANOSCALE HETEROGENEITY IN MATERIALS

Avarietyofnanostructuredsystemsexhibitdynamical

heterogeneitydriveneitherthermallyorbyadiabaticvari-

ationofexternalfields,includingmolecularswitches,1,2

polymer melts,3 colloidal suspensions,4 magnetic

domains,5-14 single-center fluorophores,15,16 biopoly-

mers,17-20andchargeandorbitaldomainsincomplex

oxides.21-24Animportantchallengeinstudyingsuchsys-

temsistheneedtoprobeaverybroadrangeoftemporal

andspatialscales.Incomplexmaterials,spontaneous

fluctuationsofelectronandspinorderingwillstartto

dominateatnanometerlengthscales,andmayprovide

aninherentlowersizelimitfordevices.Relevantmodes

inproteinfoldingandfunction,forexample,spanfrom

molecular-scalevibrationsatTHzfrequenciestomacro-

molecular-scalelibrationsonthescaleof1Hz.Dynamical

heterogeneityisoftenassociatedwithfastorultrafast

intermittentnanoscaleevents thatspawnstatistically

self-similarspatialand/ortemporalstructures.Powerlaw

dependenciesandtheabsenceofcharacteristiclength

andtimescalesareofcentralimportanceinunderstand-

ingtheemergingmacroscopicproperties,thoughthis

connectionisrarelyunderstoodindetail.

Dynamical nanoscale heterogeneity impacts a multitude of important processes, ranging from protein librations that are crucial to biological function, to superparamagnetic fluctuations that limit the density of information that can be stored on a hard drive. An important aspect of such processes is the way they connect thermally-driven ultrafast events on the nanoscale with kinetic phenomena on the macroscale. This connection has been intensely studied for many decades and in many different contexts, since it governs the emergence of complex material properties from simple microscopic interactions.

NGLS will revolutionize the ability to probe emergent phenomena through its sensitivity to very broad ranges of time and length scales, in combination with its incisive X-ray contrast mechanisms. Highly coherent X-ray pulses will enable ‘probe-probe’ correlation spectroscopy measurements of spontaneous ultrafast processes. For example, thermally driven spin flips or polaron motion in a transition metal oxide will be probed on the relevant nanometer length scale. Closely related time-series correlation spectroscopy measurements will probe longer length and time scales in these systems, where fast events cross over to domain wall or microphase boundary motion. The richness of modern nanoscience, as manifested uniformly in physical, chemical and biological materials, begs for the very diverse array of X-ray tools provided by NGLS.

Length (nm)

Wavevector (Å-1)

106 104 102 100

100

100

105

1010

1015

10-5

10-10

10-15

10-6 10-4 10-2 100

Ener

gy (e

V)

Freq

uenc

y (H

z)

XPCS NGLS

VisibleRaman

Scattering

VisibleBrillouin

Scattering

VisiblePhoton

CorrelationSpectroscopy

XXPPPCCCSSS3rd Gennneerratioon

Soouurrcceces

Inelasticcc SScatt.:X-ray

IInnnnnInI elae sticc nn eeutronn

scattt.

NNeutroonsssppiin echho

Figure27Time and length scales accessible by various experi-mental techniques. X-ray photon correlation spectroscopy operates in a key area not accessible by other techniques. NGLS will open up new frontiers in correlation spectroscopy by allowing to probe materials over a broad range of length and time scales.

3.6 DynamicalNanoscaleHeterogeneityinMaterials

50


estsofbasicsciencewiththeneedsofemergingclean,

efficient,andaffordabletechnologies.

Understandinghowtopredictandcontrolnanoscale

dynamicalheterogeneitywillrequirerevolutionarynew

toolslikeX-raylasers.Simplystated,thehighcoherence

ofanX-raylaserwillallowtheseincoherentlydrivenpro-

cessestobeprobedinunprecedenteddetailbyproject-

ingtheirinherentcomplexityintofar-fieldspeckle-diffraction

patterns,asshownschematicallyinFigure29.Withultra-

fastpulsesathighrepetitionrates,asequenceofsuch

patternscanbecollectedandanalyzedtodeterminethe

underlyingfluctuationsoveranunprecedentedrangeof

spaceandtimescales(Figure27).Ifthesystemisstaticat

thelengthscaleprobed,specklepatternscollectedatdif-

ferent timeswillbe identical,andperfectlycorrelated

witheachother.Ifthesystemisnotstatic,thespecklepat-

ternswilldecorrelate.Measuringthatdecorrelationreveals

thedynamicsthroughtheintermediatescatteringfunction

S(q,t),whichistheFouriertransformofthedynamical

structurefactorS(q,ω).Thisapproachisadirectdescen-

dentofconventionaldynamiclaserlightscattering,28-30

though with high enough spatial resolution to probe

nanoscalephenomena,andwiththepowerfulcharge,

magnetic,andorbitalcontrastmechanismsendemicto

X-raytechniques.Muchprogressinthedevelopmentof

thistechnique,(typicallyreferredtoasX-rayphotoncor-

relationspectroscopy,XPCS),hasbeenachievedoverthe

pastdecadebyspatiallyfiltering(attremendouslossof

flux)thepartiallycoherentradiationfrom3rdgeneration

synchrotronfacilities.31-38Thecoherentfluxofspontane-

ousundulatorradiationislowandthissignificantlylimits

Nanoscaledynamicalheterogeneitywillposesignifi-

cantchallengesindevelopingthecomplexmaterialsfor

next-generationnano-devices.This isalreadyamajor

issueinmagneticrecordingtechnologies,wherethermally

drivensuperparamagnetic fluctuationsdetermine the

ultimatestoragedensitythatcanbeachieved.25-27Future

deviceswillhavetoaddressissuessuchasfluctuations

thatmaylimittheperformanceofmagneticread-heads

basedoncolossalmagnetoresistanceinaspin-andorbital-

orderedmanganitematerial(depictedschematicallyin

Figure28).Conversely,understandinghowtocontrol

nanoscaleheterogeneityisanimportantcomplementto

optimizingdrivennanoscaledynamicsusing,forexam-

ple,pump-probetechniquesdescribedelsewhereinthis

proposal:thesearefastbecomingtheYinandtheYangof

nanotechnology.The X-ray correlation spectroscopy

experimentsdescribedbelow,enabledbythecababilities

ofNGLSX-raylasers,willilluminatenanoscaleordering

andfluctuationphenomenon,therebymeldingtheinter-

Magneticnanobits

Readdevice

Spin-orbitorderedmaterial

Active layer:complexmaterial

Figure28A future generation read-device. It uses complex mate-rials as active layer. Exploitation and engineering of lattice, elec-tronic and spin coupling makes the device ultra small and ultra fast.

Incoming X-rays

Illuminated area

CCD camera

qx, qzFrame 1 2 3 4 N

Magnetic sample yx

z

8

8

d

Pinhole

Figure29A series of ultrafast scattering snapshots are taken. For a static system, the speckle pattern on the detector remains the same, and individ-ual snapshots are correlated. For a dynamical system, the speckle pattern will change, and measuring the time constant for the decorrelation gives information about the intermediate scattering function S(q,t), which is the Fourier transform of the dynamical structure factor S(q,ω).

51


3.6.2 Specific Examples

Example 1: Correlation Spectroscopy Experiments in

Complex Oxides

Anoutstandingfundamentalquestionincomplexcor-

relatedmaterialsishowthevariouslowenergydegrees

offreedomarecoupled.Forexample,abilayermanga-

nitesimultaneouslyshowssomecombinationofspin,

charge,lattice,andorbitalorder.Whatarethetemporal

andspatialbehaviorsofthecollectivemodesforthese

competingorders?Howaretheseorderparameterscou-

pled?Theseordersgenerallyoccuratdifferentwavevec-

tors. Determining the thermally driven equilibrium

dynamicsatthesevectorswillilluminatethenatureofthe

fluctuatingdegreesoffreedomandthecouplingbetween

them.Inthecaseofsimplecontinuoustransitions,the

dynamicsofthevariousorderparameterscanbedeter-

minedandextrapolatedtothecriticalpointtoprobefun-

damental exponents and self-similar behaviors.The

experiment canbe repeatedatdifferentwavelengths

(e.g.,MnL-edgeandOK-edge)toprobeelementspecific

fluctuationinformation,assuggestedinFigure30.

the spatial and temporal dynamic range that can be

achieved,asshowninFigure27.Thehighcoherentpower

of4thgenerationsourcesalongwithdevelopmentoffast

2Ddetectorswilldramaticallyexpandthedynamicrange

ofXPCSinbothspaceandtime.

3.6.1 Science Case: Nanoscale Fluctuation And Dynamical Heterogeneity

Critical Gap

Theequilibriumkineticanddynamicalphenomenaof

acomplexsystemontimescalesrangingfrommillisec-

ondstopicoseconds,andonlengthscalesfromanano-

metertoamicronremainlargelyunexplored.Studiesof

dynamicalheterogeneityofthermallyandadiabatically

drivenspontaneousfluctuationsarefundamentallydif-

ferentfromtriggeredprocessesstudiedinultrafastpump-

probemeasurements.X-ray correlationspectroscopy

probesthese‘inherent’orspontaneousdynamicsontheall-

importantnanometerandlongerlengthscalewheremac-

roscopicpropertiesbegintoemerge.Nearacriticalpoint,

forexample,thermallydrivenequilibriumfluctuations

becomesignificantandleadtoself-similarbehaviorsin

spaceandtimethatproduceremarkablepropertieslike

criticalopalescence.Nanoscaleheterogeneity(notalways

nearcriticalpoints)ismanifestincomplexsystemsranging

fromproteinfunctiontodomainstructuresincomplex

magneticandsuperconductingoxides.Inthesesystems,

intermittentbehaviors(someofwhichexhibitself-simi-

larity)oftenspanbothnanometerandmicrometerlength

scalesandultrafastdynamicalandslowkinetictimescales.

Akeyfeatureofnanoscienceingeneral,istheduality

betweenreal-andmomentum-spaceproperties.NGLS

X-raylaserswillallowustocombineholographywith

correlationspectroscopytoprobethisdualityinarevolu-

tionaryway.Thiswillleadforexampletomoviesofthe

formationandevolutionofasingledomainwallnearan

orderingtransitionwithnano-topicosecondtimeresolu-

tionorofthermalfluctuationsthataretheanalogofcriti-

calopalescence.Butsuchmoviescouldalsobeanalyzed

statisticallytodeterminethespace-timecorrelationfunc-

tionG(r,t),whichistheFouriertransformofS(q,ω),sothat

nanostructuresandraredynamicaleventscanbeprobed

onthesamefootingasstatisticalpropertiesthatarecon-

nectedtousefulmacroscopicproperties.Thismeldingof

real-spacewithmomentum-spacesensitivitieswillbea

keyfeatureofmanycoherence-basedexperimentsatNGLS.

X-ray beam

CCD at OO peak

CCD at AF peak

Complexoxide device

Figure30Illustration of an experimental set up for dynamical pho-ton correlation spectroscopy at the antiferromagnetic (AF) and orbital-order (OO) Bragg peaks. The above figure shows a part of the speckle pattern obtained for a Pr0.5Ca0.5MnO3 OO peak and a bilayer AF peak. The important point is that the peaks have differ-ent ordering temperature and spatial correlation length. It is con-ceivable that the electronic order peak will have a different fluctu-ation time scale than the magnetic order peak. Probing such inher-ent electronic and magnetic fluctuation will provide insight into the spin-lattice coupling mechanism that forms the basis of correlated effects in complex oxide systems.

52


interactionoffluxlines,pinningandde-pinningeffectsas

wellasspeedandcharacteristicsof flux linemotion.

Understandingthevortexdynamicsthroughcorrelation

spectroscopywillhelpin‘pinscapeengineering’ofvorti-

ces,whichisanessentialandintegralparttomanufac-

turenextgenerationsuperconductingwires.Combining

real- andmomentum-spaceapproachesprovides tre-

mendousadvantages,inparticular,imaginginrealtime

thepinninganddepinningofvariousvortexlineswhile

alsomeasuringthecorrelationfunctionsthatdetermine

macroscopicproperties.

Example 3: Ultrasoft Modes in Complex Materials

Thereisenormousinterestincharacterizingthelow-

energymodesofbroadclassesofcomplexmaterials,

includingtwo-leveltunnelingcentersthatareubiquitous

inmostglasses,spin-andcharge-densitywavedynam-

icsinlayeredmaterials,and‘orbitalwaves’incomplex

oxides.Inmanyexperimentstodate,thedynamicsof

Example 2: Vortex Dynamics in a Superconductor

Sincethediscoveryofhightemperaturesuperconduc-

tivity,thepromiseofzeroresistancedevicesforelectric

powerapplicationsuchasgenerators,motors,andtrans-

missionlinesoperatingnear liquidnitrogentempera-

tures,hasfueledintense,worldwideresearchefforts.39

Powerapplicationssharethecommonrequirementthat

thesuperconductorhastobeabletocarrylargecurrent

densitiesinthepresenceofstrongmagneticfieldscon-

sistingoftheself-fieldofthetransportcurrentandexter-

nal fields present in motors and generators. In the

presenceofanappliedmagneticfield,typeIIsupercon-

ductorsarepermeatedbyquantizedvorticesofmagnetic

fluxasshowninFigure31(top).Themagneticinduction

inthesurroundingsuperconductingmaterialiszero.

Whena supercurrent flows, there isdissipationof

energyunlessthesevorticesare‘pinned’insomeway,as

tobeinhibitedfrommovingundertheinfluenceofthe

Lorentzforce.Onlyifthefluxlinescanbeimmobilized

willthesuperconductorsustainthehighcurrentdensities

necessaryforpracticalapplications.Thisso-calledflux

pinningarisesfromthepresenceoflocalizeddefectsor

crystallineimperfectionsthatreducetheenergyofaflux

linesuchthatittendstoremainpinnedatthislocation.

Hence,optimizingthesuperconductorforpowerapplica-

tionsinvolvesmakingthesuperconductingmaterialless

perfectbyinducingsuitabledefectsforpinningmagnetic

flux lines.This requires identifying theelectronicand

magneticpropertiesaswellasstructuralcharacteristics

ofthepinningcenters.Further,thenormalconducting

fluxlinecorehasaradiuscorrespondingtothecoher-

encelengthofthesuperconductor,ξ,whilethedimen-

sionsofthesupercurrentvorticesisdeterminedbythe

Londonpenetratingdepth,λ.ξandλarematerialdepen-

dentandthe1to100nmrange,theideallengthscalefor

softX-raytechniques.

Thedynamicsofthefluxlinelatticeinthepresenceof

anexternal fieldand transport current, aswell as its

responsetoexternalexcitations,needstobestudiedto

determinetheinteractionbetweenfluxlinesandflux-pin-

ningdefects.XMCDspectroscopyhasbeendemonstrat-

edtobeaneffectivemeanstoidentifythevorticies(as

illustratedinFigure31,bottom).Thus,XPCSstudiesat

theCuL-edge(exploitingthestrongXMCDdifferential

scatteringandabsorptioncross-sections)willbeapower-

fulprobeofthevortexstatedynamicsonthenanoscale.

Thesestudieswillprovidenovelinformationaboutthe

XMCD

(arb

. uni

ts)

XA(a

rb. u

nits

) 1.0

0.5

0.0

0.00

-0.05

930 940Photon energy (eV)

950

YBaCuO

T = 20KH = ±9T

Figure31Top: Illustration of fluxoids (red) surrounded by current vortices (green) in a type II superconductor exposed to a magnetic field (blue) (figure courtesy J. Hoffman, Harvard). By manipulating the applied magnetic field, magnetic vortices form in a Type II superconductor. When a current is applied, the Lorentz force drives dynamic behavior of the magnetic vortices. This movement dissipates energy and produces resistance thereby limiting the maximum current that can flow through the superconducting wire. Bottom: X-ray magnetic circular dichroism (XMCD) signal at the Cu L3,2 edges of YBaCuO measured in external fields of 9 T and at T = 20 K. (Figure courtesy E. Arenholz)

53


volumeσ.Thus,foraminimumsignal-to-noiseratio,the

shortestaccessibletimeinterval(measurablecorrelation

time)ΔtscalesasB-2.RecentstudiesattheALS,forexam-

ple,showthatdynamicsofnanoscaleorbitaldomainsin

acomplexoxidecanbeprobedonatimescaleof~1sec-

ond.42ThehigherbrightnessoftheNSLS-IIwillimprove

thetimeresolutioninsuchexperimentstotensofmilli-

seconds.Accessingthemicrosecondandnanosecond

timescalesthatarerelevantforfunctionaldeviceswill

requireafully-coherent,high-repetitionratesourcepro-

vidingafactorof103orgreaterimprovementinaverage

brightness. Reaching the fundamental time scales of

vibrationaldynamics,charge-transfer,andchargecorre-

lationwillrequireanultrafastsoftXraylaserincombina-

tionwithtwo-pulse(splitanddelay)probingasdescribed

belowtoestablishthecorrelationtimeofthespecklepat-

tern.Highrepetitionrateisindispensibletoensurethat

thefluxperpulseremainswithintolerablelimitswhile

maintainingthehighaverageflux.

3 .6 .3 .1 Time Series Technique: 10 ns — Many Second

Time Scale Dynamics

The‘sequentialmode’followstheprotocoldescribed

above:atemporalautocorrelationfunctiong2(q,t)ispro-

duced froma timedsequenceof specklepatterns,as

showninFigure29.Thepresentlyaccessibletemporal

rangeinthisapproachislimitedbytherepetitionrateof

presentX-rayFELs.IntheSASEmode,NGLSwilloperate

initiallyat1MHz(withanupgradepathto100MHz),dra-

maticallyexpandingtherangeofexperimentsthatcanbe

performed.The high NGLS repetition rate will make

experimentsonsystemswithverylowscatteringcontrast

possibleinboththesequentialandthesplitanddelay

modesdiscussedbelow.ThehighNGLSrepetitionrate

will also open for study the important time regime

between ~100 ns and ~30 ms.This will not be easily

probedatotherfacilitiesbecausedelaylineslongerthan

~100nsbecomeunmanageable inthesplitanddelay

approach.Also,theminimumtimescaleaccessibleinthe

sequentialXPCSapproachisdeterminedbythesource

repetitionrate.

3 .6 .3 .2 Split-Pulse Delay Line Technique:

Sub-Picosecond to Nanosecond Dynamics

Theultrafast‘split-pulsemode40,41forXPCSrelieson

superimposedpairsofspecklepatternscollectedwith

time-delayedX-raypulses.Thedecayinspecklefringe

thesemodesareprobedintheenergydomainusingquasi-

elasticscatteringofneutronsorphotons:theycansome-

timesalsobeobservedinpump-probeexperiments.The

fulltransverseandlongitudinalcoherenceandthehigh

repetitionrateofNGLSX-raylaserswillrevolutionize

suchstudiessincewewillbeabletoprobethermally-

driven(spontaneous)modesinthetimedomainwithcor-

respondingly ultrahigh energy resolution. Using the

split-pulsetechniquedescribedbelow,wewillbeableto

studynanoscalefluctuationsonafstonstimescale,cor-

responding to an the energy regime spanning 1 eV

through1μeV,andlimitedonlybythetimedelayavail-

able.Suchstudieswilllieattheforefrontofquasielastic

X-rayscatteringandwillcomplementrelatedneutron

scatteringstudieswiththeabilitytoprobesmallsamples

withthemultitudeofX-rayspectralcontrastmechanisms.

Suchexperimentswillbeenabledbythepropertiesof

NGLSandwillalsorelyonsignificantadvancesinour

abilitytomanipulatesoftX-raybeams.Inadditiontothe

needforasplitanddelaylinediscussedbelow,thefull

longitudinalcoherencewillallowheterodynedetection

experimentsthatwillconnecttime-domainXPCSexperi-

mentstoemergingveryhighresolutioninelasticX-ray

scattering techniques.Heterodynedetection isnearly

alwaysincorporatedindynamiclaserlightscatteringin

theopticalregimesinceitallowsdirectmeasurementof

the field-fieldcorrelation function,g1(q,t). In turn, for

under-damped,propagatingmodeslikethosediscussed

above,g1(q,t)providesameasure,throughFouriertrans-

formation,ofω(q)aswellasthemodedamping.

3.6.3 New Technical Capabilities

Techniquesnowunderdevelopmentatfirst-genera-

tionX-rayFEL’swillprovide the foundation formajor

advancesinXPCSresearchinthenextfewyears.40,41

Thesetechniques,combinedwiththeveryhighrepetition

rateofNGLSX-raylaserswillopenforstudyentirelynew

timeregimesofdynamicnanoscaleheterogeneitythat

arenotaccessiblewithpresentsynchrotronorlowrep-

rateX-rayFELsources.

Atpresent3rdgenerationlightsources,SXPCScapa-

bilitiesareseverelylimitedduetothelowcoherentflux

available,andthesubtletyoftheunderlyingspectralcon-

trastmechanisms.Thesignal-to-noiseratioofsuchexper-

iments scalesas t1/2Bσwith the sampling time t, the

sourcebrightnessB,andthescatteringcross-sectionper

54


Beamlines for Investigating Dynamic Nanoscale Heterogeneity in Materials

Studiesofdynamicnanoscaleheterogeneityusing

SXPCSwillrelyonthehighaveragefluxprovidedbythe

un-seededNGLSbeamline3,providingpulseswith~0.5µm

naturalcoherence length,at1MHzrepetitionrate,as

describedinSection5(Table2).Experimentsrequiring

longercoherencelength(narrowbandwidth)willexploit

theseededbeamline1operatingat100kHz(oralterna-

tively employ a monochromator on beamline 3).

Importantrequiredcapabilitiesaretunabilityacrossthe

transition-metalL-edgesinthesoftX-rayrangeandpolar-

izationcontrol.

References:

1. Liu, F., et al., Giant random telegraph signals in the carbon nanotubes as

a single defect probe. Appl. Phys. Lett., 2005. 86: p. 163102.

2. Chang, S., et al., Tunnel conductance of Watson–Crick nucleoside–base

pairs from telegraph noise. Nanotechnology, 2009. 20: p. 185102.

3. Russell, E.V. and N.E. Israeloff, Direct observation of molecular coopera-

tivity near the glass transition. Nature, 2000. 408: p. 695.

4. Weeks, E.R., et al., Three-Dimensional Direct Imaging of Structural

Relaxation Near the Colloidal Glass Transition. Science, 2000. 287: p. 627.

5. Kim, D.-H. and S.-C. Shin, Intermittency of Barkhausen avalanche in Co

nanothin films J. Appl. Phys., 2004. 95(11): p. 6971.

6. Narayan, O., Self-Similar Barkhausen Noise in Magnetic Domain Wall

Motion. Physical Review Letters, 1996. 77(18): p. 3855.

7. Cote, P.J. and L.V. Meisel, Self-organized criticality and the Barkhausen

effect. Phys. Rev. Lett., 1991. 67: p. 1334-1337.

8. Durin, G. and S. Zapperi, Scaling exponents for Barkhausen avalanches

in polycrystalline and amorphous ferromagnets. Phys. Rev. Lett., 2000. 84:

p. 4705-4708.

9. Perkovic, O., K. Dahmen, and J.P. Sethna, Avalanches, Barkhausen

noise, and plain old criticality. Phys. Rev. Lett., 1995. 75: p. 4528-4531.

10. Puppin, E., Statistical properties of Barkhausen noise in thin Fe films.

Phys. Rev. Lett., 2000. 84: p. 5415-5418.

11. Spasojevic, D., et al., Barkhausen noise: elementary signals, power laws,

and scaling relations. Phys. Rev. E, 1996. 54: p. 2531.

12. Urbach, J.S., R.C. Madison, and J.T. Markert, Interface depinning, self-

organized criticality, and the Barkhausen effect. Phys. Rev. Lett., 1995.

75: p. 276-279.

13. Vazquez, O. and O. Sotolongo-Costa, Dynamics of a domain wall in soft-

magnetic materials: Barkhausen effect and relation with sandpile mod-

els. Phys. Rev. Lett., 2000. 84: p. 1316-1319.

visibiltyisdrivenbythermallymediateddecorrelation

occuringinthetimeintervalbetweenthetwopulsesand

canberelatedtog2(q,t).Thisapproachwillprobeultra-

fast,thermallydrivendynamicsforthefirsttime,withatime

resolutionlimitedonlybytheX-raydelayline.Anexample

ofsuchanexperimentatNGLSisthedynamicformationof

magneticdomainsfromaparamagneticstate.Asthesys-

tempassesthroughtheNeeltemperature,thespinsstartto

orderandfluctuateintheformof‘spindroplets’.Thespin

fluctuationisveryfast,butasthetemperatureisfurther

loweredthedomainsstarttoforminordertominimize

theenergy.Thesplitpulsemodewillallowthetemporaland

spatialdependenceofthesespindropletstobemeasured.

3.6.4 Sample Damage and Modification at 4th Generation Light Sources:

Sampledamage/modificationduetotheintenseFEL

pulsesaresignificantissuesthathavebeenextensively

considered, and are discussed briefly inAppendix 1.

These issuesareparticularly relevant in thesplitand

delayapproach,wherethefirstX-raypulseinapaircan-

notdisruptthephenomenabeingmeasuredbythesec-

ondpulse.Thecapabilitytousemoderatepeak-power

pulses,whilemaintaininghighaveragepowerviahigh-

repetition ratewill beessential.The smaller inelastic

extinctionlengthwillmaketheproblemmoreserious

sincethepulseenergywillbedepositedinasmallervol-

ume.However,thisdecreasedextinctionlengthispartially

compensatedforbythelowerenergyperphoton.Itis

becomingincreasingclearthatonegetsthebestcontrast

indiffractiveimagingexperimentsbyusingawavelength

thatisclosertothedesiredresolutionandnotarbitrarily

shortwavelengthstoavoidradiationdamage.Theimpor-

tanceof limitingsampledamageordisruptionof the

statesbeingmeasuredmeansthatthenumberofpho-

tons/pulsethatcanbeeffectivelyusedwillbecomparable

atallFELfacilities.ThehigherNGLSrepetitionrate—up

toamilliontimeshigherthantheLCLSinNGLSSASE

mode—willmakeexperimentsonsystemswithverylow

scatteringcontrastpossible,inboththesequentialand

thesplitanddelaymodes.

55


28. Berne, B.J. and R. Pecora, Dynamic Light Scattering. 1976, New York:

Wiley.

29. Chu, B., Dynamic Laser Light Scattering. 1991, San Diego: Academic

Press.

30. Schmitz, K.S., Dynamic Light Scattering of Macromolecules. 1990, San

Diego: 1990.

31. Sutton, M., A review of x-ray intensity fluctuation spectroscopy. C.R.

Physique, 2007. 9: p. 657.

32. Lengeler, Coherence in X-ray physics. Naturwissenschaften, 2001. 88(6):

p. 249-260.

33. Livet, P., Diffraction with a coherent X-ray beam: dynamics and imaging.

Acta Cryst. A, 2007. A63: p. 87.

34. Sutton, M., Coherent X-ray diffraction, in Third Generation Hard X-Ray

Synchrotron Radiation Sources: Source Properties, Optics and

Experimental Techniques, D. Mills, Editor. 2002, John Wiley and Sons:

New York.

35. Grübel, G. and F. Zontone, Correlation spectroscopy with coherent

X-rays. J. Alloys and Compounds, 2004. 362: p. 3.

36. Mochrie, S.G. Equilibrium Dynamics of Complex Fluids studied via X-ray

Photon Correlation Spectroscopy at 8-ID at the APS. 2005; Available

from: http://8id.xor.aps.anl.gov/UserInfo/Analysis/slslecturenotes.pdf.

37. Madsen, A. X-ray Photon Correlation Spectroscopyray Photon

Correlation Spectroscopy. 2008; Available from: http://www.esrf.eu/

events/conferences/Tutorials/Slideslecture8.

38. Grübel, G., A. Madsen, and A. Robert, X-ray Photon Correlation

Spectroscopy, in Soft-Matter Characterization B. Borsali and R. Pecora,

Editors. 2008, Springer: Berlin.

39. Larbalestier, D., Gurevich, A., Feldmann, D.M., and Polyanskii, A., Nature,

2001. 414: p.368; Foltyn, S.R., Civale, L., MacManus-Driscoll, J.L., Jia, B.

Maiorov, Q.X., Wang, H., and Maley, M., Nature Mater., 2007. 6: p.631.

40. Gutt, C., et al., Measuring temporal speckle correlations at ultrafast x-ray

sources. Opt. Express, 2009. 17: p. 55.

41. Grübel, G., et al., XPCS at the European X-ray free electron laser facility.

Nucl. Instrum. Methods B, 2007. 262: p. 357.

42. Turner, J.J. and et al., Orbital domain dynamics in a doped manganite.

New Journal of Physics, 2008. 10(5): p. 053023.

14. Zapperi, S., et al., Dynamics of a ferromagnetic domain wall: Avalanches,

depinning transition, and the Barkhausen effect. Phys. Rev. B, 1998. 58: p.

6353.

15. Verberk, R., A.M. van Oijen, and M. Orrit, Simple model for the power-law

blinking of single semiconductor nanocrystals. Physical Review B, 2002.

66(23): p. 233202.

16. Issac, A., C. von Borczyskowski, and F. Cichos, Correlation between pho-

toluminescence intermittency of CdSe quantum dots and self-trapped

states in dielectric media. Physical Review B, 2005. 71(16): p. 161302.

17. Lu, H.P., L. Xun, and X.S. Xie, Single-Molecule Enzymatic Dynamics.

Science, 1998. 282: p. 1877.

18. Deniz, A.S., S. Mukhopadhyay, and et al., Lemke, Single-molecule bio-

physics: at the interface of biology, physics and chemistry. J. Roy. Soc.

Interface, 2007.6(18): p. 15-45.

19. Prakash, M.K. and R.A. Marcus, An interpretation of fluctuations in

enzyme catalysis rate, spectral diffusion, and radiative component of

lifetimes in terms of electric field fluctuations. PNAS, 2010. 104: p. 15982.

20. Chen, S.-J., RNA Folding: Conformational Statistics, Folding Kinetics, and

Ion Electrostatics. Annual Review of Biophysics, 2008. 37(1): p. 197-214.

21. Alers, G.B., A.P. Ramirez, and S. Jin, 1/f resistance noise in the large

magnetoresistance manganites. Appl. Phys. Lett., 1996. 68: p. 3644.

22. Hardner, H.T., et al., Non-Gaussian noise in a colossal magnetoresistive

film. J. Appl. Phys., 1997. 81: p. 272.

23. Podzorov, V., et al., Mesoscopic, non-equilibrium fluctuations in inhomo-

geneous electronic states in manganites. Europhys. Lett., 2001. 55: p.

411-7.

24. Raquet, B., et al., Noise Probe of the Dynamic Phase Separation in

La2/3Ca1/3MnO3. Phys. Rev. Lett., 2000. 84: p. 4485.

25. Weller, D. and A. Moser, Thermal effect limits in ultrahigh-density mag-

netic recording. Magnetics, IEEE Transactions on, 1999. 35(6): p. 4423-

4439.

26. O’Grady, K. and H. Laidler, The limits to magnetic recording — media

considerations. Journal of Magnetism and Magnetic Materials, 1999.

200(1-3): p. 616-633.

27. McDaniel, T.W., Ultimate limits to thermally assisted magnetic recording.

J. Phys: Cond. Matter, 2005. 17: p. R315.

56

3 . SCIENCE DRIVERSQUANTUM MATERIALS

remarkablepropertiesofthesematerials.Thisisanambi-

tious goal, with tremendous potential impact across

diversetechnologyareas:fromefficientenergytransport,

storage,andconversion;tolow-power/high-speedinfor-

mationprocessingandcommunication;tohigh-density

informationstorage;tomaterialsandnano-structures

withengineeredthermal,mechanical,andelectricalprop-

ertieswithmyriadapplications.

Collective Modes

Thescientificchallengeistounderstandhowexotic

andpowerfulpropertiesofquantummaterials“emerge”

fromthecollectiveorcoordinatedbehaviorofthecon-

stituentcomponents.Thesearepropertiesthatarenot

predictablebyconsideringtheindividualparticles(e.g.

3.7 QuantumMaterials

“Quantummaterials”refersbroadlytosystemsthat

arenotadequatelydescribedbysimplesingle-electron

band-models.Suchmodelsandrelatedtheoriesprovided

foundationalknowledgeforthesemiconductorrevolu-

tionofthe20thcentury.Quantummaterialsarepromising

materialsforthe21stcentury,forwhichwesorelylackan

equivalent knowledge foundation.These materials

includeunconventionalsuperconductors,multiferroics,

topologicalinsulators,colossalmagnetoresistancecom-

pounds,andnano-structureswheresurface/interface

effectsandquantumconfinementgiverisetonewphysics,

newproperties,andimportantnewfunctionalities.

NGLSwillenablequalitativelynewapproaches for

understandingquantummaterials.Thisnewknowledge

willbeessential inordertodeveloptheprinciplesfor

directedmaterialsdesignandsynthesistoexploitthe

“Quantum materials” are materials in which electrons — through quantum entanglement — behave collectively in ways we are unable to predict from the reductive models and experimental approaches that guided the development of 20th century semiconductor technologies. Materials in which electrons are naturally quantum-entangled, such as high-Tc superconductors and colossal magnetoresistive manganites, have been at the heart of some of the greatest surprises in 20th century material science. New quantum material systems exhibiting unique emer-gent properties are being discovered every year. Ideas inspired by these materials compose a large part of the innovative landscape at the frontier of modern electronics, including quantum information technologies, super-conducting electrical grids, and nano-device engineering. However, better understanding and control of the materials themselves is essential to develop their potential for these applications.

NGLS X-ray lasers will provide qualitatively new experimental capabilities to observe the energetically fragile many-electron dynamics of quantum materials. The high repetition rate and high peak brightness proposed for NGLS will enable new nonlinear photoemission techniques that directly probe electron correlations. Photon hungry spectroscopies such as Resonant Inelastic X-ray Scattering (RIXS) will finally achieve the requisite energy and momentum resolution to characterize correlated states for effective comparison with theoretical predictions. Ultrafast time resolution will enable the observation of correlated states as they develop, and as they respond to specific excitations of the material. Importantly, the availability of high repetition rate makes it practical to inves-tigate these fragile states with moderate pulse energies (while maintaining high average power) in order to avoid disrupting the states being measured.

Direct probes of charge correlations and their dynamics have been long-recognized as a critical capability gap of modern materials science. Bridging this gap requires the capabilities of NGLS X-ray lasers, and will propel the application of quantum materials in technology areas ranging from efficient energy transport, to low-power/high-speed information processing, to high-density information storage.

57


NGLSwillenableentirelynewapproachesfordirectly

probing collective or emergent behavior in quantum

materials, approaches that are not available through

existingtechniquesandfacilities. Inthefollowing,we

presentselectedexamplesof futureexperiments that

illustratethescientificimpactofNGLSX-raylaserson

ourunderstandingofquantummaterials.

Amongthemostexcitingmaterialsstudiedtodayare

ultrathinatomicfilmssuchasgraphene,3layeredcom-

poundsincludingthehigh-Tcsuperconductors4,5(cuprates

andpnictides),andcolossalmagnetoresistivematerials

(layered manganites).Theoretical and experimental

researchoverthelastthreeyearshasledtotheidentifica-

tionofnew“topologicalinsulator”statesofmatterinwhich

electronswithinonenanometerofacrystalsurfacehave

uniqueandrobustpropertiesthatarehighlyvaluedfor

devices,suchasnovelsuperconductingandmagnetic

states.6-8Allofthesematerialsachievefunctionalproperties

frominteractingelectronsthatmovepredominantlyalong

layersintheircrystalstructure.Furthermore,inallcases

existingmeasurementscanprovideonlylimitedinforma-

tionaboutthekeyelectronicprocesses.Highaveragebright-

ness,ultrafastpulses,andhighlyphasecoherentX-rays

fromNGLSwillmakeitpossibletotakeX-raytechniquesinto

newregimesoftime-,energy-,space-,momentum-,and

spin-resolution,providingcriticalinformationtounderstand

bothartificiallyandnaturallynano-structuredquantum

materials(Figure33)atthescienceandtechnologyfrontier.

electrons,atomsetc.)operatinginisolation.Thepara-

digmforunderstandinganinteractingelectronsystemin

termsofthechargedcollectivemodesdatesbacktothe

1950’streatmentbyPinesandNozieresoftheinteracting

electrongas.Theydescribedthelow-energyfermionsas

Landauqausiparticles,andidentifiedtheelementarycol-

lectiveexcitationasthewell-knownplasmon.Theformer

areobserved,forexample,asapeakintheone-particle

spectralfunction,A(k,ω),andthelatterasapeakinthe

two-particle,dynamicstructurefactor,S(q,ω).

Remarkably,whilethestudyoffermionicquasiparticles

inmodernquantummaterialsisnowwelladvanced,weare

still lackinganeffectivemeanstostudythecollective

modes. Angle-resolved photoemission spectroscopy

(ARPES)measuresdirectlyA(k,ω), and in thepast15

yearshasemergedasthesinglemostpowerfulprobeof

quantummaterials.However,S(q,ω)the essential observ-

ableofaninteractingelectronsystem,hashardlybeen

measuredattherelevantscaleinanyquantummaterial.

Theproblemisthat,becausethegroundstatesofquantum

materialsarisefromasubtlebalanceamongcompeting

interactions,therelevantcollectivemodesappearatmod-

estenergy,typically1to100meV(seeFigures32and36).

ModerninelasticX-rayorelectronscatteringspectrometers

lackthecombinationofphotonfluxandenergyresolution

requiredtomeasureS(q,ω)inthisrange.Theabsenceofa

meanstomeasurecollectivemodesrepresentsanenor-

mousgapinourunderstandingofquantummaterials.

–40

1

–2 0 2 4

0.0000.0100.0200.0120.0160.020

2

1

-1

0

-2

0.0 0.5

0.0

-0.2

-0.4

-0.60.3 0.4 0.5

1.0

(a)

(b)

(c)

qs qe q

Figure32 Two examples of theoretical predictions of collective excitations in S(q,ω). Left: Collective mode predicted by a unified field the-ory of the Mott state.1 Right: Collective mode characteristic of superfluid defects in a smectic, stripe state.2 Because S(q,ω) has never been measured in a quantum material in the relevant energy regime, none of these (or any other) predictions have ever been tested.

58


gaps,itisdifficulttodistinguishsuperconductinggaps

fromthoseresultingfromotherbroken-symmetrystates,

suchaschargeandspindensitywaves.Infact,exactly

thisdifficultyhasledtotheprolonged“pseudogap”con-

troversy in high-Tc superconductivity, where we are

unabletodistinguishbetweennascentfluctuatingsuper-

conductivityandcompetingformsoforder.

Superconductingcoherenceappearsinthe“anoma-

louspropagator”—ratherthanthesingle-particlepropa-

gator described above.The anomalous propagator is

related to the probability amplitude that the system

remainsinitsgroundstateifweremoveanelectronfrom

thestate|k↑>attimezeroandanotherfrom|-k↓>ata

latertime.Aspectroscopybasedonnon-lineartwo-pho-

tonARPEScandirectlyprobetheanomalouspropagator

andtherefore,two-electronquantumcorrelationssuchas

superconductingcoherence.Thebrightness,pulsedura-

tion,repetitionrate,andwavelengthtunabilityofNGLS

makeitaperfectplatformfromwhichtocarryoutsuch

measurements.

Ouranalysisoftwo-photonARPESfollowsdirectlythe

standardtreatmentoftwo-photonabsorptioninnonlin-

earoptics.Two-photonabsorptionproceedsfromaground

statetoafinalstateviaanintermediatevirtualstate,with

thefirstphotoncreatingthe

virtual intermediate state,

andthesecondphotonpro-

motingthesystemfromthe

intermediatetofinalstate.

Inthesecond-ordernonlin-

earARPESprocessillustrated

3.7.1 Understanding Charge Pairing: Two-photon Nonlinear ARPES Spectroscopy

Thecomplexityofquantummaterials,aswellastheir

potentialutility,canbetracedtothepresenceofcompet-

inginteractionsbetweenspin,charge,andlatticedegrees

offreedom.Theseinteractionsgenerateamultiplicityof

broken-symmetryphases,suchascharge/spindensity

wavesandsuperconductivity, aswell asmoreexotic

phasesthathaveyettobeobserved,suchasd-density

waveandcurrentlooporder.Inthissectionwedescribea

new spectroscopy termed “two-photon nonlinear

ARPES”toprobemany-bodyquantumsymmetrybreak-

ing,whichwillbecomepossiblewiththeuniquecombi-

nation of (controllable) high power density, high

repetition rate, and tunable soft X-rays provided by

NGLS.

Asdiscussedearlier,ARPESdirectlymeasures the

one-electronspectraldensityfunction,A(k,ω),whichis

relatedtothesingleparticle-propagator.Fromthespec-

tral intensity,allsingle-particlepropertiessuchasthe

momentum-resolveddensityofstates,quasiparticlelife-

time,anddispersionrelation(renormalizedbandstruc-

ture) are obtained. However, because A(k,ω) is a

one-particlefunctions,ARPESdoesnotdirectlyprobecol-

lectivemodesandmulti-particlecorrelations,andiseffec-

tively blind to a wealth of two- (and multi-) electron

propertiessuchassuperconductingcoherence,exciton

pairing,spindimerization,andlocalvalencebondforma-

tion.WhileARPESdoesobservetheopeningofenergy

Correlated Phenomena

Control/Design

Cooper pairformation

Charge density/orbital waves

Antiferromagnetism Ferromagnetism

Spinliquid

Stripes/Checkerboardorder

Orbitalwaves

Electronic phaseseparation

Superconductivity andmagnetism

Coupled charge/Spin order

Opto-magnetics

Ferroelectricity andferromagnetism

UnconventionalSuperconductivity

ColossalMagnetoresistanceMulti- ferroics

Rich and Novel Electronic Phenomena

Figure33 Collective electronic states and dynamical modes that lead to enhanced material properties for next generation applications.

Time-resolved

non-linear ARPES

59


delayshouldthenyieldtheadditionalinformationabout

thequasiparticlecoherencetime.

TheconditionforobservingnonlinearARPESisthat

thesecond-photonmustfollowthefirstwithinaspace-time

intervaldeterminedbythequasiparticlemean-freepath

andlifetime.Forhigh-Tcsuperconductorstheseparameters

are~100nmand~1psrespectively.Therequiredminimum

peakfluenceistherefore~1022photons/cm2/s,whichcor-

respondstoaninstantaneouspowerof~100kW/cm2.

Thispowerrequirementiswellabovethedamagethresh-

oldforaCWsource,orevena~100pspulsedsource.

Withpulsesofupto500fsdurationfromNGLS(~10meV

energyresolution,usingatime-compensatedmonochro-

mator),focusedto100µm,~106photonsperpulseare

neededtoapproachthecoherentnonlinearregime.TheinFigure34,thegroundstateistheBardeen-Schrieffer-

Coopergroundstate,|BCS>.Inthevirtualintermediate

state,onephotoelectronisejected,leavingbehindaqua-

siparticle,orunpairedelectron.Thesecondphotonthen

ejectsthisunpairedelectrontoreachafinalstatewith

twophotoelectrons.Thefinalstateisthegroundstateof

thesuperconductor,albeitwithonelessCooperpair.The

keypoint is that two-photonabsorptionprovides the

extraenergytobreaktheCooperpair,withtheexcess

photonenergysharedbythetwoejectedelectronsina

coherentprocess.

Figure35contraststhephotoemissionsignalfroma

superconductorinthecaseoflinear(one-photon)and

nonlinear(two-photon)ARPES.InlinearARPESonlythe

occupiedstatesbelowtheFermienergyareobserved.

The spectral density is zero at the Fermi energy and

appearsatEF-Δ.Inadditiontothegap,onemayobservea

faint“backfolding”oftheband,whichisaweaksignature

ofpairing.Incontrast,thereisaclearsignatureofsuper-

conductingcoherenceinnonlinearARPES,showninthe

right-handplotofFigure35.Wepredictapeakinspectral

densitysharplylocalizedinbothenergyandmomentum.

Thespectraldensityappearsatthemid-gapenergyand

inanarrowrangeofmomentum,Δk,ofordertheinverse

ofthesuperconductingcoherencelength.

Thereareavarietyofwaystodistinguishthelinear

andnonlinearARPESsignalsexperimentally.Perhapsthe

mostdirectandinformative is tomeasuretheARPES

spectraldensityresultingfromapairofX-raypulsesasa

functionoftheirrelativedelay.Whenthetwopulsesare

coincidentintimethereisanadditionalnonlinearsignal

resultingfromtheircoherentsuperposition.Measuring

thechangeinthenonlinearARPESsignalasafunctionof

|BCS > +2 photoelectrons(final state)

|BCS > +1qp + 1 photoelectron(intermediate state)

|BCS > +(intial state)

∆

EF

EF

kF

Unoccupied

Correlatedstates

Spectral density—linear ARPES

Spectral density—nonlinear ARPES

Spectral density at Fermi level reveals superconducting coherence

Ener

gy

Occupied

Momentum

εκ

Ener

gy

Momentum

Figure34 Illustration of 2-photon ARPES spectroscopy of super-conductors: initial ground state, intermediate state, and final states with two emitted electrons.

Figure35 Contrasting the ARPES spectral density in linear (top) and nonlinear ARPES (bottom).

60


(seeSection3.8foracomparisonofacquisitiontimesfor

time-resolvedARPES).Laser-basedharmonicsourcesare

notcontinuouslytunable,andwhiletheycanprovidethe

requisitepeakpower,orthehighrepetitionrate,theycannot

providebothsimultaneouslywithresolutioninthe10meV

range.*

3.7.2 Collective Excitations: Energy-Domain Resonant Inelastic X-Ray Scattering (RIXS)

ResonantinelasticX-rayscattering(RIXS)isapower-

fulapproachwiththepotentialtoprobecollectivecharge

excitations,revealingacompletemapofS(q,ω)withboth

energyandmomentum(q)resolutionspanningtheentire

Brillouinzone(BZ).9-11InasimplesemiconductorRIXS

revealsexcitationsacrossabandgap,showingthekinetic

regimethatcanbeaccessedviachemicaldoping.When

quantumstructureandnanoscaleinhomogeneityareadded

tothesystem,newclassesofcollectiveexcitationsappear

atlowenergycorrespondingtothebreakingofquantum

entanglementormodificationofthespatialdistribution

ofcharge(“chargetransfer”excitations).8-13Furthermore,

RIXSisanelement-specificprobeofbulkproperties,with

sensitivitytothealteredelectronicandstructuralenvi-

ronmentatinterfaces,12anessentialcapabilityforunder-

standingcompositesandnano-structuredmaterials.

However,RIXScapabilitiesareseverelylimitedbythe

fluxofpresentX-raysourceswhichprovideonlysparse

‘slices’ofS(q,ω)coveringasmallpartoftheBZwithrath-

ercoarseenergyresolution(~100meV).Whilespectrom-

etersatmodernsynchrotronscanachievemeVenergy

resolutionwithhigh-orderBraggmonochromators(and

gratingmonochromatorsinthesoftX-rayrange),this

comesattheseverepenaltyofdiscarding99.998%ofthe

beamintensity,leavingonly108or109photons/secfor

experiments,fluxcomparabletothatfromalab-scale

rotating anode X-ray source; and requiring weeks to

monthofdataacquisitionforacomprehensivedataset

evenatcoarseenergyresolution.Thisisnotnearlysuffi-

cientforstudyingcollectiveelectronicexcitations~kBT,

andisadirectconsequenceofthefactthatsynchrotron

sourcesaretemporallyincoherent.

high-repetitionrateofNGLSinconjunctionwithanarray

ofmomentum-andenergy-resolved3D time-of-flight

(TOF)analyzers(seeSection3.8)willenablerapiddata

acquisition and discern small nonlinear signals from

background,whiletheshortpulsesreducetheaverage

poweronthesampletoonly~100mW/cm2whichistypi-

callybelowthedamagethreshold.Thecombinationof

MHzrepetitionrate,timeandenergyresolution,andtun-

ability(importantforoptimizingphotoemissioncross-

sectionswithsufficientmomentumtospantheentire

Brillouinzone)arenotavailablefrompresentsynchro-

tron,X-rayFEL,orlaser-HHGX-raysources.

Two-photonnonlinearARPESenabledbyNGLSwillbe

apowerfulnewtooltounderstandsuperconductivityin

complexcorrelatedmaterials.Keyattributesinclude:

• The appearance of a new spectral feature at the

chemicalpotentialthatsignalstheonsetofsupercon-

ductingcoherence.

• Thespectralweightofthisfeatureisdirectlypropor-

tionaltothesuperfluiddensity.

• Themomentumspacewidthofthenonlinearspectral

densitymeasures thesuperconductingcoherence

length.

• Cross-correlationoftwoX-raypulsesyieldsadirect

measureofthesuperconductingquasiparticlecoher-

encetime.

Requirementstwo-photonARPESasdescribedabove

arebeyondthecapabilitiesofcurrentsynchrotrons,X-ray

FEL’s,andlaser-basedharmonicsources.Theyinclude:

• moderatepeakpowers—minimumoftwophotons

withinaquasi-particlelifetimeandmean-freepath

• transform-limitedpulses—toachieveresolutionin

the10meVrange

• highrepetitionrate—toavoidspace-chargebroad-

ening,andtodiscernthespectralsignatureofthe

Cooperpairsfromthebackground(viaenergyand

momentum-resolvingtime-of-flightspectrometer)

• tunability—tooptimizephotoemissioncrosssection

forsensitivitytocoherentstates

Synchrotronsourcesprovidetherequisiterepetition

rateandtunability,butcannotprovidethenecessarypeak

power.X-rayFEL’sprovidetherequisitepeakpower,but

onlyafractionofthisisusable(inordertoavoidspace-

chargebroadening),andthelowrepetitionratesofcur-

rentFEL’sleadstoimpracticalacquisitiontimesforARPES

*Forexample,toachieve>106photonsonthesample(at100eV,10meVbandwidth)at>100kHz,onerequiresanominal1kWaveragepowerlaser—assuming10-5conversionefficiency(perharmonic),0.4eVnominalharmonicbandwidth,andx100lossinatime-compensatedsoftX-raymono-chromator.

61


3 .7 .2 .1 Collective Excitations that Define a

Superconducting Gap

Insuperconductors,theenergyandmomentumquan-

tizationofcollectiveexcitationsatthesuperconducting

gapencodes the fundamental interactions that cause

superconductivity.13Theseexcitationscanbecreatedby

perturbingthephasecoherenceinthesuperfluidofelec-

trons,takingtheformofawhirlpool-likevortex(forexam-

pleseeFigure40,right)oraripple-likegapexcitationsuch

asismodeledinFigure38.Theenergyscaleofthesuper-

conductinggapinnoteworthyhigh-Tccupratesandiron

pnictidescanrangefrom20-50meV,14,15whichcannotbe

measuredwithadequateenergyresolutionandmomen-

tumrangeatexistingRIXSbeamlines,butwillbecom-

pletely characterized with an array of q-resolved,

high-resolutionspectrometers(seeFigure46)inconjunc-

tionwithhighaveragefluxavailableattheNGLS.

Mappingthemomentumdependenceof thesuper-

conductinggapcollectivemodescanqualitativelyreveal

thesymmetryof theorderparameter (e.g.s-waveor

d-wave)andthelengthscaleofCooperpairing.Witha

material-specificmodelsuchasshowninFigure38,one

candirectlyfittheinteractionsthatcreatesuperconduc-

tivity,withthegoalofrelatingthemtochemicalcomposi-

tioninamaterialclass.Agreatdealofrecentinterestin

thehigh-Tc“pseudogap”phaserelatestotheunresolved

question of whether quantum fluctuations above the

superconductingtransitionareaprecursorofsupercon-

ductivity or represent a competing form of quantum

order.16Understandingpseudogapbehaviormaybean

importantroutetodevelopingmaterialswithhighercriti-

caltemperatures.Currenttechniques(e.g.visiblelight

spectroscopiesandSTM)lackmomentumresolutionand

observeonlyasinglestructuredenergygapinboththe

pseudogapandsuperconductingphase.Incontrast,RIXS

providesatwo-dimensionalenergyvs.momentummap

ofthegapcollectivemodesandstructureofeachphase

revealingessentialinsighttotheinteractionsthatunder-

pinsuperconductivity.Mostfieldtheoriesofcorrelated

electronsystems,inparticularthedopedMottinsulator,

involvespecificpredictions forS(q,ω) atmeVenergy

scales.Theabilitytomeasurethisquantitywillunravel

theintricacyofemergentphenomenaandrevolutionize

condensedmatterphysics.

NGLSsoftX-raylaserswillovercometheseexperi-

mentalbarriersbyprovidingthreetofourordersofmag-

nitudehigheraveragephotonfluxinpulsesclosetothe

Fouriertransformlimit(i.e.longitudinallycoherent,with

narrowbandwidth,seeSection5,Table2).Forexample,

NGLSbeamline1willprovide1011photons/pulse,with

<50meVbandwidthpriortoanymonochromator.The

highrepetitionrateenablesexperimentsatmoderate

peakpowerstoavoiddamagetothesample.

3 eV

1 eV

Mott Gap,C-T Gapd-d excitations,Orbital Waves

PseudogapsOptical phononsMagnonsLocal Spin–FlipsSuperconducting gapSpin resonance modes

0

100 meV

–3 –2 –1

Energy loss (eV)

0 1 2

(e) 2008 E = 0.13 ev

RIXS spectra of La2CuO4 at Cu L3 -edge

Figure36Energy scales of some important types of collective excitation. All of these modes can be probed by RIXS and ARPES using the high energy resolution proposed for NGLS.

Figure37 Resolution sets the paradigm: RIXS measurements with 0.1eV resolution currently allow separate charge excitations to be resolved in a cuprate superconductor. With meV resolution at NGLS, low energy excitations such as magnons, superconducting gap excitations and many others will be visible, as well as line-shape features from the entanglement between those modes and electronic excitations. (Figure courtesy of G. Ghiringhelli and L. Braicovich)

62


functionality,8,18suchasthequantumcomputingcapa-

bilitiesillustratedinFigure40(right).Thesearelikelyto

definefuturegenerationdevicearchitectures,andunder-

standingtheirelectricandmagneticfieldresponsewith

chemicalspecificityisthekeytonotonlydefinetheirper-

3 .7 .2 .2 Emergent Behavior at Interfaces

Whilemultifunctionalmaterialsdevelopedfromcom-

posites and nano-structured material interfaces are

emergingasthemostpromisingcandidatesfornext-gen-

erationelectronics,welacktherequisitetoolstoprobe

suchheterogeneousandentangledelectronicstates.As

showninFigure39,X-rayspectroscopieshavetheunique

capabilitytoselectivelymeasureelectronicpropertiesat

aninterface.Forexamplethelifetime,energy,andspatial

propertiesofcollectiveexcitationssuchasinteraction-

dressedelectrons,charge-transferexcitations,andspin

wavescanbedirectlyprobedviaRIXS.9,10Collectiveexci-

tationsinvolveatransientdipolemomenttowhichX-rays

couple,withawiderangeofcharacteristiclengthscales.

UsingshortwavelengthX-raysatNGLSwillmakeitpos-

sibletoprobecollectiveexcitationsonatunablelength

scalesapproachinginteratomicspacings,whichiscriticalfor

materialpropertiesbutcannotbeaccessedwithvisibleor

ultravioletlight.Probingwithresonance-tunedX-raysalso

targetsachemicallyspecificlocation,12makingitpossi-

bletoisolatehowmaterialpropertiesarelinkedtothe

quantumstateofelectrons,andtotracetheconnection

betweenchemicalcompositionanddesiredphenomena.

Thelocalchargegradientanddistinctsiteenergiesat

interfacesmakeitpossibletotargetspecificsitesbytun-

ingthephotonenergyandscatteringgeometry.Ithas

beenshownthatsuperconductorsandmagnetsfabricated

withinheterostructuresandcompositesexhibitenhanced

A

FY

YBCO cap layer

LCMO cap layer

TEY

Mn edge

Cu edge

e-

e-

h

C

H

v

b

Interface cluster

c

a

MnO(1)

O(2)

O(3) Cu

Ba

Ba

Y

ba

Figure39 Targeted measurements (A) X-rays with energy tuned for element and orbital sensitivity can be targeted to observe the electrons at an interface. (B) An atomic structure studied by X-ray scattering at a manganite-cuprate interface. (Graphic from Reference 17)

2.8002.2751.9501.6251.3000.97500.65000.32500

0 4 5 6 70.0

0.5

1.0

1.5

2.0

2.5

3.0

1 2 3

Figure38Contour plot of the predicted RIXS spectral function for a collective excitation of the cuprate superconducting gap for a range of q from (0, π) to (π, π) versus energy (given in units of ~5 meV). (Image from Reference 13)

Momentum

Majorana

Superconductor

Ener

gy

Tim

e

Figure40Low energy collective modes define new functional materials: Recent measurements have shown that collective elec-tronic vortex modes on the surface of a superconducting doped topological insulator (CuxBi2Se3, bands in left panel) are a natural platform for quantum computing. (right) A quantum computing operation can be performed by “braiding” vortices. Band structure theories suggest that these “Majorana fermion” collective vorti-ces exist, but current RIXS spectrometers lack the time and ener-gy resolution to study collective modes linked to the supercon-ducting phase. (CuxBi2Se3 data from Reference 8)

63


orbitalhybridization,etc.)asthecorrelationdevelops.

Suchstudieswillopenanewdimensionandnewunder-

standingofmaterialpropertiesbeyondthatpossiblewith

static(ortime-averaged)measurementsasafunctionof

temperature,pressure,doping,appliedmagneticfield,

isotopesubstitution,etc.Itisalsoacrucialsteptoward

addressingtwograndchallenges:(1)tounderstandcom-

plexmaterialsystemsoutofequilibrium,and(2)touse

tailoredexcitationtocontrolemergentbehaviorincomplex

correlatedsystemsinordertoachievedesiredproperties.

Inmany ionicsolids,photoexcitationresults in the

generationofacharge-transferexcitoninwhichchargeis

partiallytransferredfromoneatomicsitetoaneighbor-

ingone.Animportantexampleofthisisthecupratefam-

ily towhich thehigh-Tcsuperconductorsbelong.The

enhancedmobilityofpairedchargesrequiresthatoptical

excitationsinsuchdopedMottinsulatorsbetreateddif-

ferentlyfromtheirbandcounterparts. Inthisareaour

presentunderstandingiscriticallylacking.Figure41(left)

showsthenearbandgapabsorptionofSr2CuO2Cl2,a

two-dimensional,spin-1/2Heisenbergantiferromagnet,

and the corresponding real-space excitation. Due to

strongCoulombcorrelations,eachCu3dorbitalisoccu-

piedbyasingleholeinequilibrium.Theinsulatorexhibits

a2eVabsorptionpeak,wherephotoexcitationcorre-

sponds toÅscalespatial transferof theCuhole toa

superpositionofsurroundingO-2porbitals.

Oneexample(ofanentireclassofexperiments)isto

coherentlydrivecharge-transfer(CT)excitonswithafew-

formancefactorsbutalsoimprovetheminaninformed

waythroughthedevelopmentofphysicalmodels.Inaddi-

tion,externalelectricalormagnetic fieldsare readily

incorporatedinRIXSexperimentsasameanstotune

material properties and introduce desired collective

behaviors(e.g.electricfieldstopolarizeorbitalorder,or

andmagneticfieldstocreatevorticesinasuperconductor).

3.7.3 Collective Excitations: Time-Dependent (Pump-Probe) Approaches

3 .7 .3 .1 Pump-Probe Attosecond Dynamics of Collective

Excitations in Complex Materials

Thepropertiesofstrongly-correlatedelectronmateri-

alsemergefromcomplexelectronicgroundstateswhere

strongCoulomb interactionsbetweenchargecarriers

givesrisetoanon-rigidbandstructure,i.e.theenergy/

momentumdistributionofelectronicstatesdependson

theoccupationsofspecificatomicorbitals.Theenergy

scaleoftheseinteractionsrangesfrommeVtoeV,corre-

spondingtodynamicsthatdevelopon100’soffemtosec-

ondstoattosecondtimescales.Here,NGLScanprovidea

revolutionarycapabilitytounderstandthesesystemsvia

ultrafastmeasurements,inwhichtailoredcoherentexci-

tationsperturbthesystemoutofequilibriumontime-

scalesshorterthantheunderlyingcorrelations.NGLS

ultrafastX-rayscanthenbeusedtodisentangletheinter-

actions by probing the evolving electronic structure

(bonding,chargedistribution,spin/magneticmoment,

Sr2CuO2CI2

Photon energy (eV)

1.6

1.5

1.0

0.5

520

-8 -4-4Time (fs)

Ener

gy (e

V)En

ergy

(eV)

0 4 8

540

560

900

920

940

960Copper

Cu-O charge state

Oxygen

0.01.8 2.0 2.2

Abso

rptio

n α

(105 c

m-1

)

15 K

400K

X-ray O K-edge X-ray O K-edge

X-ray Cu L-edgeX-ray Cu L-edge

Figure41Left: Charge-transfer exciton peak in Sr2CuO2Cl2.19 Middle: Illustration of the related attosecond real-space transfer in the CuO2 plane probed via two-color X-ray pulses. Right: Simulated differential absorption spectra at the Cu and O X-ray L-edges, revealing the coherent Cu-O polarization directly on the sub-femtosecond timescale with element specificity.

64


tionandspectralresolution(withinthetransformlimit)

arecriticalcapabilitiesthatarealsounavailableusing

presentultrafastsources.Moreover,forprobingcorrelat-

edelectronstructureitis,moreover,essentialthatthe

highaveragefluxbedeliveredathighrepetitionratesin

ordertokeeptheflux/pulseinatolerablerangesoasnot

toadverselyaffectthecollectivestatesbeingmeasured

(seeAppendix1forfurtherdiscussion).

3 .7 .3 .2 Pump-probe: Control of Time-Reversal

Symmetry in Topological Insulators

The prediction20,21 and subsequent experimental

observations22-27ofso-called3Dtopologicalinsulators,

buildingonthetheoreticalunderstandingofthe2Dquan-

tumspinHalleffect,28havestartedanavalancheoffer-

vent activity in both theoretical and experimental

investigationsofthesematerials.Thesesystemsrepre-

senttheexistenceof“topologicalorder”inthesolidstate

(asopposedtomorecommonsymmetry-breakingorder)

anddependonthetime-reversalinvariancepresentin

non-magneticmaterials.Theresulting2Dlinearmetallic

surfacestatesofmasslessDiracfermionsarestrongly

robustagainstmanyperturbations,withtheirgapless

naturefullyprotectedbythetime-reversalinvariance.7

Thedemonstratedmomentumspin-lockingofthesurface

state,23wherestatesarestronglyspinpolarizedalonga

spatialdirectiondeterminedbythedirectionoftheircrys-

talmomentum(depicted inbothrealandmomentum

spaceinFigure42),enhancetheexcitementoverthefun-

damental physics present. Of equal interest are the

wealth of potential device applications ranging from

spintronicstoquantumcomputing.

Thecreationof a transientnearbymagnetic state,

breaking time-reversal symmetry, provides a unique

opportunitytodirectlyobserveandunderstandthere-

establishmentoftopologicalorderfromabrokensym-

metrystate.Whilehigh-resolutionspin-resolvedARPES

experimentshavemadestrongprogressinexploringthe

energy,momentum,andspindependenceoftheelec-

tronicstructuresintopologicalinsulators,21-26littlehas

beendonetodirectlyexploretheirtimedependenceand

interactionswithsurfaceinhomogeneity.Inadditionto

fundamentalunderstanding,thetemporaldependenceof

theseelectronicstructuresawayfromequilibriumiscen-

traltothedevelopmentofdeviceapplication.Thepossi-

bility of pump-probe based time-resolved RIXS, and

time-andspin-resolvedARPESprovidestheuniqueability

cyclenear-IRpulse.Onatto-

second time scales, the

oscillatingIRelectricfieldis

almoststationary.Thus,the

effect of light waves as a

time-dependent perturba-

tionof thecorrelatedelec-

tronic structure will be

directlyobservableviatran-

sientsoftX-rayabsorption

spectroscopyand/ortime-

resolvedRIXS.Absorption

edges can be exploited to

providetheelementalspeci-

ficity, forexamplebetweenCuandOusingtwo-color

attosecondpulses.Thiswillenableforthefirsttimethe

directobservationofthecoherentpolarizationbuildup

anddephasingduringcharge-transferandstabilizationin

theexcited-stateofahighlycorrelatedmaterial.These

willbemanifestasquantumbeatsontheattosecondtime

scale(Figure41,right)andascoherentRabioscillations

atopticalfieldintensitiessufficientforcompletetransi-

tions between ground-state and CT-exciton states.

Resolving the charge-transfer dynamics will provide

important new insight to the Cu-O correlations and

Coulombinteractionsrelevantforawiderangeofcom-

plexmaterials.

Theimportanceofultrafasttime-resolvedmeasure-

mentsinunderstandingthesesystemsderivesfromthe

capabilitytousetailoredexcitationtoperturbsystems

outofequilibriumontime-scalesshorterthantheunder-

lyingcorrelations,andthendisentangletheinteractions

byprobingtheirtimeresponseasthecorrelationdevelops.

TheunimaginablyshorttimescaleaccessiblewithNGLS

willprovideuniqueinsightintothefundamentaldynam-

icsofelectronicwavefunctionsandinteractionsinsolids

bytracingdirectlyelectronicpolarizations,populations,

andbandstructurewithelementalspecificityas they

evolveonattosecondandfew-femtosecondtimescales.

AtNGLS,thestabilityaffordedbyasuperconducting

linac(operatinginCWmode)willenablesynchronization

ofthesub-femtosecondFELpulsestowithinanoptical

cycleoftheCEPstabilizedexcitationpulses.Thelowaver-

agefluxofcurrenttable-topultrafastsoftX-raysources

presentsaseverelimitationforbothoptical-pumpX-ray

probe, and X-ray pump X-ray probe experiments.

Polarizationcontrolandanabilitytotradeofftimeresolu-

Two-color X-ray probe

X-ray pump, X-ray probe

High-resolution RIXS



see Section 4.3

65


insulatorcrystalswithmagneticimpurities.26Figure43(a)

and(b)showthelinear,masslessDiracdispersionwith-

outmagneticdopingandthemassiveandgappedDirac

dispersions with magnetic doping, respectively.This

methodofbreakingthetime-reversalsymmetry,however,

isintrinsicallyastatictechnique,andthereforecannot

allowtheobservationofthesystemsasthetime-reversal

symmetryphaseisre-establishedfromanearbybroken-

symmetryphaseonfundamentaltimescales.Suchchem-

icaldopingalsointroducesadditionalimpurityscattering

andchangesthechemicalpotential.

Highintensitypulsesofcircularlypolarizedlightpro-

videameans tobreak time-reversalsymmetryonan

ultrafasttimescale.Thismethodhasdistinctadvantages

overmagneticchemicaldopingasitavoidsthecomplica-

tions of impurities and modification of the chemical

potential.Additionally,theshorttime-scaleofthepulses

immediately allows the direct study of the dynamics

betweenphaseswiththeuseofX-rayprobepulsesand

RIXSortime-andspin-resolvedARPES(asdiscussedin

Sections3.8.2.2and3.8.3).Suchanexperimentisdepict-

edinFigure44.Spinresolutionisfundamentalinthese

materialsforprobingthecharacteristicspin-momentum

texturesofthetopologicalstate,andtheresultingspin

dynamicsinthepresenceoftransientsymmetrybreak-

ing. Preliminary measurements show that significant

timereversalsymmetrybreakingispossiblewithalaser

pumpperturbation.Collectiveexcitationsbetweenthe

surfacestateDiracconesarehighlysensitivetotheX-ray

polarizationandscatteringgeometryattheL2/3andM2/3

resonanceedges.Thesefactorsreflectthespinpolariza-

tionandcanbeusedtoidentifyagapopenedthrough

timereversalsymmetrybreaking,inlargepartbecause

spinsneartheDiracpointflipdramaticallyfromin-plane

toout-of-planeperpendicularorientationswhenagapis

openedfrommagneticperturbation.8

Magneticsymmetrybreakingwillreconfigurethespin

orientationoftopologicalDiracsurfaceelectrons,chang-

ingthebalanceofinteractionsthatsetstheenergyof

excitations that involve the simultaneous collective

motionofmanyelectrons(plasmons).Thestrongentan-

glementbetween2DDiracelectronsandplasmons(com-

biningtoform“plasmarons”)isthereforeexpectedto

generateastrongsignaturevisibleinhighfluxX-rayscat-

teringmeasurements.TheNGLSistheonlyproposed

sourcewithsufficientaverageflux,repetitionrate,and

versatilityforthistypeofinvestigation.Thecreationofa

to directly probe the energy, momentum, and spin-

dependentdynamicsofelectronicexcitationsintopologi-

calinsulators.

Thetime-reversalinvarianceofthesesystemsiscen-

traltothesurfacestatesofmasslessandmetallicDirac

fermions. Breaking the time-reversal symmetry with

magneticfluxcanthereforeresultinmassiveDiracfermi-

onsinstead,withagapformingattheDiracpoint,and

canleadtofunctionalenhancementssuchastherecord-

ingofstablequantuminformation(Q-bits,seeFigure

43c). Such breaking of time-reversal symmetry has

recentlybeenachievedbychemicallydopingtopological

a Real space b Momentum (k) space

Figure42 Unique spin texture of the topological insulator surface state. (a) In real space, electrons flow along the sample surface with their spins locked perpendicular to their direction of travel, and oppositely flowing electrons have opposite spins. (b) In momentum space, the surface state electronic bandstructure forms a linear Dirac-like cone. The electrons have spin polarization locked tan-gential to constant energy contours, as depicted at the Fermi level.

Perturbed TI surface Superconductor

a

Bi2Se3 Dirac cone

.4

–1 10

Momentum (Å–1)

.2

0

Bind

ing

ener

gy (e

V)

b c

B

Topo surface

Figure43 (a) ARPES measured spin-integrated dispersion of the topological insulator Bi2Se3, showing a linear, massless, Dirac cone dispersion, with the intersection at the Dirac point ensured by time-reversal symmetry. (b) Chemically doped with magnetic atoms, time reversal symmetry is broken causing a gap at the Dirac point. (c) When magnetic perturbations and superconductiv-ity are combined on a topological insulator, vortices at the surface act as stable bits of quantum information, Q-bits. (Images from Reference 8)

66


states,suchasentangledplasmonsandelectrons(“plas-

marons”)ingraphene,29andatthesurfacesoftopologi-

calinsulators.Theselow-energyexcitationscomedirectly

fromthequantumelementsthatareofinterestfordevice

applications.

Some of the most immediate questions that time-

resolvedRIXScouldanswer,using thecapabilitiesof

NGLS,havetodowithhowtheelectrondynamicsand

entanglementinaquantumsystemchangewithtimein

pump-probeexperiments.Ithasbeenwellestablished

that ultrafast X-ray pulses can be used to measure

(“probe”)themeltingandreformationoflowtempera-

tureorderedstatesafterexposuretoan intense laser

“pump”.However,itcanbechallengingtoobtainmean-

ingfulscientificinformationfromthesemeasurements,

becausethequantumstateafterlaserexposureisnearly

impossibletopredictormodelfromfirstprincipletheo-

ries.Analyzingthetime-evolutionofRIXSscatteringfol-

lowingalaserpumpwillgreatlyclarifythepicture.When

valenceelectronlevelsaredepopulatedbyalaserpulse,

thevacantstatesareexpectedtobevisibleaslowenergy

peaksintheRIXSspectrum,30,31givinganimmediate

metricofhowtheelectronconfigurationhasbeenper-

turbed.Throughsuchmeasurements,themeltingandre-

establishmentoforderedstatescanbedirectlycompared

totheevolutionofthelowenergywavefunction,giving

muchmoretractiontotheoreticalmodels.

Manyinterestingquantumstatessuchassupercon-

ductivityandtopologicalinsulatorordercannotbestud-

ieddirectlyinpresent-daytimeresolvedX-rayscattering

experimentsbecausetheirfunctionalstatesaredistin-

guishedbycollectiveelectronicbehaviorsthathaveno

correspondingstructuralphasetransition.Inthesecases,

themostdirectwaytoobservethetimeevolutionwillbe

tomeasuretime-resolvedchangesininelasticcollective

modeswithhighfluxtechniquessuchasRIXSand2-pho-

tonARPES.

RIXSspectroscopicmeasurementsprovidemultifac-

etedinformationabouttheelectronicstatethatisinde-

pendent of any detailed theoretical model. The

momentum-andenergy-axiswidthofRIXSfeaturespro-

videameanstoevaluatethetimeevolutionoftheelec-

tronicmeanfreepathandscatteringrate.Thedegreeto

whichtheenergyofRIXSfeaturesdependsonmomen-

tumrepresentshowstronglyelectronsarelocalizedina

material(e.g.byspinororbitalorder).TheRIXSsignalat

largemomentainparticularisthoughttobedominated

nearbymagneticstate,breakingtime-reversal,starting

fromatopologicalinsulator,isjustoneexampleofhow

NGLSwillrevealthephysicsofcompetingorprecursor

phasesincomplexmaterials.

3 .7 .3 .3 . Pump-probe RIXS

Theavailabilityofultrafastpulsesathighrepetition

ratefromNGLSwillopenentirelynewapproachesfor

understandingcollectivedynamicsinthetimedomain.

Forexample,ultrafasttime-resolvedRIXSmeasurements

performed at NGLS will measure collective electron

dynamicsinresponsetotailoredexcitations:vibrational

excitations,THzexcitations,transientquasiparticlecre-

ation,andcharge-transferexcitations.Thisissubstantially

moreinformativethanpresenttime-resolveddiffraction

(elasticscattering)studiesthatprobeonlyasinglelong-

rangeorderparameter,andwillmakeitpossibletostudy

the ordering and perturbation-response dynamics of

functionalstates ina farmore flexibleandphysically

informativeway,evenformaterialssuchastopological

insulatorsandsuperconductorsthathavenostaticorder.

Following inelasticmodes in the timedomainwill

extendpump-probetechniquestoavarietyofordered

statesandquantumpropertiesthatcannotcurrentlybe

trackedinthetimedomainduetoalackofchargeorlat-

ticesuperstructure.Tonamejustafewpossibilities,atthe

10meV energy scale one could measure momentum-

resolvedelectronicexcitationsacrossasuperconducting

gapinhigh-Tcsuperconductors,13ordiscerncollective

excitationfeaturescausedbyunusuallow-dimensional

Optical circular pump:

100 fs

X-ray probe pulse

• Breaks TRS

• Leaves TI in transientnon-zero magnetic state Dynamics:

• Gap?

• Spin texture?

0

Time/Spin ARPES

500

Figure44 Schematic of a pump-probe time and spin resolved ARPES experiment. A circularly polarized pump pulse alters the fundamental symmetry of the system, and a synchronized and variably delayed X-ray pulse will induce photoemission, and the photoelectrons are properly analyzed.

67


expecttoseethequantumshake-upcauseenergyshifts,

ahigherprevalenceoflowenergyexcitations,andbroad-

erfeaturewidthsdevelopinginthesecondhalfofthe

pulse.The“melting”oforderedstateslikesuperconduc-

tivityormagnetismwillbemanifestbythedisappear-

anceoftheircorrespondingcollectiveexcitations,and

could be monitored in tandem with other collective

modesforafemtosecond-by-femtosecondrecordofthe

quantumstate.

3 .7 .3 .5 Multi-dimensional Spectroscopy of

Collective Excitations

NonlinearX-rayexperimentswillrevealnotonlythe

interactionsthatgovernthecorrelatedgroundstate,but

alsothenatureofnon-equilibriumstatesofrelevancefor

arangeofnext-generationelectronicmaterials.Optically-

drivenexperimentsalongwithmoresophisticatedultra-

fastnonlinearsoftX-rayprobes,suchasCoherentX-ray

RamanSpectroscopy(CXRS),core-holecorrelationspec-

troscopy,four-wavemixing,andrelatedmulti-dimensional

spectroscopytechniquesinthesoftX-rayregimewillpro-

videthefirstcompletepictureofelectroncorrelationsin

thesematerials.Figure45illustratesmulti-dimensional

spectroscopy ina transition-metaloxide.X-raypulse

sequencestunedtotheO1s→2pandCu2p→3dreso-

nancesprobecorrelationsbetweenO-2pandCu-3dlevels

viacore-holecorrelationspectroscopy.35Alternatively,

X-raypulsesmayprobelocalizedd-dtransitions,andfol-

lowvalencechargeflowbetweentheCuandOsitesvia

coherentX-rayRamanspectroscopy.36Thedevelopment

ofnonlinearX-rayscienceisoneofthemostambitious

goalsforNGLS.Thiswilltrulyrevolutionizeourunder-

standingofcomplexmaterialsbyenablingthefullimple-

mentationofmulti-dimensionalX-rayspectroscopyasa

probeofmany-bodycorrelationsinquantummaterials.A

moredetaileddescriptionofthesetechniques,andthe

informationtheyprovideisgiveninSection4.3.

Beamlines for Nonlinear ARPES, Ultrafast, and High-resolution Experiments in Quantum Materials

Two-photonARPESexperimentsinquantummaterials

willrelyprimarilyonNGLSbeamline1,providing280eV

photonenergies(betterthan50meVresolutioninlong-

pulseoperationwithoutamonochromator)asdescribed

inSection5(Table2).Theuseofhigh-repetitionrateat

byexcitons(closelyboundstatesofelectronsandholes),

which are sharply defined in energy and should be

strongly affected by the degree of confinement.

Measurementsofthesepropertiesinpump-probeexperi-

ments may provide a time-resolved view of how the

onset of a complex low temperature ordered state

restrictstherelativelyfreeelectronkineticsofdisordered

materials,highlightingthespecificquantumcharacteris-

ticsofacomplexmaterial.RIXSistypicallyseveral(e.g.

3)ordersofmagnitudeweakerthanresonantelasticscat-

tering,withgreatvariabilitydependingonthespecific

material and resonance to be investigated. However,

evenprobingaverylimitedparameterrange,theseprop-

ertiesmakeitacriticalmethodtounderstandthefemto-

secondtimeevolutionofcomplicatedwavefunctions.

3 .7 .3 .4 Time-domain RIXS: Phase Profile

Measurements of Dynamic Structure

Theavailabilityoftunablephase-coherentpulseswith

highaveragebrightnessatNGLSoffersqualitativelynew

approachestoprobecollectiveexcitationsinmaterials.In

particular, nonlinear interference-based pump-probe

techniques suchas frequency-resolvedopticalgating

(FROGandcross-correlated“XFROG”)32,33canbeused

toextractboththeintensityandtemporalphaseofthe

scatteredX-raypulsestodeterminethedynamicstructure

factorwithunprecedenteddetailthatisnotavailablefrom

conventionalRIXSbasedontemporallyincoherentpulses.

Asanincidentphotonprobesacollectiveexcitation

viascatteringinamaterial,theoutgoingphotonisphase

shifted,i.e.thefingerprintofthephase-dependentcollec-

tiveexcitationspectrumS(q,ω,φ)isimprintedonthetem-

poralphaseofthescatteredX-raywave.Thisinformation

canbeextractedbycharacterizingthetemporalphase

andintensityofthescatteredwave.Keyrequirements

includetemporalcoherencethatisrepeatablefrompulseto

pulse,toenabletheaccumulationofsignalovermultiple

pulses.

In recentyears, sophisticated techniques forpulse

characterization(withattosecondresolution)havebeen

developedanddemonstratedintheEUVregime,andare

now being extended to the soft X-ray range.34This

approachprovidesimportantnewinformationaboutthe

“birth”ofcollectiveexcitations,andhowtheyevolveon

theattosecondtofemtosecondtimescales.Forexample,

comparingphaseinformationinthefirst250fsandfinal

250fsofanintense500fsscatteredpulse,onemight

68


strongnon-resonantscatteringmodes(e.g.phonons)by

atleastthreetofourordersofmagnitude,enhancingthe

contrastofweakresonantchargeexcitationmodesinthe

<100meVrange.

Enhancedenergyresolutioninexistingspectroscopies

hasbeenahallmarkofscientificadvancement,analo-

goustothewaythatMoore’sLawhasdefinedtheadvanc-

ing capabilities of computers. Dramatically higher

averagephotonfluxmakesitpracticaltousehigherreso-

lutionspectrometers,significantlyadvancingthecapabil-

ityofcurrentRIXSmeasurementtechnologiesthatare

limited to~0.1eV resolution.Figure46 illustratesan

advancedhigh-efficiencyRIXSend-stationdesignedto

measuremultiplemomentumdirectionssimultaneously

athighenergyresolution.

NGLSbeamline3inconjunctionwithanarrayofmomen-

tum-andenergy-resolved3DTOFanalyzers(seeSection

3.8)willenablerapiddataacquisitionanddiscernsmall

nonlinear signals from background (while avoiding

space-chargeeffects).Inthiscase,highenergyresolution

willbeprovidedbyamonochromator.RIXSexperiments

onquantummaterialsattransition-metalL-edgesinthe

softX-rayrangewillsimilarlyrelyonthehighenergyres-

olution(andhighaverageflux)ofNGLSbeamline1(with-

out a monochromator) and/or beamline 3 with a

monochromator.Attosecondvisible-pump,X-ray-probe

spectroscopyexperimentswillrelyprimarilyontheseed-

edNGLSbeamlines1and2.Theseexperimentswilluse

one-color(andinsomecasestwo-color)X-rayprobesto

followvalencechargedynamicsviaXASandXESattran-

sition-metalL-edgesandOK-edgeinthesoftX-rayrange.

Multi-dimensionalspectroscopyexperimentswillrelyon

two-colorsub-femtosecondcapabilitiesofbeamline2.

Tunabilityandvariablepolarizationwillbeimportantfor

nearlyallquantummaterialsstudies.

Technical Considerations — RIXS

TheRIXStechniqueisveryphoton-hungry,makinga

highrepetitionratecriticaltoavoiddisruptingthelow-

energycollectivestatesbeingmeasured.Basedonrecent

measurementsattheLCLS,itappearsthatasafeupper

boundforphotonfluxmaybeintherealmof1mJ/cm2,

sufficienttoallowroughly108photonsineachpulseincident

onthesamplewithamoderatelyfocusedbeam(~30μm

X40μm).Ata1MHzpulserate,thisconfigurationwill

produceasignalthatisthreetofourordersofmagnitude

strongerthanexistingbeamlines,*andallowconsider-

ableleewaytoimprovethestateoftheartexperimental

resolution.A90°scatteringbranchanalyzerwillsuppress

O-1s Cu-2p

Valence coupling

Time

Sample

t1 t2 t3

k1

k1

k2

k2

k3

k3

k4

k4

kS

Figure45 Schematic multi-dimensional spectroscopy in transition-metal oxide. X-ray pulse sequences tuned to the O-1s and Cu-2p probe correla-tions between O-2p and Cu-3d levels via core-hole correlation spectroscopy.35 Alternatively, X-ray pulses tuned to the O 1s -2p and Cu 2p-3d transitions may probe localized d-d transitions, and charge flow between the Cu and O sites via cohrerent X-ray Raman spectroscopy.36

–15°0°

15°

Figure46 Schematic of a high efficiency RIXS end-station, config-ured to simultaneously measure multiple momenta with enhanced energy resolution.

* Current spectrometers used for non-resonant measurements can achieve resolution ~1 meV (e.g. at APS Sector 3 among others) by sacrificing roughly four orders of magnitude in scattered signal intensity, which renders them unsuitable for synchrotron-based RIXS.

69


10. Kotani, A. and S. Shin, Rev. Mod. Phys., 2001. 73: p. 203–246.

11. Abbamonte, P., et al., Resonant Inelastic X-Ray Scattering from Valence

Excitations in Insulating Copper Oxides. Physical Review Letters, 1999.

83(4): p. 860.

12. Chakhalian, J., et al., Orbital Reconstruction and Covalent Bonding at an

Oxide Interface. Science, 2007. 318(5853): p. 1114-1117.

13. Lee, P.A. and N. Nagaosa, Collective modes in the superconducting

ground states in the gauge theory description of the cuprates. Physical

Review B, 2003. 68(2): p. 024516.

14. Carlson, E.W. and e. al., The Physics of Conventional and Unconventional

Superconductors, ed. K.H.B.a.J.B. Ketterson. 2002, Berlin: Springer-

Verlag.

15. Wray, L., et al., Momentum dependence of superconducting gap, strong-

coupling dispersion kink, and tightly bound Cooper pairs in the high-Tc

(Sr,Ba)1-x(K,Na)xFe2As2 superconductors. Physical Review B, 2008.

78(18): p. 184508.

References:

1. Leigh, R.G., P. Phillips, and T.-P. Choy, Hidden Charge 2e Boson in Doped

Mott Insulators. Physical Review Letters, 2007. 99(4): p. 046404.

2. Cvetkovic, V., et al., Observing the fluctuating stripes in high-Tc super-

conductors. EPL (Europhysics Letters), 2008. 81(2): p. 27001.

3. Geim, A. and K. Novoselov, Nobel Prize in Physics. 2010.

4. Abrikosov, A.A., V.L. Ginzburg, and A. J. Leggett, Nobel Prize in Physics,

2003.

5. Bednorz, G. and K. Müller, Nobel Prize in Physics, 1987.

6. Hasan, M.Z. and C.L. Kane, Topological Insulators. arXiv:1002.3895v2,

2010.

7. Moore, J., Topological Insulators: The next generation. Nature Physics,

2009.5(6): p. 378-380.

8. Wray, L.A., et al., Observation of topological order in a superconducting

doped topological insulator. Nat Phys, 2010. 6(11): p. 855-859.

9. Hasan, M.Z., et al., Electronic structure of Mott insulators studied by

inelastic X-ray scattering. Science, 2000. 288(5472): p. 1811-1814.

RIXS Experiments at 1 meV

Thefollowingestimatesillustratethescientificneedforahigh-repetition-rateultrafastXraylasertoinvestigate

low-energycollectiveexcitationsincorrelatedmaterialsviaRIXS:

Required integrated flux on the sample: ~1018 photons


Time resolution

StorageRing 102[2] 5x108 200 days 100ps

PulsedFEL 108[1] 102 1000 days ~ps

NGLS 108[1] 106 3 hrs ~ps

[1] Fluence limit:~1mJ/cm2toavoiddisruptionofelectronicproperties

(e.g.1keV,108ph/pulse,50μmfocalspot⇒ ~1mJ/cm2)

includesmonochromatorlosseswithSASEFELoperation

Fouriertransformlimit:1meV⇔2psec




Rep.rate 5x108Hz

Pulseduration 100ps



Rep.rate 6x106Hz

Pulseduration ~1ps

70


27. Kane, C.L. and E.J. Mele, Quantum spin Hall effect in graphene. Physical

Review Letters, 2005. 95(22).

28. Moore, J.E. and L. Balents, Topological invariants of time-reversal-

invariant band structures. Physical Review B, 2007. 75(12).

29. Bostwick, A. and e. al., Science, 2010. 328: p. 999-1002.

30. Tohyama, T., K. Tsutsui, and S. Maekawa, Theory of RIXS in strongly cor-

related electron systems: Mott gap excitations in cuprates. Journal of

Physics and Chemistry of Solids, 2005. 66(12): p. 2139-2144.

31. Li, Y.W., et al., X-ray imaging of dispersive charge modes in a doped Mott

insulator near the antiferromagnet/superconductor transition. Physical

Review B, 2008. 78(7): p. 073104.

32. Trebino, R., Frequency-Resolved Optical Gating: The Measurement of

Ultrashort Laser Pulses. 2002: Springer.

33. Trebino, R. and e. al., Rev. Sci. Instrum., 1997. 68: p. 3277.

34. Thomann, I., et al., Characterizing isolated attosecond pulses from hol-

low-core waveguides using multi-cycle driving pulses. Opt. Express,

2009. 17: p. 4611-4633.

35. Schweigert, I.V. and S. Mukamel, Coherent ultrafast core-hole correla-

tion spectroscopy: X-Ray analogues of multidimensional NMR. Phys Rev.

Lett., 2007.99(16): p. 163001.


linear local probe for electronic excitations. Phys Rev. Lett., 2002. 89(4):

p. 043001.

16. McElroy, K., Nature Physics, 2006. 2: p. 441-442.

17. Damascelli, A., Z. Hussain, and Z.X. Shen, Reviews of Modern Physics,

2003. 75: p. 473-541.

18. Fert, A. and P. Grünberg, Nobel Prize in Physics, 2007.

19. Lövenich, R., et al., Evidence of phonon-mediated coupling between

charge transfer and ligand field excitons in Sr2CuO2Cl2. Physical Review

B, 2001. 63(23): p. 235104.

20. Fu, L., C. Kane, and E. Mele, Topological insulators in three dimensions.

Physical Review Letters, 2007. 98: p. 106803.

21. Hsieh, D., et al., Observation of Time-Reversal-Protected Single-Dirac-

Cone Topological-Insulator States in Bi2Te3 and Sb2Te3. Physical Review

Letters, 2009. 103(14): p. 146401.

22. Hsieh, D., et al., A topological Dirac insulator in a quantum spin Hall

phase. Nature, 2008. 452(7190): p. 970-U5.

23. Hsieh, D., et al., A tunable topological insulator in the spin helical Dirac

transport regime. Nature, 2009. 460(7259): p. 1101-U59.

24. Hsieh, D., et al., Observation of Unconventional Quantum Spin Textures

in Topological Insulators. Science, 2009. 323(5916): p. 919-922.

25. Chen, Y.L., et al., Experimental Realization of a Three-Dimensional

Topological Insulator, Bi2Te3. Science, 2009. 325(5937): p. 178-181.

26. Chen, Y.L., et al., Massive Dirac Fermion on the Surface of a Magnetically

Doped Topological Insulator. Science, 2010. 329(5992): p. 659-662.

71

3 . SCIENCE DRIVERSSPIN AND MAGNETIZATION AT THE NANOSCALE

digmsthatwilldefineinformationandenergytechnolo-

giesofthe21stcentury.

Toaccomplishthis,criticalgapsinourunderstanding

ofspinandmagnetizationdynamicsmustbebridged,

andforthispurposethecapabilitiesofNGLSsoftX-ray

laserwillbeindispensible.Inparticular,NGLSwillenable

incisiveprobingofspindynamicsonthefundamental

timescalesofexchangeinteractions(1-100fs)andspin-

orbitcoupling(~1ps).Mostimportantly,thecapabilityfor

probingthesefundamentaltimescaleswillbecombined

with:

• nanoscalespatialresolution—imaginglocalspin

structures

• momentumandenergyresolution—understanding

k-dependentspinscattering

• elementspecificity—distinguishingtherolesofspe-

cificinnercoreelectrons,e.g.intransition-metalions

• magneticorderingsensitivity—distinguishingferro-

andanti-ferromagneticorderviaX-raymagneticlin-

eardichroism(XMLD)

Magnetisminnovelmagneticmaterialsarekeycom-

ponentsofmoderntechnologiesrangingfromadvanced

permanentmagnetsinelectricitygenerationanduse,to

computerharddrives,toscientificandmedicalimaging.

Themanipulationofspinandchargeonananometer

lengthscaleformsthefoundationformoderninforma-

tionprocessingandstoragetechnology.Todate,thevora-

cious demand for higher-speed and higher-density

informationprocessing,storage,andretrievalhasbeen

metbyexponentialimprovementsintechnologyoverthe

pastseveraldecades.Thispaceofadvancementisnow

approachingsignificantlimitationsthattestourfunda-

mentalunderstandingofspinandmagnetizationdynam-

icsatthenanoscale.Atthesametime,therolesofspin

andmagnetization,andthecouplingofspinandcharge

incomplexmaterialsgivesrisetonewphenomenaand

newmaterialpropertiessuchasunconventionalsuper-

conductorsandtopologicalinsulators.Akeychallengeis

tounderstand,manipulate,andexploittheseproperties

toprovideafoundationfornewdevicesandnewpara-

Achieving a fundamental understanding of spin and magnetization dynamics at the nanoscale is essential to meet the future demand for higher-speed and higher-density information technologies. New concepts to manip-ulate the spin of the electrons on nanometer length scales will provide a foundation for new devices and new paradigms that will define information and energy technologies of the 21st century.

NGLS will be indispensible to bridge critical gaps in our understanding of spin and magnetization dynamics. In particular, NGLS will enable incisive probing of spin dynamics on the fundamental time scales of exchange inter-actions (1-100 fs) and spin-orbit coupling (~1 ps) in combination with element-specific nanoscale spatial resolu-tion, and momentum and energy sensitivity. The high repetition rate, high energy resolution, and ultrafast pulses at NGLS will bring unprecedented experimental capabilities to perform time resolved spin-polarized ARPES in spin flip processes. Femtosecond two-color experiments with polarized soft X-rays will reveal the distribution of spin and angular momenta and experiments are envisioned, where one of the two pulses can be utilized as a source for generating spin accumulations with a subsequent probing of the spin dynamics in non-magnetic metals.

NGLS will provide unique insight into the fastest manipulation of spins by photons and will open a path to utilize pure spin currents in loss-free future electronic devices.

3.8 SpinandMagnetizationattheNanoscale

72


alsomissing.Directmeasurementofthetransientnon-

equilibriumelectronicstructure,withfullenergy,momen-

tum,spin,andtimeresolutioniscriticalforelucidating

theseissues,andtime-andspin-resolvedARPESoffers

exactlythesecapabilities.Time-resolveARPESmeasure-

mentswillprovidethefirstreal-timeglimpseofspin-flip

scatteringprocesseswhicharebelievedtoproceedvia

“hotspots,“i.e.regionsintheBrillouinzoneofhighspin-

orbitcoupling(seeFigure48).

• spin/orbitsensitivity—quantifyingandseparating

magneticgroundstatepropertiesviaX-raymagnetic

circulardichroism(XMCD).

3.8.1 Ultrafast Manipulation of Magnetism and Spin

Prospectsforcontrollingmagnetismontheultrafast

time scale were highlighted by seminal experiments

morethanadecadeagothatdemonstratedlight-driven

sub-picosecond demagnetization of a ferromagnetic

metal.1Thesesurprisingresultschallengedconventional

paradigms,andevidencedourlackofunderstandingof

spinandmagnetizationdynamicsonfundamentaltime

scales. Moreover, they sparked new interest into

approachesforultrafastopticalcontrolofmagnetism.2,3

Theconceptofall-opticalswitchingofmagnetization(see

Figure47)illustratestheprospectsforpushingmagnetic

data storagespeeds into the fs region.However, the

microscopicmechanismbehindthisswitchingprocess

remainselusive.Otherintriguingneweffectsappearon

thehorizonsuchastherecentlyreportedchangeofthe

magnetic anisotropy energy barrier during ~200 fs

intenseelectricalfieldpulses.4However,itsmicroscopic

originisevenlessexploredandunderstood.

Theessenceofmagnetismisangularmomentumas

demonstratedclearlybytheEinstein-deHaaseffect.5We

needtounderstand:

• Whatarethechannelsforultrafastangularmomen-

tumtransferto,fromandwithinthespinsystem?

• Doestheangularmomentumcomefromlightorisit

providedbyotherreservoirssuchasthelattice?

3 .8 .1 .1 Ultrafast Manipulation of Magnetism and Spin:

Time- and Spin-Resolved ARPES

Whileall-opticalpump-probetechniquesareableto

takestroboscopicimagesoftheactualmagnetization,

keyquestionsremainaboutthemicroscopicspinand

electrondynamicsontime-scalesrangingfromtheultra-

fastcontrolpulse to thecompletionofmagnetization

reversal.Forinstance,theexactmechanismforthebal-

anceofangularmomentumwhichmustoccurbetween

the light, the electron spins, and the lattice remains

unknown.Afullunderstandingofhowthesystemisable

tocompletethemagnetizationreversalthroughademag-

netizedstate,longafterthecontrolpulseisremoved,is

Figure47Demonstration of all-optical magnetization reversal on GdFeCo alloy where the magnetization of adjacent regions can switch only by changing the helicity of the incident fs laser pulse.6

K

+q

K

X-L

spsp

d

Kx (Å-1)

K y (Å-1

)

Figure48Region of the Ni bulk Brillouin zone as measured by synchrotron based ARPES (H. Durr, unpublished) showing the regions of spin-orbit “hot spots”, i.e. those points where spin up and down bands cross, leading to reduced magnetic moments and increased spin-orbit coupling. It is believed that spin-flip scattering into such “hot spots” provides an efficient avenue to spin angular momentum transfer to lattice excitations,2 q. Spin and time-resolved ARPES at NGLS will provide the first time access to observing such processes in real time.

73


showsthefirstreportedbranchingofspinandorbital

momentevolutionsfollowingfemtosecondlaserheating

ofCoPdalloys,usingultrafastX-raydichroismtofollow

thespinandorbitevolution.8Thistypeofexperimental

probewilldirectlyviewthemechanismswithintheelec-

tronic structure that provide the required angular

momentumbalanceandallowultrafastmagnetization

control.Thisinturncanprovideinsightforoptimizing

materialdesignstoimproveperformance,andincrease

spatialdensityforeventualdeviceapplications.

In complex materials, a powerful experimental

approachistransienttwo-colorscatteringorholographyas

illustratedinFigure50.ThisexploitssoftX-raydichroism

andsourcecoherencetofollowspinandorbitalordering,

andtheirevolutioninresponsetotailoredmaterialexcita-

tion.Inmultiferroicsforexample,thisapproachwillpro-

videinsighttothecoupledorderparameters.Thistypeof

experimentsaimsatspatiallycorrelatingcoupledorder

parametersandobtainingsnapshotsoftheirtemporal

evolution.Becauseofdynamicfluctuationsoftheorder

parameters, thecorrelationhastobeperformedona

pulsebypulsebasisusingtwoX-rayprobecolors(see

Figure50)ordifferentX-raypolarizations.

3.8.2 Spin Accumulations and Currents

Presentsemiconductorelectronicdevicesanddata

storagetechnologiesrelyprimarilyonchargecurrentsto

transmitandstoreinformation.Powerconsumptionand

A time-delayed ultrafast X-ray pulse, following an

ultrafastopticalorTHzexcitationpulse,withfullanalysis

oftheresultingphotoelectronenergy,angle,andspin,

will capturenot just the time-resolvedmagnetization

state,but reveal theentirespin-dependentelectronic

structure.Suchmeasurementswillenableadirectcor-

roborationofcontroversialinitialreportsonlaser-induced

localizationofthevalenceelectronicstructureinmaterials

likemetallicNi.7

3 .8 .1 .2 Ultrafast Manipulation of Magnetism and Spin:

Dynamic Soft X-Ray Scattering

X-raysoffertheuniqueadvantageofallowingusto

probespinandorbitalmomentumseparately.Figure49

Sz

Lz

T=280 ± 20 fs

–55

–67

0.2

0 1Delay (ps)

2

0.4

0.6

0.8

T=280 ± 20 fs

Orbits Spins

Time

Spin

and

orb

ital m

omen

ts (ħ

per

ato

m)

Figure49 Materials composed of alternate layers of Co and Pd introduce a spin orientation perpendicular to the layers which is preferred for magnetic data recording. Femtosecond optical puls-es were used to alter the electronic level population leading to a change in electron orbits (blue arrows and ellipses) and essential-ly quenching the magnetic anisotropy responsible for the perpen-dicular spin orientation. Femtosecond X-ray pulses were used to probe the change in orbital motion(blue symbols) and follow the spin rotation into the layer plane (red symbols).8

Probe spin & orbital distributions

Holography

Electrons

Lattice

Spins

THzpump

X-rayL3probe

X-rayL2probe

Figure50Two-color coherent scattering experiments to probe spin and orbital contributions to magnetic order, and their evolu-tion in response to tailored excitation, e.g. THz excitation of the electronic system (figure courtesy H. Durr).

74


tingisrathersmall.10NGLSX-raylaserswillachievethe

goalofdirectimagingofspinaccumulationduethecom-

binedcapabilitiesofhighrepetitionrate(forhigh-sensi-

tivitymeasurementsandhighaverageflux)withtime

structurefortransientspectroscopymeasurements.

Furthermore,theabilitytoperformtwo-colorX-raypulse

experimentswillenablemeasurementsanalogoustoopti-

calpump-probeexperimentsinsemiconductors,sincea

circularpolarizedX-raybeamcanalsobeutilizedasasource

forgeneratingspinaccumulations.Thelatterisparticularly

interesting,sincethesub-pspulselengthswillenable

directinvestigationofspindynamicsinnon-magneticmet-

als,wherethespinrelaxationtimeistypicallyafewps.14

ThusfutureX-rayinvestigationsofspinaccumulations

inmetalswillresultinasignificantlymoredetailedunder-

standing of spin-dependent effects.This is important

sincetheimpactofspincurrentsgoesfarbeyondthe

spintronics paradigm. Namely, spin currents can be

observed in insulators,19 theycanbecoupledtoheat

transport,20andtheycanresultinangularmomentum

transport,whichimpactsmechanicalmotion.21Thusthey

offerawidevarietyofnovelopportunitiesforenergycon-

versionatthenanoscale.

3 .8 .2 .1 Spin Accumulations and Currents:

Magnetic Nano-spectroscopy

Currentapproachestostudyspinandmagnetization

dynamicson fundamental lengthand timescalesare

inherentlyandseverelylimited.Thefastestfemtosecond

heatdissipationarenowmajorobstaclestofurthermin-

iaturization,withpowerdensitiesofmodernprocessors

exceeding100W/cm2.Moreover,theprojectedenergy

demandfromconsumerelectronicsin2030,basedon

today’stechnologies,willrequiretheequivalentof230

additionalnuclearpowerplants.Onepromisingcandi-

date to replace existing charge-based technologies

(CMOS)exploitsspincurrentsandaccumulations,which

isthefoundationforspintronicsdevelopment.9

Mostpracticalimplementationsofactualspintronic

devicesaremetal-based,butatthesametime,adirect

imagingofeitherspincurrentsoraccumulationsinthese

structuresremainselusive.10Additionally,theconceptof

purespincurrents,whicharenotaccompaniedbyanet

chargecurrent,hasrecentlyreceivedincreasedatten-

tion.11Fromafundamentalpointofviewthesepurespin

currentsprovidedirectinsightintospin-dependentphys-

ics and are completely undisturbed by charge trans-

port.12-14 For technological applications, they offer

significantpotentialadvantages,suchasreducedpower

dissipation,absenceofOerstedstrayfields,anddecou-

plingofspinandchargenoise.

Purespincurrentscanbegeneratedthroughnon-local

electricalinjection,opticalinjection,spinpumpingfroma

precessingferromagnet,andviatheandspinHalleffect.15

Tounderstandthespindynamicsofsuchnon-equilibrium

spinaccumulations,adirectimagingcapabilityenabled

byNGLSX-raylaserswillbeamajoradvance.Thisisevi-

dent frompast researchonsemiconductingsystems,

wheresuchimagingispossibleintheopticalregimedue

tostrongmagneto-opticeffectsandlongspindiffusion

lengths.16-18Questionsofparamountsignificancetoboth

thebasicunderstandingandeventualtechnicalutilization

ofspincurrentscouldbeimmediatelyanswered:

•Howdospincurrentscoupletochargecurrents?

•Dospincurrentsinteractwithheattransport?

•Canweexploitangularmomentumtransfer?

•Whatisthespatialandtemporalspinflowcreatedby

spinpumping?

X-rayimagingwithelementspecificmagneticcontrast

through X-ray circular dichroism is a very promising

approach.Theachievablespatialresolutioniscomparable

orbetterthaneventheshortestspindiffusionlengths

encountered inmostmetals.However,so far there is

insufficientX-rayspectralsensitivitytospinaccumula-

tions,sincethespin-dependentchemicalpotentialsplit-

Current CMOS technology– inherent energy losses

Log

(l S-D

)

VT VG-S

CMOS

Mott-FET

Figure51 I-V characteristic of current CMOS technology illustrating the relatively large required switching voltages (and inherent energy losses) in comparison with Mott-FET transitions.

75


spinaccumulationswithasubsequentprobingof the

spindynamicsinnon-magneticmetals,wherethespin

relaxationtimeistypicallyafewps.

Avarietyofimagingtechniques,whichhavealready

beendevelopedatcurrentsources(X-rayholography23

orzoneplatebasedfullfieldX-raymicroscopy24),canina

similarwaybeimplementedatNGLStotakesnapshot

imagesofnanoscaleultrafastspindynamics.

EstimatesfortheheatloadontoaFresnelzoneplate

objectivelenstoachievehighspatialresolutioninasin-

glepulseexperimentindicatethatthetemperaturerisein

theopticalelementwillbelessthan100degrees,which,

basedontheU41microscopeatBESSY,canbeeasilytol-

eratedbyexistingzoneplatetechnologies.24

3 .8 .2 .2 Spin Accumulations and Currents:

Time- and Spin-resolved ARPES

Inthepastdecades,angle-resolvedphotoemissionspec-

troscopy(ARPES)hasprovedtobeanextremelysuc-

cessfulandpowerfultechniqueforadvancingthe

understandingofcomplexcorrelatedelectronsystems,

includinghigh-temperaturesuperconductors(HTSC),25-30

colossalmagnetoresistive(CMR)manganites,31-33gra-

phene,34-37andtopologicalinsulators.38-42Thedevelop-

mentofspin-resolvedARPEStechniqueshassparked

rapidlygrowinginterestinthelastfewyearsduetoits

relevanceforprobingspinphysicsforpotentialdevice

applications,andagrowingpoolofhigh-impactresults

laserexperiments,limitedbythewavelengthofoptical

light,lacksufficientspatialresolution.Thehighestresolu-

tionmicroscopiesaremanyordersofmagnitudeaway

fromaccessing fundamental timescales. Inaddition,

opticaltechniqueslacksensitivitytoseparatespinand

orbitalmoments,andareunabletodistinguishbetween

different typesofmagneticordering.SoftX-ray tech-

niquesareaperfectmatchintermsofelementspecificity,

sensitivitytospinandorbitalmoments,andspatialreso-

lution.Forexample, the latestdevelopmentsofX-ray

opticshasdemonstrateda<10nmspatialresolution.22

Slicingexperimentsatcurrentsynchrotronscanaccess

thefew100soffemtosecondregime,however,thelossin

intensityistremendousandinsufficient(bymanyorders

ofmagnitude)toenablesnap-shotcapabilitiesforimag-

ingorspectroscopicanalyticaltoolsatthenanoscale.This

gapcanonlybefilledwithanextgenerationsoftX-ray

source.

Thehighrepetitionrate,thehighenergyresolution

andtheultrafastpulsesatNGLSwillbringunprecedent-

edexperimentalcapabilities.Timeresolvedspin-polar-

izedARPESwillallowthestudyofspinflipprocesseswith

energyandmomentumresolution,whichareessentialto

identifyspin-flipprocessesresponsibleforthemagneti-

zationreversal inanall-opticalprocess.Femtosecond

two-colorexperimentswithpolarizedsoftX-raysincom-

binationwithimagingandmicro-spectroscopywillreveal

thedistributionofspinandangularmomenta.Oneofthe

twopulsescanalsobeutilizedasasourceforgenerating

FM

Spin

wav

e

NMFigure52 Spin Hall effects generate transverse spin currents from charge currents even in non-magnetic materials.

Figure53 Pure spin currents generated in nonmagnetic metals (NM) via spin pumping from spin waves in a ferromagnet (FM).

76


exampleseeReferences59,60),aswellasthefunda-

mentallengthscalesofthemanynanoscaleandnano-

structured systems of current interest. Physical

restrictionsonthelocationsofthezoneplatesorother

opticsrequiredforthenanoscalebeamsrequirephoton

energies≥100eV,wherehigh-harmonicgenerationlaser

basedsystemsaresimplyunabletoprovidethehigh

averagefluxrequiredforARPESexperimentswithtime,

spin,andspatialresolution.

Thefullpotentialoftime-,space-,andspin-resolved

ARPEStoattackthekeyquestionsintheelectrondynam-

icsofnextgenerationmaterialsrequiresasourcewith

shortpulsesatthetimescalesinquestion(10-500fs),

tunabilityintherangeof280–1000eV,highaverageflux,

andhighrepetitionrates(~1MHz).Thesestrictrequire-

mentsarefulfilledonlybyNGLSspecifications.Thegain

inourabilitytodirectlyobservechargeandspindynam-

icsandtheirresponsestoexternalstimuliwillbepara-

mountforourgoalsofdevelopingmoreprecise,faster,

moreefficient,andmorereliablecontrolovermaterials

andtheircapabilities.

3.8.3 Instrumental and Technical Considerations for Time- and Spin-Resolved ARPES

Spin-resolvedARPESisinherentlyalowcount-rate

experimentaselectronspin-polarimetersarequiteineffi-

includingthediscoverybyspin-ARPESoftopological

insulators.39,40,43-48

AswithX-rayscattering,highphotonfluxalongwitha

highrepetitionratefromNGLSX-raylaserswillallow

spin-resolvedARPEStobeperformedwithtime-resolu-

tion through pump-probe techniques. Initial research

developing (non-spin resolved) pump-probe time-

resolvedARPESstronglysuggestthatitcanbeasuperb

probeofelectrondynamics,49-51althoughthelimitations

ofthecurrentprobesourcesarerestrictive.

Initialtime-andspin-resolvedexperimentshavebeen

performedwithtable-toplasersystemsprovidingboththe

pumpandprobepulses(forexample,seeReferences52

and53),howevertheseexperimentsareseverelylimited

bytheprobesourceintermsoflowphotonenergyand

lackof tunability.HighresolutionARPESexperiments

alsorestrictthenumberofusablephotonsperpulsedue

tosignificantspace-chargebroadeningoftheoutgoing

photoelectrons.54-58Combinedwiththerequirementof

highaveragefluxforsuchcount-starvedexperiments,

thisresultsintheabsoluteneedforahighrepetitionrate

source,farbeyondwhatlaser-basedsystemsandother

FELsourceswillbeabletoprovide.Theseissuesarenot

minor, but become full“show-stoppers” for the vast

majorityofexperimentsthatcanbeenvisioned.

TheconceptofspatiallyresolvedARPESexperiments

performedwithnanoscalefocalspotshasalsobecome

importantduetothelargespatialinhomogeneityofmany

“correlatedelectron” systemsof current interest (for

FEL

Plane mirror

Sample stage

CCDdetector

Condenser

Zone plateobjective

Figure54 X-ray optical setup of a full-field X-ray microscope for single pulse imaging. The FEL radiation is collected by a special condenser — a beam shaping diffractive optical device about 1 mm in diameter — which forms a homogeneous illumination of the object field. (Figure courtesy G. Schneider, BESSY)

77


anarrayofmultiple(possiblyupto10)analyzersmount-

edatdifferentemissiondirectionsforsimultaneoususe.

Theresultingmassivelyparalleldataacquisitionwould

greatly improve the speed and scope of successful

experiments.TheultrafastpulsewidthofNGLS,com-

pared with current synchrotron sources, could even

allowforimproveduseoftheTOFtechniqueovercurrent

setupsthatarelimitedbythepulsewidthsofthelight

source.Withmuchshorterpulsewidthsprovidedby

NGLS,andimprovedelectrondetectors(e.g.Reference

64),significantlysmallerTOF-basedanalyzerscouldbe

utilizedwhilestillachievingthesameenergyresolution.

Thispossiblesizereductioncouldbequiteimportant,

especiallywhenconsideringtheconceptofanentire

arrayofmultiplexedanalyzers.

Beamlines for Studies of Spin and Magnetization Dynamics

Visible/THz-pump,X-ray-probestudiesofmagnetiza-

tionandspindynamicswillrelyprimarilyontheseeded

NGLSbeamlines1and2asdescribedinSection5(Table2).

Theseexperimentswilluseone-color(andinsomecases

two-color)X-rayprobestofollowmagnetizationandspin

dynamicsviaX-raydichroismanddichroicscattering

effectseffectsattransition-metalL-edgesinthesoftX-ray

cient.Inadditiontohighrepetitionrateandothercapa-

bilitiesprovidedbyNGLSX-raylasers,theseexperiments

willalsorequireinstrumentationthatmaximizesdetec-

tionefficiency.ThetimingstructureofNGLSpulses,in

additiontoprovidingtimeresolutionthroughpump-and-

probetechniques,naturallyallowstheuseoftime-of-

flight(TOF)basedphotoelectronanalyzersthatcanbe

moreefficientthanthemorefrequentlyusedhemispheri-

calanalyzers,duetomultiplexingdetectionintheenergy

dimension.61-63

AnewlydevelopedTOF-basedspin-ARPESanalyzer61

(Figure55)thatincludesaphotoelectronbandpassfilter

allowsforhigherrepetitionratesources(>10MHz)and

betterenergyresolutions(<10meV)thanaretraditionally

possible,andillustratesanidealinstrumentalapproach

forthistypeofexperiment.Theenhancedefficiencyofthe

uniquespinpolarimeterincludedinthisdesign61furthers

improvestheseexperiments.Indeed,thisinstrumenthas

alreadysuccessfullytakenspin-resolved(nottime-resolved)

ARPESdataatbeyondstate-of-the-artresolutionswhile

makinguseoftheALStwo-bunchmodeat~3MHzrepeti-

tionrate,61thusdemonstratingthefeasibilityofthistype

ofexperimentwithinNGLSrepetitionratecapabilities.

Akeyconceptinmaximizingthepossiblethroughput

ofsuchexperimentswouldbetoadditionallymultiplex

detectionintheangular(momentum)dimensionthrough

e–

Figure55 Left: Schematic of a time-of-flight (TOF) based, high efficiency spin-resolved photoemission spectrometer, recently developed at the ALS. The spin resolution is obtained through low energy exchange scattering, depicted in the inset. From Reference 61. Right: Depiction of a high efficiency ARPES endstation with multiplexed TOF analyzers for massively parallel data acquisition over an extremely wide angu-lar range. Such a setup could provide the efficiency required in such count-starved experiments, where the allowed photon flux per pulse is also restricted by space charge considerations.

78


withoutamonochromator,and<10meVresolutionwith

amonochromator,asdescribedinSection5(Table2).The

highrepetitionrateprovidedbybeamline3willbeimpor-

tantforhigh-sensitivitymeasurements(whileavoiding

space-chargeeffects).Inthiscase,highenergyresolution

willbeprovidedbyamonochromator.

range.Tunabilityandvariablepolarizationwillbeofpara-

mountimportancefornearlyallstudiesofmagnetization

andspindynamics.

Time-andspin-resolvedARPESexperimentswillrely

primarilyonNGLSbeamline1,providing280eVphoton

energies—<50meVresolutioninlong-pulseoperation

Time- and Spin-Resolved ARPES Experiments at 10 meV

Thefollowingestimatesillustratethescientificneedforahigh-repetition-rateultrafastX-raylasertoinvestigate

spinandmagnetizationdynamicsviatime-andspin-resolvedARPES:

Required integrated flux on the sample: ~1017 photons


Time res .solution


PulsedFEL 106[1] 102 10,000 days ~fs

NGLS 106[1] 106 1 day ~fs

[1] Fluence limit:space-chargelimitations(distortionofphotoemissionspectra)

includes10xmonochromatorlosseswithSASEFELoperation

Fouriertransformlimit:10meV⇔200fs


[3] Rate limit:<107Hz,determinedbytime-of-flightenergyanalyzer



Rep.rate 5x108Hz

Pulseduration 100ps



Rep.rate 6x106Hz

Pulseduration ~1ps

79


24. Schneider, G., et al., X-Ray Microscopy at BESSY : From Nano-

Tomography to Fs-Imaging, in 9th International Conference On

Synchrotron Radiation Instrumentation, J.-Y. Choi and S. Rah, Editors.

2007: Daegu, Korea. p. 1291-1294.

25. A. Damascelli, Z. Hussain, and Z.X. Shen, Angle-resolved photoemission

studies of the cuprate superconductors. Rev. Mod. Phys, 2003. 75: p. 473.

26. A. Lanzara, et al., Evidence for ubiquitous strong electron–phonon cou-

pling in high-temperature superconductors. Nature, 2001. 412: p. 510.

27. G.H. Gweon, et al., An unusual isotope effect in a high-transition-temper-

ature superconductor. Nature, 2004. 430: p. 187

28. H. Ding, et al., Spectroscopic evidence for a pseudogap in the normal

state of underdoped high-Tc superconductors. Nature, 1996.382: p. 51.

29. T. Kondo, et al., Competition between the pseudogap and superconduc-

tivity in the high-Tc copper oxides. Nature, 2009. 457: p. 296.

30. Z.X. Shen, et al., Anomalously large gap anisotropy in the a-b plane of

Bi2Sr2CaCu2O8+δ. Phys. Rev. Lett., 1993. 70: p. 1553.

31. D. S. Dessau, et al., k-Dependent Electronic Structure, a Large “Ghost”

Fermi Surface, and a Pseudogap in a Layered Magnetoresistive Oxide.

Phys. Rev. Lett., 1998. 81: p. 192.

32. N. Mannella, et al., Nodal quasiparticle in pseudogapped colossal mag-

netoresistive manganites. Nature, 2005. 438: p. 474.

33. Y. D. Chuang, et al., Fermi Surface Nesting and Nanoscale Fluctuating

Charge/Orbital Ordering in Colossal Magnetoresistive Oxides. Science,

2001. 292: p. 1509.

34. A. Bostwick, et al., Observation of Plasmarons in Quasi-Freestanding

Doped Graphene. Science, 2010. 328: p. 999.

35. S. Y. Zhou, et al., Metal to Insulator Transition in Epitaxial Graphene

Induced by Molecular Doping. Phys. Rev. Lett., 2008. 101: p. 086402.

36. S. Y. Zhou, et al., Substrate-induced bandgap opening in epitaxial gra-

phene. Nature, 2007. 6: p. 770.

37. T. Ohta, et al., Controlling the Electronic Structure of Bilayer Graphene.

Science, 2006. 313: p. 951.

38. D. Hsieh, et al., Nature Materials, 2008. 452: p. 970.

39. D. Hsieh, et al., A tunable topological insulator in the spin helical Dirac

transport regime. Nature, 2009.460: p. 1101.

40. D. Hsieh, et al., Observation of Unconventional Quantum Spin Textures in

Topological Insulators. Science, 2009. 323: p. 919.

41. Y. L. Chen, et al., Experimental Realization of a Three-Dimensional

Topological Insulator, Bi2Te3. Science, 2009. 325: p. 178.

42. Y. L. Chen, et al., Massive Dirac Fermion on the Surface of a Magnetically

Doped Topological Insulator. Science, 2010. 329: p. 659.

43. A. Varykhalov, et al., Electronic and Magnetic Properties of

Quasifreestanding Graphene on Ni. Phys. Rev. Lett., 2008. 101: p. 157601.

44. A. Varykhalov, et al., Quantum Cavity for Spin due to Spin-Orbit

Interaction at a Metal Boundary. Phys. Rev. Lett., 2008. 101: p. 256601.

References:

1. E. Beaurepaire, et al., Ultrafast Spin Dynamics in Ferromagnetic Nickel.

Phys. Rev. Lett., 1996. 76: p. 4250.

2. B. Koopmans, et al., Explaining the paradoxical diversity of ultrafast

laser-induced demagnetization. Nature Materials, 2010. 9: p. 259.

3. A. V. Kimel, et al., Ultrafast non-thermal control of magnetization by

instantaneous photomagnetic pulses. Nature, 2005. 435: p. 655.

4. S. J. Gamble, et al., Electric Field Induced Magnetic Anisotropy in a

Ferromagnet. Phys. Rev. Lett., 2009. 102: p. 217201.

5. A. Einstein and W.J.d. Haas, Experimental Proof of Ampère’s Molecular

Currents. Deutsche Physikalische Gesellschaft, Verhandlungen, 1915. 17:

p. 152-170.

6. C. D. Stanciu, et al., All-Optical Magnetic Recording with Circularly

Polarized Light. Physical Review Letters, 2007. 99: p. 047601.

7. C. Stamm, et al., Femtosecond modification of electron localization and

transfer of angular momentum in nickel. Nature Materials, 2007.6: p. 740.

8. C. Boeglin, et al., Distinguishing the ultrafast dynamics of spin and orbital

moments in solids. Nature, 2010. 465: p. 458.

9. S. D. Bader and S.S.P. Parkin, Spintronics Annu. Rev. Cond. Matter Phys.,

2010. 1: p. 71.

10. O. Mosendz, et al., Imaging of lateral spin valves with soft x-ray micros-

copy. Phys. Rev. Lett. B, 2009. 80: p. 104439.

11. C. Chappert and J.V. Kim, Electronics free of charge. Nature Phys., 2008.

4: p. 837.

12. G. Mihajlovic, et al., Absence of the Giant Spin Hall Effect. Phys. Rev. Lett. ,

2009. 103: p. 166601.

13. O. Mosendz, et al., Quantifying Spin Hall Angles from Spin Pumping:

Experiments and Theory. Phys. Rev. Lett., 2010. 104: p. 046601.

14. G. Mihajlovic, et al., Negative Nonlocal Resistance in Mesoscopic Gold

Hall Bars. Phys. Rev. Lett., 2010. 104: p. 237202.

15. A. Hoffmann, Pure spin-currents. Phys. Status Solidi C, 2007. 4: p. 4236.

16. J. M. Kikkawa and D.D. Awschalom, Lateral drag of spin coherence in

gallium arsenide. Nature, 1999. 397: p. 139.

17. Y. K. Kato, et al., Observation of the Spin Hall Effect in Semiconductors.

Science, 2004. 306: p. 1910.

18. S. A. Crooker, et al., Observation of the Spin Hall Effect in

Semiconductors. Science, 2005. 309: p. 2191.

19. Y. Kahiwara, et al., Transmission of electrical signals by spin-wave inter-

conversion in a magnetic insulator. Nature, 2010. 464: p. 262.

20. K. Uchida, et al., Observation of the spin Seebeck effect. Nature, 2008.

455: p. 778.

21. G. Zolfagharkhani, et al., Nanomechanical detection of itinerant electron

spin flip. Nature Nanotechn., 2008. 3: p. 720.

22. Chao, W. and e. al., Sub 10 nm soft x-ray microscopy. in preparation, 2010.

23. S. Eisebitt, et al., Lensless imaging of magnetic nanostructures by X-ray

spectro-holography. Nature, 2004. 432: p. 885-888.

80


45. Hochstrasser, M., et al., Phys. Rev. Lett., 2002. 89: p. 216802.

46. J. W. Wells, et al., Nondegenerate Metallic States on Bi(114): A One-

Dimensional Topological Metal. Phys. Rev. Lett., 2009. 102: p. 096802.

47. J.H. Dil, et al., Rashba-Type Spin-Orbit Splitting of Quantum Well States in

Ultrathin Pb Films. Phys. Rev. Lett., 2008.101: p. 266802.

48. Ke He, et al., Direct Spectroscopic Evidence of Spin-Dependent

Hybridization between Rashba-Split Surface States and Quantum-Well

States. Phys. Rev. Lett., 2010. 104: p. 156805.

49. F. Schmitt, et al., Transient Electronic Structure and Melting of a Charge

Density Wave in TbTe3. Science, 2008. 321: p. 1649.

50. L. Perfetti, et al., Time Evolution of the Electronic Structure of 1T-TaS2

through the Insulator-Metal Transition. Phys. Rev. Lett., 2006. 97: p.

067402.

51. L. Perfetti, et al., Ultrafast Electron Relaxation in Superconducting

Bi2Sr2CaCu2O8+δ by Time-Resolved Photoelectron Spectroscopy. Phys.

Rev. Lett., 2007. 99: p. 197001.

52. A. B. Schmidt, et al., Spin-Dependent Electron Dynamics in Front of a

Ferromagnetic Surface. Phys. Rev. Lett., 2005. 95: p. 107402.

53. M. Cinchetti, et al., Spin-Flip Processes and Ultrafast Magnetization

Dynamics in Co: Unifying the Microscopic and Macroscopic View of

Femtosecond Magnetism. Phys. Rev. Lett., 2006. 97: p. 177201.

54. A. Pietzsch, et al., Towards time resolved core level photoelectron spec-

troscopy with femtosecond x-ray free-electron lasers. New Journal of

Physics, 2008. 10: p. 033004.

55. Graf, J., et al., Vacuum space charge effect in laser-based solid-state

photoemission spectroscopy. J. Appl. Phys., 2010. 107: p. 014912.

56. Passlack, S., et al., Space charge effects in photoemission with a low

repetition, high intensity femtosecond laser source. Journal Of Applied

Physics, 2006. 100: p. 024912.

57. S. Hellmann, et al., Vacuum space-charge effects in solid-state photo-

emission. Phys. Rev B., 2009. 79: p. 035402.

58. X.J. Zhou, et al., Space charge effect and mirror charge effect in photo-

emission spectroscopy. Journal Of Electron Spectroscopy And Related

Phenomena 2005. 142: p. 27.

59. M. Fäth, et al., Spatially Inhomogeneous Metal-Insulator Transition in

Doped Manganites. Science 1999. 285: p. 1540.

60. K. M. Lang, et al., Imaging the granular structure of high-Tc superconduc-

tivity in underdoped Bi2Sr2CaCu2O8+δ. Nature, 2002. 415: p. 412.

61. C. Jozwiak, et al., A high-efficiency spin-resolved photoemission spec-

trometer combining time-of-flight spectroscopy with exchange-scatter-

ing polarimetry. Review Of Scientific Instruments, 2010. 81: p. 053904.

62. L. Moreschini, et al., A time-of-flight–Mott apparatus for soft x-ray spin

resolved photoemission on solid samples. Rev. Sci. Instrum., 2008. 79: p.

033905.

63. O. Hemmers, et al., High-resolution electron time-of-flight apparatus for

the soft x-ray region. Rev. Sci. Instrum., 1998. 69: p. 3809.

64. A. Vredenborg, W. G. Roeterdink, and M.H.M. Janssen, A photoelectron-

photoion coincidence imaging apparatus for femtosecond time-resolved

molecular dynamics with electron time-of-flight resolution of σ = 18 ps

and energy resolution ΔE/E = 3.5%. Rev. Sci. Instrum., 2008. 79: p. 063108.

81

3 . SCIENCE DRIVERSBIOLOGICAL SYSTEMS: IMAGING DYNAMICS AND FUNCTION

decadesintime,frompicosecondstomilliseconds.Such

dynamicsarecentraltothefunctionofbiologicalsystems

andmacromolecularmachines,rangingfromtheenzymes

responsibleforDNArepairandreplication,toribosomes

responsibleforproteinsynthesis,todynamicmembranepro-

teinchannelsandsignalingcomplexes,toorganellsandthe

hierarchicalstructuresofthecell—thefundamentalunitoflife.

Inordertoadvancetheunderstandingofbiological

processesatacellularlevel,detailedknowledgeofthe

structureofverylargemacromolecularmachinesisan

absolutenecessity.Thehigh-repetition-rateX-raylasers

atNGLSwillenableentirelynewmethodsforimaging

biomolecules innativeenvironments (solution rather

thancrystal),capturingthedynamicsofmoleculesduring

theirfunctionalcycles,andvisualizingofcellularcompo-

nentsintheirnativecontext.Thedreamof“imagingbio-

logicalfunction”willberealizedforthefirsttime.

In the following, we present a few representative

examplesofkeyareasofbiologythatwillbetransformed

Modernsynchrotronsandmacromolecularcrystallog-

raphymethodshaverevolutionizedthefieldofstructural

biologybyenablingtheroutinestructuredeterminationof

isolatedorsimplecomplexesofmacromolecules.These

structuralmodels1arethefoundationforunderstanding

fundamentalprocessesinbiology,2,3andthedevelop-

mentofnewtherapeuticdrugs4,5andnovelclassesof

nano-materials.6Thesignificanceofthisisevidencedby

severalNobelPrizesinrecentyears.

However,conventionalstructuraldeterminationsof

biologicalmacromoleculesdisregardthepresenceofhetero-

geneousconformationsbyassumingidenticalobjects.

Moreover,currentX-rayandelectronmicroscopy(EM)7

structuraldeterminationmethodsdependongrowingordered

crystalsofproteins,whichnecessarilyinhibittheinvesti-

gationofdifferentconformations,andthedynamicmotions

thatconnectthem.Atthesametime,itiswellknownthat

biologicalfunctionisprofoundlyinfluencedbysubtle(and

notsosubtle)changesinconformationthatspanmany

Biological function is profoundly influenced by changes in molecular conformation that span many decades in time, from picoseconds to milliseconds. Such dynamics are central to the function of biological systems and macromolecular machines, ranging from the enzymes responsible for DNA repair and replication, to ribosomes responsible for protein synthesis, to dynamic membrane protein channels and signaling complexes, to organ-elles and the hierarchical structures of the cell — the fundamental unit of life. An enhanced understanding of the role of dynamics in biological function has both fundamental and practical importance, ranging from under-standing how cells work to the design of better therapeutics.

Modern synchrotrons and scattering / diffraction techniques have revolutionized the field of structural biology by enabling the routine structure determination of isolated or simple complexes of macromolecules. The high-rep-etition-rate of NGLS will enable entirely new methods for imaging biomolecules in native environments (solution rather than crystal), capturing the dynamics of molecules during their functional cycles, and visualization of cellular components in their native context. Of paramount importance will be the ability of NGLS to deliver ultrafast X-ray pulses at high repetition rate. This, in combination with newly developed sample delivery systems and X-ray detectors, will make it possible to collect billions of images of single molecules and whole cells. From these images it will be possible to probe thousands of conformational states accessible to biomolecules in solution.

NGLS will provide for a greater understanding of biomolecular dynamics and structure in native environments, and a wealth of information to guide the development of better therapeutics.

3.9 BiologicalSystems:ImagingDynamicsandFunction

82


However,thismethodprovidesonlystaticsnapshotsof

reactionstatesthathappentocrystallize,andhenceis

incapableoffullymappingoutthecompleteconforma-

tionalevolutionofintact,functionalholoenzymes.The

approachesofferedbyNGLSoffer anunprecedented

opportunitytorisetothischallenge.Furthermore,suc-

cessfulapplicationofthesemethodstotopoisomerases

willopenanewwindowin to thestudyofany large,

dynamicmacromolecularcomplex.Suchassemblieslie

attheheartofthemolecularoperationsofthecellandlife

asweknowit.

An example of another such system is the“initio-

some,”alargecomplexresponsibleforassemblingthe

machinery(the“replisome”)necessarytocopyDNAfor

cellproliferation.13Thesecomplexesassembleinastep-

wisemanner,againdependentonATP,butoveramuch

largerregionofDNA.Theyfurtherwrap,deform,andmelt

DNAasanintegralpartofthepathwaytowardreplisome

assembly.Thestructuresformedduringthisprocessare

large(borderingon1MDa),dynamic,14and—giventhe

flexibilityofDNA—impossibletocrystallize.Anabilityto

imagethetransient-assemblystatesonthewaytorepli-

someformationwillonlybepossiblebydynamicsingle

moleculeimagingmethodssuchasdescribedinSection

4.1basedontheuniquecapabilitiesofNGLSX-raylasers.

bythecapabilitiesofNGLS

forimagingheterogeneous

ensembles and conforma-

tionaldynamicsofbio-mole-

culesinnativeenvironments.

InSection4.1wedescribein

further detail the enabling

techniques of fluctuation

X-ray scattering (fXS), bil-

lion-snapshot coherentdif-

fractive imaging, and

associated advanced com-

putationalmethodsofmani-

fold mapping, that will be

realized for the first time

using the high-repetition-

rateX-raylasersatNGLS.

3.9.1 DNA Repair

TypeIItopoisomerasesareessentialenzymesrequired

forunknottinganddisentanglingDNAinthecell.8Both

reactionsarecarriedoutbythetopoisomerase-mediated

breakingofoneDNAsegment,andthepassageofasec-

ondDNAthroughthebreak.Followingpassage,thebro-

kenDNAisresealedtopreventdamagetothegenome.

Howtype II topoisomerasescarryoutstrandpassage

faithfullyandrapidlyisnotunderstood.Theenzymemust

coordinatelarge-scaleatomicmovements(ontheorder

of30-150Å)tosuccessfullynavigateoneDNAthrough

another.Achemicalco-factor,ATP,isrequiredforactivity.

HowchemicalenergyreleasedfromthehydrolysisofATP

ischanneledintomechanicalmotionisnotunderstood.

Sucheventsarecentralnotjusttotopoisomerases,but

alsototheoperationsofallmolecularmachines.

TypeIItopoisomerasesalsoarethetargetofnumer-

ousantibiotics9andanti-canceragents10usedintheclinic.

HowthesedrugsactattheactivesiteforDNAcleavage

andATPhydrolysishasbeenestablished.Whatisless

clearishowdrugsaffecttheconformationalcyclingof

theprotein.Thereisanintriguingpossibilitytodevelop

newclinically-valuableagentsthatinterferespecifically

withtopoisomeraseandDNAmovement,asopposedto

chemistry.

New imaging methods are needed to tackle these

problems.Crystallographyhasbeeninvaluableindeter-

miningatomic-resolutionmodelsoftopoisomerasepieces.11,12 Figure56 Hexameric helicases.

Coherent diffractive imaging



Heterogeneous ensembles

Spontaneous dynamics

Native environments


see section 4.1

83

3 . �SCIENCE�DRIVERSBIOLOGICAL�SYSTEMS: �IMAGING�DYNAMICS�AND�FUNCTION

The interface between the two ribosomal subunits, the

30S and 50S subunits in bacteria, consists of salt water to

a large extent.15 The number of direct interactions

between the two subunits, which are primarily RNA, was

much lower than expected. This may explain why ribo-

some function is incredibly sensitive to the salts in the

solution around it. The ability to image the ribosome and

its dynamics, under physiological conditions in solution,

will provide a crucial understanding of the effects of local

environment on structure and stability. The ability to

image thousands of conformational states will show the

full trajectory of ribosome movement during its cycle, and

potentially the production of the growing protein chain.

3.9.3� Membrane�Proteins

Cells are surrounded by membranes that separate the

cellular interior from the outside world. The lipids and

proteins that dominate the composition of membranes

exhibit a characteristic architecture in which the lipids

adopt a bilayer arrangement (~40 Å thick) penetrated by

integral membrane proteins. The flows of molecules,

energy and information across this barrier are mediated

by the integral membrane proteins embedded in the

bilayer. Membrane proteins represent a fertile area for

structural study; while an estimated 25% of the proteins

encoded in the genomes of organisms are membrane

proteins, they constitute less than 1% of the current

entries in the Protein Data Bank.16 The biological signifi-

cance of membrane proteins is mirrored in their pharma-

cological significance, since membrane proteins

represent the targets for a majority of the most popularly

prescribed drugs.17

3.9.2� Protein�Synthesis

Proteins are manufactured by ribosomes, molecular

machines that translate mRNA into a chain of amino acids

specified by the nucleotide sequence originating from the

DNA. The molecular machinery involved in this produc-

tion process, from transcription to translation, has been

extensively researched and has led to two Nobel prizes

over the last five years. However, a full understanding of

the entire protein production process, involving the inter-

actions between many molecular machines is lacking. In

order to understand the protein synthesis cycle, many

groups have been striving to obtain atomic-resolution

images of the intact ribosome synthesizing a protein.

Cate et al., have determined the first “frames” of the reac-

tion cycle by solving the crystallographic structures of

two intact ribosomes from the model organism

Escherichia coli.15 The ribosome crystals contained two

ribosomes per asymmetric unit, which provided two dif-

ferent snapshots of the molecule’s reaction cycle. This has

provided clues as to how the ribosome moves along mes-

senger RNA (mRNA), the genetic template for protein

synthesis. Comparison of the two ribosome structures

revealed movement of the head of the small ribosomal

subunit that appears to helps complete one translocation

step along the mRNA. Using models and lower-resolution

structures of the ribosome, it has been possible to pro-

pose a sequence of steps in translocation that finishes

with the swiveling of the small subunit head to allow the

mRNA and the transfer RNAs to move by one step.

Swiveling of the head may be driven by elongation-factor

G (EF-G), which uses one guanosine triphosphate (GTP)

molecule to catalyze the stepping along the mRNA.

CPS13

S19

L9

E

E

P AL11

5

L11L1

Head

Head

3

Figure57Ribosome�ratcheting.

84


between‘open’and‘closed’states.Thisconformational

switchingmaybegatedinresponsetochangesinmem-

brane potential,22 ligand binding,23 or application of

mechanicalforces.24Structuralstudieshavebeenableto

visualizeendpointsand,rarely,intermediatestatesin

conformationalswitching.(e.g.Figure58)

Voltage-gatedchannels20representoneofthebasic

circuitelementsofneurobiology,andtheirresponseto

changesinmembranepotentialservesasakeyeventin

thepropagationofelectricalsignalsthroughthenervous

system.Fortypicalpotentialsof~0.1V,significantelectric

fields,inexcessof107V/m,aremaintainedacrossthe

membraneduetothethinnessofthebilayer.Overthe

pastdecade,thestructuralframeworkforaddressingthe

openingandclosingofvoltage-gatedchannelshasbeen

developed,20butthedynamicsofthisprocess(thetem-

poralresponseofthestructuralchanges)hasbeenlack-

ing. It shouldalsobenoted that the influenceof the

membranepotentialonaproteinrequiresamembrane,

and these effects cannot be studied with detergent

extractedproteinsintheabsenceofamembrane.Time-

resolvedstructuralstudiescharacterizingtheresponseof

membraneembeddedvoltage-gatedchannels(andthe

membrane)tochangesinmembranepotentialwillbe

essentialinestablishingthemechanisticdetailsofthis

fundamentaleventincellsignaling—howdoesaprotein

domainmovethroughthemembranetocreateanall-or-

nonechangeinchannelconductancewithoutcompro-

misingtheintegrityofthemembrane?

3.9.4 Nanogeobiology

Recentadvancesinmicrobiologyhavedemonstrated

thatmicro-organismsareintimatelyinvolvedinthetrans-

formationofinorganicmineralsatornearthesurfaceof

theEarth.25Microbe-catalyzedelectrontransfer(ET)to

metalionsinthesemineralsliesattheheartofmanyof

thesetransformations,whichareperformedbyredox-

activeproteinsthatefficientlytransportelectronsover

longdistanceswithminimallossoffreeenergy.26These

redoxproteinsareexquisitelytunedforfacileETtodifferent

solid-phaseelectronacceptors.Understandingthenatural

diversityandimpactofmicrobialredoxproteins,how

microorganismshaveshapedtheEarthovergeologic

periodsandhowtheycontinuetodoso,isabasicresearch

goalinbiogeochemistry.Becausemicroorganism-mineral

ETprocessesinfluencethedistributionandmobilityof

Thesamepropertiesthatenablemembraneproteins

tofunctioninthisheterogeneousmilieualsohavepro-

found consequences for their structural analysis.

Reflectingthesmallmembranesurfaceareatocellular

volumeratio,membraneproteinsaregenerallypresentin

lowabundance,andextractionfromthemembranetypi-

callyinvolvestheuseofdetergentstosolvatethehydro-

phobicsurfaces.Asaconsequence,themostdetailed

structuralinformationonmembraneproteinsistypically

intheirdetergentextractedstate.18Whiledetergentshave

beenextremelyuseful,theyarenotalwaysfaithfulmim-

icsofthemembranebilayerandcanperturbthestructure

and dynamics of membrane proteins. New methods

enabledbyNGLSX-raylasers,thatprovideinformation

onthestructureanddynamicsofmembraneproteinina

native-likebilayerenvironment, includingproteolipo-

somesorsupportedbilayers,willbetransformative.

Thisinterplaybetweendynamicsandfunctionisbeau-

tifullyillustratedbyionchannels:acollectionofintegral

membraneproteins thatmediate the transmembrane

passageofionsintheirenergeticallyfavoreddirection.19

Ionchannelsarekeyelementsofsignalingandsensing

pathways, including nerve cell conduction, hormone

response, and mechanosensation.The characteristic

propertiesofionchannelsreflecttheirconductance,ion

selectivity,andgating.Ionchannelsareoftenspecificfor

aparticulartypeofion(suchaspotassium20orchloride21)

oraclassofions(suchasanions)andaretypicallyregu-

latedbyconformationalswitchingoftheproteinstructure

Step 1

Step 2

α = -63Expanded: η = 49 R = 3.0

α = -42Closed: η = 30 R = 1.2

α = -68Open: η = 52 R = 11.0

TM1-

TM1´

cro

ssin

g an

gle,

|α| (

º)

TM1 tilting angle, η (º)Pore

radiu

s, R (Å

)

75

70

65

60

55

50

45

3035

4040

5055 0

24

68

1012

14

SI–Sn

40

StepSteppS

p 1S

p

StSStep 2Step 2

α = -63Expanded: η = 49 R = 3.0R

α = -42Closed:o η = 3030 RR = 1.21RR

O

SI–SnS

Figure58 Different conformational states of a mechanosensitive ion channel moving from closed to open24.

85


betweenproteinsandsolidsistounderstandhowprotein

structureandconformationaldynamicsmediateintrapro-

teinandinterfacialET.Becauseofthedearthofcomputa-

tional and experimental tools, there is virtually no

mechanisticinformationoninterfacialET.

Multi-hemec-typecytochromesexpressedbyFe(III)-

reducingbacteria(Geobacterspp.,Shewanella oneiden-

sis) are among the best-characterized redox-active

microbialproteinsthatinteractwithmineralsandsoluble

metalions.29Physiologicalstudiesofthesebacteriahave

identified a number of outer membrane multi-heme

c-typecytochromesthatinteractwithmineralsurfaces.

OmcA and MtrC, decaheme c-type cytochromes

expressedbyS. onedensis,havebeenshowntobindtoa

hematitesurface,andratesofelectrontransfertothe

mineral surface have been measured.30 However no

high-resolution structure of MtrC or related proteins

exists to rationalize thepathwayofelectron transfer.

Consequently,thedetailedmechanismsofETareoften

derivedfrommodelstudiesonsimplerredox-activepro-

teinsthatareeasiertohandleandforwhichcrystallo-

graphicstructurescanbeobtained31(Figure59).Such

modelsystemscannotaddressstructure—activityrela-

tionshipsinrelevantsystems.Forexample,itisknown

thatsubtlechangesintheaxialcoordinationgeometryof

theFecenterinthehemegroup,causedbyaminoacids

substitutionsremote fromtheaxialhemeaminoacid

ligands,mayexertsignificantchangesintheredoxprop-

ertiesandratesofelectrontransfer.32

Amajorcontributiontoourunderstandingofthese

complexnanogeobiologicalsystemswillbenewmeth-

ods,enabledbyNGLSX-ray lasers, todetermine the

structureofenvironmentallyandtechnologicallyimpor-

tant electron transfer proteins, including multi-heme

contaminants,andthecarboncycle,understandingthem

iscrucialforsafeguardingournaturalenvironment.

Moreover,thereisanenormouspotentialforharness-

ingETproteinsformyriadbiotechnologies.Biological

engineersatLBNLhaverecentlyusedthesenaturally-

occurringproteinstorelayelectronicsignalsfromliving

systemstosyntheticinorganicmaterials.27Byelucidating

theprinciplesunderlyingprotein-inorganicET,thiseffort

will specifically contribute to one of the DOE Grand

Challenges:“totaptheexistingworldofbiologicalnano-

technologybyconstructingmolecularlevel,functional

interfacesbetweenlivingsystemsandsyntheticsoft-mat-

terandsolid-statetechnology.”28Themostsignificant

challengeforunderstandinganddirectingelectronflow

Figure59Soluble tetraheme c-type cytochrome used as model for electron transfer at a protein-mineral interface.31

Optical pump

Photoactivatedelectron donor

Single protein-clustercomplexes

θ

Electrontransfer protein

Polyoxometalateelectron acceptor

X-ray probe

Figure60 Schematic representation of optical pump-probe experiment with sensitizer-protein-acceptor bioconjugate.

86


low resolution electron microscopy images. The dynamic

imaging capabilities of NGLS X-ray lasers present

unprecedented opportunities to characterize whole func-

tional carboxysomes; observe dynamic structural changes

during catalysis; understand the cellular context in which

carboxysomes assemble; and track changes in carboxy-

some structure in response to environmental changes.

Carboxysomes may be viewed as complex CO2 fixa-

tion machines that self-assemble into an architecture that

is reminiscent of icosahedral viral capsids. The carboxy-

some shell is composed of thousands of copies of hexa-

meric shell proteins that tile to form the facets of the

shell.36,37 A second protein forms pentamers; 12 of these

provide the “defects” necessary to close a hexagonal

layer into an icosahedral shell38 (Figure 61c). Packaged

within the carboxysome shell are approximately 250 cop-

ies of L8S8 RubisCO,34 a highly abundant but inefficient

enzyme that utilizes Ribulose 1,5-bisphosphate (RuBP) as

a substrate to convert CO2 into 3-phosphoglycerate, the

key step in the Calvin cycle39 (Figure 61d). Also contained

within the carboxysomes are approximately 80 copies of

a carbonic anhydrase (CA) which converts bicarbonate

into CO2.36 The encapsulation of RubisCO with CA inside

of the carboxysomes enhances CO2 fixation by elevating

the local CO2 concentration around RubisCO and mini-

mizing the competing reaction of RubisCO with oxygen.

Because carboxysomes are found in all cyanobacteria

and many chemoautotrophic bacteria, they are a key

component of the global carbon cycle.

New time-resolved X-ray imaging techniques are

required for a full understanding of the structure and

catalytic dynamics of CO2 fixation by the carboxysome.

For example, structural data at nanometer resolution for

a single carboxysome will reveal how RubisCO and

c-type cytochromes such as OmcA. Dynamic pump-probe

studies that visualize the structural changes that occur on

the timescales of intraprotein and interfacial ET are

essential.

Experiments that can visualize the structural dynamics

of the protein-inorganic material interface before and

during intermolecular electron transfer will vastly expand

our understanding of ET. To determine how the kinetics

and pathways of electron transfer are altered by associa-

tion with a mineral surface, the sensitizer-protein biocon-

jugate could be complexed to a polyoxometallate cluster,

which can serve as a model for a metal oxide mineral

(Figure 60). These kinds of studies will provide the first-

ever systematic understanding of the structure and kinet-

ics of interfacial ET. We envision that this fundamental

science will enable rational engineering of the redox pro-

tein / crystal interface, and in turn have significant applica-

tions in biosynthesis, bioenergy, and biosensing.

3.9.5� Carboxysomes

Carboxysomes are self-assembling proteinaceous organ-

elles that play a key role in bacterial CO2 fixation (Figure

61). They range in size from approximately 90-170 nm in

diameter and in mass from 100-350 MDa33-35 and encap-

sulate hundreds of copies of two key enzymes of CO2 fix-

ation: RubisCO and Carbonic Anhydrase (CA) (Figure 61c

and d).34,36 Given the urgent global need to reduce CO2

emissions and develop CO2 sequestration technologies,

there is great interest in understanding carboxysomes

and in utilizing these organelles to enhance CO2 capture

and fixation rates in bio-engineering applications. Thus far,

structural information about the carboxysomes has been

limited to structures of isolated component proteins or

A B C D O2

HCO3 -

HCO3 - CO2

3-PGA

RuBP

2PG

Figure61�(A�and�B)�Transmission�electron�micrographs�of�carboxysomes�in�a�dividing�cyanobacteria�(A)�and�single�carboxysome�(B)�Scale�bars,�50nm�(Figures�from�Reference�37)�(C)�Predicted�model�of�a�whole�carboxysome.�RubisCO�(green),�Carbonic�Anhydrase�(red),�single�domain�hexameric�shell�proteins�(dark�blue)�tandem�domain�shell�proteins�(light�blue)�and�pentameric�shell�proteins�(yellow)�(D).�Schematic�of�CO2�fixation�reactions�inside�the�carboxysome.�CA,�carbonic�anhydrase;�PGA,�phosphoglycerate;�RuBP,�ribulose�bis-phosphate.

87


events during neurotransmission are highly regulated

and subject to stimulated changes.41 In the presynaptic

terminal, these changes modulate the releasable pool

and the release probability of synaptic vesicles. The

molecular components involved in neurotransmitter

release interact in a hierarchical fashion: Some compo-

nents have mutual pairwise interactions, some compo-

nents have interactions that are restricted to adjacent

partners, and some components or groups of compo-

nents are spatially separated by compartments. In addi-

tion, some of the interactions are sequential. This

complexity allows the neuron to create multiple regula-

tory mechanisms for neurotransmitter release.

Brunger and colleagues have applied a combination of

structural and biophysical studies to understand the

molecular basis for neurotransmitter release. Structural

information about complexes between the individual

molecular components has been obtained by X-ray crys-

tallography,42 NMR,43 and electron microscopy44 methods.

This information has provided the framework for investi-

gations targeted at the functional and dynamic aspects of

the system, using single-molecule fluorescence micros-

copy techniques.45-47 The experimental studies have then

been used to guide computational simulations designed

to capture intermediate conformations in fusion, other-

wise inaccessible experimentally.48

Despite this wealth of experimental and computational

information the detailed interactions between this complex

machinery is still not understood in the context of the cell.

Therefore, a complete understanding of the fusion process

requires the visualization of membranes fusing in the

presence of the driving proteins in the cell. Cellular imaging

methods that can focus into similar regions of cells from

many snapshots are needed to capture this process and

describe the conformational changes involved. The capa-

bilities of NGLS will make it possible to image entire syn-

aptic vesicles, synaptosomes (synaptic vesicles docked to

plasma membrane fragments), and of the minimal fusion

machinery that exhibits calcium-triggered complete fusion

activity with fast kinetics in an in vitro system recently

developed by Brunger and colleagues. Of key importance

will be the ability to collect billions of images of fusing

cells and “zoom in” on topologically similar regions in

order to generate a consensus image and the conforma-

tional variability around this average (see Section 4.1.2).

CA pack together inside a functional carboxysome to

mediate substrate and product channeling. Docking

atomic level structures of RubisCO, CA and shell proteins

into a nanometer scale resolution map of the whole car-

boxysome will provide a detailed view of how RubisCO

and CA are arranged for CO2 fixation, and how they interact

with the protein shell. We also anticipate that the internal

structure of the carboxysome may change with available

light, the metabolic state of the cell or the state of biogene-

sis; a powerful result of single particle imaging will be the

ability to compare reconstructions of carboxysomes from

many cells, or cells grown under different environmental

conditions. This will make it possible to characterize con-

served and heterogeneous features of carboxysome

structure and to identify structural changes that occur in

response to changing environmental conditions.

Nanometer resolution structures of carboxysomes will also

provide an initial foundation to understand how the struc-

tures of carboxysome proteins change during catalysis.

Moving to the cellular level, how are these organelles

integrated into the metabolic and regulatory processes of

the cell and which structural elements in the cell help pro-

mote and stabilize their assembly? This is particularly rel-

evant for bioengineering experiments because

understanding macromolecular self-assembly is one of

the frontiers of synthetic biology. Knowing how carboxy-

somes self-assemble and identifying associated cellular

changes that enhance assembly will guide experiments

to reconstruct or even improve these CO2 fixation

machines in vitro. Studying cells growing under different

environmental conditions using experimental techniques

that do not require subsequent freezing or fixation could

also led to significant advances in understanding the bio-

logical conditions that lead to carboxysome assembly.

Given their regular architecture and intermediate size

between whole cells and single particles, carboxysomes

offer an ideal system for developing organelle and cellular

imaging methods that could later be applied to more

complex or designed systems for encapsulating other

metabolic processes in carboxysome shells.

3.9.6� Membrane�Fusion

The protein-mediated fusion of lipidic membranes is a

critical process in biology.40 The pre- and postsynaptic

88


X-rayFELbaseddiffractionorscatteringexperiments

onisolatedmolecules,complexesorwholecellstherefore

mustrelyontheuseofultrashortX-raypulsesthatareable

toovercomethedamagelimit.Inthelasttenyearstheo-

reticalcalculationshaveindicatedthatultrashortpulses

couldbesuccessfullyusedtoobtaindiffractionpatterns

beforeradiationdamagedestroysthesample(“diffract

anddestroy”).54,55RecentexperimentsatFLASHand

LCLSusingpulsesinthe10to100fsrangeappeartocon-

firm this theory for macromolecular nanocrystals

(J.Spence,unpublished).Imagingofbiologicalsamples

atNGLSwillrelyonitsabilitytogenerateextremelyshort

X-raypulsesinthefemtosecondregimeatMHzrate.

Beamlines for Imaging Biological Dynamics and Function

Biologicalimagingresearchwillrelyon“diffractand

destroy”methodsusingthe3rdand5thharmonicswith

thehighestfluxperpulseontheseededNGLSbeamline1

at100kHzrepetitionrates,andontheun-seededSASE

beamline3,atMHzrepetitionrates(ashigh-speeddetec-

torsallow)asdescribedinSection5and6.6.Choiceof

wavelengthwillbedeterminedbybalancingthescatter-

ingefficiencyandtherequiredresolutionforparticular

samples.Akeycomponentoftheseexperimentswillbea

high-speedparticleinjectorsynchronizedtotheCWSCRF

linac(seeSection4.1.3),provideforsamplereplacement

onapulse-by-pulsebasis.

References:

1. H.M. Berman, et al., Nucleic Acids Research, 2000. 28: p. 235-242.

2. Sobolevskly, M.P. Rosconi, and E. Gouaux, Nature, 2009(462): p. 745-756.

3. K.C. Hsia, G.B., A. Hoelz, P. Stavropoulos, Cell, 2007. 131: p. 1313-1326.

4. Partley, R., Expert Opin Pharmacother, 2009. 3: p. 503-512.

5. Winger, J., et al., The structure of the leukemia drug imatinib bound to

human quinone reductase 2 (NQO2). BMC Structural Biology, 2009. 9(1):

p. 7.

6. Ballister E.R., et al., Proc. Natl. Acad. Sci. U.S.A., 2008. 105: p. 3733-3738.

7. T.R. Shaik, et al., Nature Protocols, 2008. 3: p. 1941-1974.

8. AJ, S. and B. JM, Q Rev Biophys, 2008. 41: p. 41-101.

9. Mitscher, L.A., Bacterial topoisomerase inhibitors: quinolone and pyri-

done antibacterial agents. Chem. Rev., 2005. 105: p. 559–92.

10. McClendon, A.K. and N. Osheroff, DNA topoisomerase II, genotoxicity,

and cancer. Mutat. Res. 2007, 2007. 623: p. 83-97.

11. Berger, J.M., et al., Structure and mechanism of DNA topoisomerase II.

Nature, 1996. 379: p. 225-32.

3.9.7 Radiation Damage in Biological Imaging

Oneofthemostseriousproblemsencounteredinthe

imagingofbiologicalsamplesisthedamagegeneratedby

theincidentX-raysorelectrons.49Stronglyionizingradia-

tiongeneratesprimaryandsecondarychemicaldamageto

macromoleculesresultinginbrokenchemicalbonds.In

thecontextofmacromolecularcrystallographythisphe-

nomenonhasbeenthesubjectofseveralstudies (see

Reference50forareview),leadingtoestimatesofmaxi-

mumX-raydoseformacromolecularcrystals.51Practical

measureshavebeendevelopedtomitigatetheeffectsof

damage,mostnotablytheuseofcryo-coolingtodramati-

callyreducetherateofdiffusionofdamagingfreeradicals

throughoutthecrystal.52Morerecentlyresearchershave

usedmicrobeamstoilluminateonlyverysmallregionsof

thecrystalinordertolimittheregiondamaged.53However,

ultimatelydamageisinescapableandproportionaltothe

numberofincidentphotons.

a

b

Figure62Figure 62. Model of Ca2+-triggered vesicle fusion by SNAREs and synaptotagmins. (A) Membrane stalk induced by SNARE complex formation prior to Ca2+ arrival between a synaptic vesicle (top membrane) and the plasma membrane (bottom mem-brane). Syntaxin, red; synaptobrevin, blue; SNAP25, green; com-plexin, yellow; synaptotagmin 1, magenta; phospholipids, gray car-toons; cholesterol, cyan. (B) Fusion pore after Ca2+ (yellow spheres) binding to synaptotagmin 1. Upon Ca2+ influx, the synap-totagmin 1 Ca2+ binding loops coordinate Ca2+ with anionic phos-pholipids, penetrating the lipid headgroup region. Simultaneously, the SNARE complex fully zippers by an unknown mechanism, likely involving complexin as well. (From Reference 47)

89


31. Kerisit, S., et al., Molecular computational investigation of electron-

transfer kinetics across cytochrome-iron oxide interfaces. Journal of

Physical Chemistry C, 2007. 111(30): p. 11363-11375.

32. Springs, S.L., S.E. Bass, and G.L. McLendon, Cytochrome b(562) variants:

A library for examining redox potential evolution. Biochemistry, 2000.

39(20): p. 6075-6082.

33. Schmid, M.F., et al., Structure of Halothiobacillus neapolitanus carboxy-

somes by cryo-electron tomography. J Mol Biol, 2006. 364(3): p. 526-35.

34. Iancu, C.V., et al., The structure of isolated Synechococcus strain

WH8102 carboxysomes as revealed by electron cryotomography. J Mol

Biol, 2007. 372(3): p. 764-73.

35. Iancu, C.V., et al., Organization, structure, and assembly of alpha-car-

boxysomes determined by electron cryotomography of intact cells. J

Mol Biol, 2010. 396(1): p. 105-17.

36. Heinhorst, S., G.C. Cannon, and J.M. Shively, Carboxysomes and car-

boxysome-like inclusions, in Microbial monographs. 2006, Springer-

Verlag: Berlin, Germany. p. 141-165.

37. Kerfeld, C.A., et al., Protein structures forming the shell of primitive bac-

terial organelles. Science, 2005. 309(5736): p. 936-8.

38. Tanaka, S., et al., Atomic-level models of the bacterial carboxysome

shell. Science, 2008. 319(5866): p. 1083-6.

39. Andersson, I. and T.C. Taylor, Structural framework for catalysis and reg-

ulation in ribulose-1,5-bisphosphate carboxylase/oxygenase. Arch

Biochem Biophys, 2003. 414(2): p. 130-40.

40. Jahn, R., T. Lang, and T.C. Südhof, Membrane Fusion. Cell, 2003. 112: p.

519-33.

41. Sudhof, T.C., The synaptic vesicle cycle. Annu Rev Neurosci., 2004. 27: p.

509-47.

42. Sutton, R.B., et al., Crystal structure of a SNARE complex involved in syn-

aptic exocytosis at 2.4 Å resolution. Nature, 1998. 395: p. 347-353.

43. Fiebig, K.M., et al., Folding intermediates of SNARE complex assembly.

Nature Structural Biology, 1999. 6: p. 117-123.

44. Furst J, S.R., Chen J, Brunger AT, and Grigorieff N., Electron cryo-micros-

copy Structure of N-ethyl Maleimide Sensitive Factor at 11 Å Resolution.

EMBO Journal, 2003. 22: p. 4365-4374.

45. Weninger, K., et al., Single molecule studies of SNARE complex assem-

bly reveal parallel and anti-parallel configurations. PNAS, 2003. 100: p.

14800-14805.

46. Bowen, M.E., et al., Single molecule observation of liposome-bilayer

fusion thermally induced by SNAREs. Biophys. J., 2004. 87: p. 3569-3584.

47. Vrljic, M., et al., Molecular mechanism of the synaptotagmin-SNARE

interaction in Ca2+-triggered vesicle fusion. Nat. Struct. Mol. Biol., 2010.

17: p. 325-331.

48. Kasson, P.M., et al., Ensemble molecular dynamics yields submillisecond

kinetics and intermediates of membrane fusion. PNAS, 2006. 103: p.

11916-11921.

12. Dong, K.C. and J.M. Berger, Structural basis for gate-DNA recognition

and bending by type IIA topoisomerases. Nature, 2007. 450: p. 1201-5.

13. O’Donnell, M., Replisome architecture and dynamics in Escherichia coli.

J Biol Chem., 2006. 281: p. 10653-6.

14. Yao, N.Y. and M. O’Donnell, Replisome structure and conformational

dynamics underlie fork progression past obstacles. Curr Opin Cell Biol.,

2009. 21: p. 336-43.

15. Zhang, W., J.A. Dunkle, and C. J.H., Structures of the ribosome in inter-

mediate states of ratcheting. Science, 2009. 325: p. 1014-7.

16. Berman, H.M. et al., The Protein Data Bank. Acta Crystallogr D Biol

Crystallogr., 2002. 58: p. 899-907.

17. Davey, J., G-protein-coupled receptors: new approaches to maximise

the impact of GPCRS in drug discovery. Expert Opin. Ther. Targets, 2004.

8: p. 165–170.

18. Poolman, B., et al., Functional analysis of detergent-solubilized and

membrane-reconstituted ATP-binding cassette transporters. Methods

Enzymol., 2005. 400: p. 429-59.

19. Cooper, E.C. and L.Y. Jan, Ion channel genes and human neurological

disease: Recent progress, prospects, and challenges. PNAS, 1999. 96: p.

4759-4766.

20. Doyle, D.A., et al., The structure of the potassium channel: molecular

basis K+ conduction and selectivity. Science, 1998. 280: p. 69-77.

21. Dutzler et al., X-ray structure of a ClC chloride channel at 3.0 Å reveals

the molecular basis of anion selectivity. Nature, 2002. 415: p. 287-294.

22. Jiang, Y.X., et al., The principle of gating charge movement in a voltage-

dependent K+ channel. Nature, 2003. 423: p. 42-4.

23. Kawate, T., et al., Crystal structure of the ATP-gated P2X(4) ion channel

in the closed state. Nature, 2009. 460: p. 592-8.

24. Liu, Z., C.S. Gandhi, and D.C. Rees, Structure of a tetrameric MscL in an

expanded intermediate state. Nature, 2009. 461: p. 120-4.

25. Weber, K.A., L.A. Achenbach, and J.D. Coates, Microorganisms pumping

iron: anaerobic microbial iron oxidation and reduction. Nature Reviews

Microbiology, 2006. 4(10): p. 752-764.

26. Fredrickson, J.K. and J.M. Zachara, Electron transfer at the microbe-

mineral interface: a grand challenge in biogeochemistry. Geobiology,

2008. 6(3): p. 245-253.

27. Jensen, H.M., et al., Engineering of a synthetic electron conduit in living

cells. Proceedings of the National Academy of Sciences of the United

States of America, 2010. inpress.

28. BESAC, Directing Matter and Energy: Five Challenges for Science and

the Imagination., U.D.B.E.S.A. Committee, Editor. 2007.

29. Shi, L.A., et al., The roles of outer membrane cytochromes of Shewanella

and Geobacter in extracellular electron transfer. Environmental

Microbiology Reports, 2009. 1(4): p. 220-227.

30. Lower, B.H., et al., Specific bonds between an iron oxide surface and

outer membrane cytochromes MtrC and OmcA from Shewanella

oneidensis MR-1. Journal of Bacteriology, 2007. 189(13): p. 4944-4952.

90


52. Hope, H., Cryocrystallography of biological macromolecules: a generally

applicable method. Acta Crystallogr B. , 1988. 44: p. 22-6.

53. Stern, et al., et al., Reducing radiation damage in macromolecular crys-

tals at synchrotron sources. Acta Crystallogr D Biol Crystallogr. , 2009.

65: p. 366-74.

54. Neutze, R., et al., Potential for biomolecular imaging with femtosecond

X-ray pulses. Nature, 2000. 406: p. 752.

55. Hau-Riege, S.P., R.A. London, and A. Szöke, Dynamics of X-Ray Irradiated

Biological Molecules. Phys. Rev. E, 2004. 69: p. 051906.

49. Henderson, R., The potential and limitations of neutrons, electrons and

X-rays for atomic resolution microscopy of unstained biological mole-

cules. Q Rev Biophys., 1995. 28: p. 171-93.

50. Garman, E.F., Radiation damage in macromolecular crystallography:

what is it and why should we care? Acta Crystallogr D Biol Crystallogr.,

2010. 66: p. 339-351.

51. Owen, R.L., E. Rudiño-Piñera, and E.F. Garman, Experimental determina-

tion of the radiation dose limit for cryocooled protein crystals. PNAS,

2006. 103: p. 4912-7.

New techniques enabled by NGLS4

revealawealthofadditionalstructuralinformation,con-

tainedinfluctuationsaboutthemeanSAXSsignal(i.e.

modulationsinthenominallyradially-symmetricSAXS

signal),thatcannotbeobtainedbyconventionaltech-

niques.UnlikeSAXS/WAXS,whichprojectsthreedimen-

sionsontoone,thesnapshot“WAX”patternsfromNGLS

willshowtwo-dimensionalvariation,andsoprojectthree

dimensionsontotwo,providingmuchmoreinformation.

Thepowerfulapproachdescribedabove,isknownas

“fluctuationX-rayscattering”(fXS)andwasoriginally

proposedbyKamthirtyyearsago.3,4Todate,thismethod

hasbeenconsideredintractable—notduetothespeedof

thepulseperse,northedifficultyoftheanalysis,butthe

requirementofmeasuringatleastonescatteredX-rayper

particlepersnapshot,withinarotationaldiffusiontime.A

recentexperimentaltest5attheAdvancedLightSource,

usingafixeddistributionofgoldnano-rodsasamodel

two-dimensional system has shown that an ab-initio

reconstructionispossiblewithoutrelyingonanyofthe

modelingassumptionsnormallyrequiredfortheanalysis

ofSAXSdata.However,themodelexperimentusingTMV

(whichisstillahighlyfavorablecase)inwateratroom-

temperatureisstillunfeasibleonexistingstoragerings.

ThecapabilitiesofNGLSX-raylaserswillfinallyenablethis

powerfulfXSapproachfordetermining3Dstructuresof

macromoleculesintheirnativeenvironments.

Theory: TheresurgenceofinterestinKam’scorrelation

averagingapproachhasnothappenedinavacuum.The

long-plannedsinglemoleculediffraction(SMD)method

(diffract and destroy) poses significant data analysis

4.1.1 Fluctuation SAXS — Molecular Structures in Native Environments

Understandingbiologicalprocessesatacellularlevel

requiresdetailedknowledgeofthestructureanddynam-

icsoflargemacromolecularmachines.Wepresentlylack

thenecessarytoolsfordynamicimagingofsuchbiologi-

calmachines—inoperation,innativeenvironments,and

atthenanoscale.Small-angleandwide-angleX-rayscat-

tering (SAXS/WAXS) techniques1 offer tremendous

potential,andarepresentlybeingusedtogreatadvan-

tageincombinationwithcrystallography.2However,the

fullpotentialofthesescatteringtechniquesremainsunre-

alizedbecausetherequiredexposuretimesatpresent

X-raysourcesareordersofmagnitudelongerthanthe

rotationaldiffusiontimesofthesampleparticles.Thusthe

scatteringdataaresphericallyaveraged,ineffectproject-

ingthethreedimensionsofthestructureontoonedimen-

sion.The restricted information available from these

one-dimensionalprojectionsisinsufficientforunambigu-

ousstructuredetermination.

NGLSX-raylaserswillprovidethefirstcapabilityfor

high-quality SAXS/WAXS with exposure times much

fasterthanrotationaldiffusiontimescales,therebyelimi-

natingthesphericalaveraginglimitation(e.g.<140nsfor

atestsampleoftobacco-mosaicvirus,TMV,size~300nm

x20nm,inpureroom-temperaturewater—rotational

diffusiontimeswillbeps-nsforsmallermacromolecules).

Thehigh-speedSAXS/WAXSscatteringsnap-shotswill

4.1 Imagingstructureandfunctioninheterogeneousensembles

92

4 . �NEW�TECHNIQUES�ENABLED�NY�NGLSIMAGING�STRUCTURE�AND�FUNCTION�IN�HETEROGENEOUS�ENSEMBLES

recorded of different areas and the autocorrelation

around a single q ring was calculated for each pattern. The

averaged autocorrelations “converged” toward the auto-

correlation of one particle. This derivation of a one-parti-

cle pattern from a measurement of an ensemble of

particles is the essence of correlation averaging (Figure

63). The final step consisted of phasing this data to recon-

struct a two-dimensional image of one typical gold rod.

This work represents the first successful ab initio experi-

mental demonstration of the Kam fXS method, albeit in

two dimensions.8,9

A more ambitious experiment was carried out by

Howells et al.,10 at the TROÏKA hard X-ray beam line at the

European Synchrotron Radiation Facility. This was an

attempt at a full 3D fXS experiment but using highly vis-

cous solvents and low temperatures to slow the rotation

of the particles. Goethite and TMV samples were used and

correlations were recorded. Data analysis is in progress.

4.1.1.2 WhatHasBeenDone?Theory:

A full 2D theory was described and used successfully

to reconstruct a set of simulated diffraction patterns of

K-channel membrane protein molecules in a membrane,

again with orientations random about one axis.5 The

same theory was used to reconstruct the set of patterns

from the gold rod samples in the experiment cited above. A

3D theory, still using spatial correlation averaging, was also

developed for the SMD experiment and used successfully

to reconstruct the 3D image of a protein molecule.11

Recent theoretical developments show that ab-initio

image reconstruction of three-dimensional objects (ran-

domly oriented) is possible from fXS data, using algo-

rithms developed for protein crystallography and

coherent diffractive imaging. Furthermore, current algo-

rithms available for shape reconstruction from SAXS

data can be extended to incorporate fXS data. Although

the computational complexity of the problem is signifi-

cant, modern computational approaches, such as hard-

ware acceleration of specific time-consuming operations,

are expected to eliminate the most significant computa-

tional bottlenecks.

In order to determine the structure of intermediate

states during the reaction cycle of macromolecular

problems that have been under study. The idea to use cor-

relation averaging in that case was rediscovered indepen-

dently by Saldin and coworkers.6 Indeed it may be that

SMD data can only be reconstructed by correlation aver-

aging.7 The reconstruction in fXS requires the solution of

two phase problems, one to get from the experimentally

determined autocorrelation function to the 3D single-par-

ticle diffraction pattern and a second one to invert the dif-

fraction pattern to retrieve the 3D image (as in coherent

diffraction imaging). However, in spite of this increase in

activity, the latest round of publications has not included

a solution of the full 3D reconstruction problem for a

modern fXS experiment, even in simulations.

4.1.1.1 WhatHasBeenDone?Experiment:

Good progress has been made on the two-dimension-

al analogue of fXS, in which identical particles lie in a

plane differing only by rotation about an axis parallel to

the X-ray beam. An fXS experiment has been done at the

ALS in which 80-nm-long gold rods were dispersed on

membranes, the rod orientations being random about

one axis only. About a hundred diffraction patterns were

SimulationData

20

10

0

-10

0˚ 90˚ 180˚ 270˚ 360˚

SimulationData

20

10

0

-10

0˚ 90˚ 180˚ 270˚ 360˚

Figure63�The�upper�panel�shows�the�q�ring�autocorrelation�of�a�single�snapshot�of�many�80�nm�cylinders�(dotted)�and�the�calculat-ed�autocorrelation�of�a�single�cylinder.�The�lower�panel�shows�the�effect�of�averaging�together�121�different�measured�snapshots�and�again�the�calculated�autocorrelation�of�a�single�cylinder.�The�aver-age�over�snapshots�converges�toward�the�single-particle�pattern.�(Figure�from�Reference�8)

93

4 . �NEW�TECHNIQUES�ENABLED�NY�NGLSIMAGING�STRUCTURE�AND�FUNCTION�IN�HETEROGENEOUS�ENSEMBLES

• High average photon flux, and high flux / pulse:

Number of scattering particles and scattering efficiency

is limited. Proposed NGLS flux per pulse is compara-

ble to exposing roughly one second on state of the art

SAXS beamlines, indicating that with sufficient data,

high resolution 2D fXS patterns can be efficiently

obtained. Scattering patterns from multiple expo-

sures can be summed together (with appropriate pro-

cessing of individual patterns).

• Coherence: A fully coherent beam provides significant

simplifications in data analysis of mixtures and

doesn’t impede analysis of samples with a single

structural species.

The proposed parameters of NGLS X-ray lasers fit the

above requirements. The optimal choice of energy for an

fXS experiment will be determined by the nature of the

problem investigated. The available energy ranges from

hard X-rays (using harmonics) to the water window

(wavelength approximately ~2 nm) allow for the imaging

of relatively small systems such as enzymes (using hard

X-rays) or very large systems such as viruses or large

macromolecular assemblies, such as polysomes.

NGLSCapabilitiesforfXS

• Fast X-ray pulses to exploit the “diffract and destroy”

method for single molecules or ensembles.

• Samples can be in buffer in physiological conditions,

without viscosity enhancers.

• Samples can be at room temperature, no need for

cooling to slow the particle diffusion.

• Higher X-ray flux at NGLS means that fXS can be

effectively applied to smaller and more symmmetric

macromolecules.

• Can study time dependent or triggered processes in 3D.

• Can deal more effectively with heterogeneity (non-

identical particles).

• Resonant scattering (near edges) possible in principle.

• The amount of information provided by an fXS measure-

ment is roughly equivalent to knowing the 3D autocor-

relation of the unknown structure at SAXS resolution.

This will be orders of magnitude more information

than that provided by the spherically-averaged pat-

terns of standard SAXS, allowing one to determine

many more parameters with higher accuracy.

machines, the theory of fXS can be extended following

the lines of Andersson et al.,1,3 The latter work demon-

strates that a time resolved series of WAXS data from an

evolving mixture of species, as observed in time resolved

measurements, can be decomposed into curves of indi-

vidual species of (meta) stable intermediates (Figure 64).

It can be shown that a similar technique can be applied to

fXS data, enabling the investigation of time resolved

structural changes of large macromolecular machines in

near native environments.

4.1.1.3 ExperimentalConsiderationsforfXS

SourceRequirements:

• Ultrafast X-ray pulse durations: scattering patterns

must be measured with X-ray exposures that are fast-

er than molecular rotational diffusion times (typically

sub-picosecond under native conditions)

-1ns100ps316ps1ns3.16ps10ns31.6ns100ns316ns1us3.16us10us31.6us100us316us1ms3.16ms10ms

0.0x100

-3.0x104

-6.0x104

-9.0x104

-1.4x105

a

b

0.0 0.5 1.0q[A-1]

1.5 2.0 2.5

0.0 0.5 1.0q[A-1]

1.5 2.0 2.5

Mb†•COMb*•COMb•COMbThermal

Mb*•CO-Mb†•COMb•CO-Mb*•COMb-Mb•COThermal(static)

Figure64Time�dependent�evolution�of�SAXS/WAXS�patterns�of�CO-Myoglobin�dissociation�(a).�Numerical�analyses�of�these�pat-terns�results�in�curves�of�individual�species�(b).12

94

4 . NEW TECHNIQUES ENABLED NY NGLSIMAGING STRUCTURE AND FUNCTION IN HETEROGENEOUS ENSEMBLES

machines,andprovideunprecedented,statisticallyvali-

datedaccesstotheoperationofmolecularfactories.

Radiationdamageseverelylimitstheinformationthat

canbeobtainedfromasinglecopyofsoft-matterobjects

beforetheyaredestroyed.Toboosttheweakscattered

signal,currentstructuraltechniquesaveragedatagath-

eredfrommanyobjectsassumedtobeidentical.This

includeswell-establishedtechniquessuchascrystallog-

raphyandcryo-EM,15aswellas“scatter-and-destroy”

methodsrecentlyenabledbyX-rayFEL’s.7,16-19

Thereismountingevidence,however,thatstructural

variabilityisnotonlycommonatthemolecularlevel,but

alsoimportanttofunction,andthatinsoft-matter,“struc-

ture”isneitherstatic,norimmutable.20-26Understanding

structuralvariabilityanddynamics,whilevitalforcon-

trollingsoft-matterprocesses,hasremainedelusive.

Molecularmachines,suchasenzymes,undergo—

ofteninitiate—conformationalchanges.Snapshotsof

molecularmachinesthusnecessarilyrepresentultralow-

signalsightingsofmembersofconformationallyhetero-

geneousensembles,whichcannotbesimplyaveraged.

Understandingthestructureanddynamicsofheteroge-

neousensemblesrepresentsakeyfrontierinsoft-matter

science.

The ability to collect and comprehend information

from systems spanning a broad heterogeneity landscape

is essential, if we are to control complex dynamic pro-

cesses for efficient energy conversion, carbon sequestra-

tion, and enzymatic reactions honed by nature over

millennia.

4 .1 .2 .1 Accessing Dynamic, Heterogeneous Systems by

Manifold Mapping

Randomsnapshotsobtainedfrommembersofahet-

erogeneousensemblearecorrelated—otherwisethere

wouldbenothingtocharacterizetheset.Intheabsence

ofnoise,thesecorrelationsforcethesnapshotstolieona

hypersurface—amanifold—whosedimensionalityis

determinedbythenumberofdegreesoffreedomavail-

abletothesystem.Arotatingrigidbodyobservedinfar-

fielddiffraction,forexample,hasthree(orientational)

degreesoffreedom,andthusproducesathree-dimen-

sionalmanifold.19Theadditionaldegreesoffreedomofa

dynamic,non-rigidsystemgiverisetohigherdimensional

manifolds27(Figure65).Similarly,thereactioncoordi-

natesofanongoingprocessarereflectedinthedimen-

sionalityofthedatamanifold.27

Sample Environment and Data Aquisition .

Utilizingaliquidwatermixingjet13runninginasingle

fileacrosstheNGLSbeam,combinedwithsuitablechoices

ofsubstrate,proteinconcentration,jetvelocityanddelay

lines,wecanobtainfXSdataatintermediatetimepoints

withinthedutycycleofamacromolecularmachinethat

canspanfromsub-nanosecondstoseconds.Thehigh

repetitionrateofNGLSwillallowustocollecthundreds

ofthousandsofpatternsatsuitablychosentimepoints

alongthecontinuousordropletbeam.NGLSwilloperate

inthe“diffract-and-destroy”mode,inordertoavoidany

effectsofradiationdamageonthediffractionpatterns.14

Givendataratesassociatedwiththeseexperiments,

efficientdetectortechnologyisinstrumentaltothesuc-

cessoftheseexperiments.Anexcitingpossibilityisthe

developmentofhardwaresolutionsfordatareduction

(seeSection6.6).ObtainingfXSdatainvolvesthecompu-

tationofangularcorrelationsofthescatteringpatterns.If

thisessentialstepcanbeperformedonthedetectorwith

dedicatedhardwarewhilethedataiscollected,asignifi-

cantinfrastructurebottleneckisresolved.

4 .1 .1 .4 Outlook

ThefXStechniqueoverlapswithsinglemoleculedif-

fractionstudiesthattypicallyprovidemoredetailedinfor-

mationthantheensemble-basedfXSmethod.Duetothe

underlyingexperimentaldesignofanfXSexperiment,

thetechniquelendsitselftoahigherlevelofautomation.

Assumingaconservative,particleinjectorlimited,data

acquisition rate of 10 kHz, 10 million images can be

obtainedwithin20minutes,sufficienttoassembleahigh

resolutionfXSdataset.Giventhehigh-throughputnature

oftheexperiment,akintoindustrystandardproteincrys-

tallographicdataacquisition,automated,high-through-

putfXScanbeanessentialtoolinthediscoveryofnovel

drugsactingonmembraneproteinsorotherlargemac-

romolecularmachinesnotamenableforroutinebiophys-

icaltechniquesemployedinstructurebaseddrugdesign.

4.1.2 Giga-shot Imaging of Heterogeneous Ensembles with Manifold Mapping

NGLSwillproduce~1010diffractionsnapshotsperday.

Combinedwithadvancedmanifold-basedanalyticaltech-

niques,thiswillelucidatetheroleofheterogeneityinbio-

logical systems, enable 4D imaging of molecular

95


fundamentalunderlyingsymmetries.29Theseso-called

isometries,previouslyassociatedwithcertainmodelsof

theuniverseingeneralrelativity,36directlyrevealthenat-

ural,physicallymeaningfuleigenfunctionsofdatamani-

folds produced by scattering. Projection on these

eigenfunctionsallowssuperbnoisediscrimination,while

theisometriesthemselvesprovidea“compass”fornavi-

gationonthemanifold.Thisunprecedentedcapability

canbeusedtoefficientlyidentifydatamanifoldsinthe

presenceofoverwhelmingnoise,andextractphysically

meaningfulinformationfromthem.

Manifolds swept out by random sightings of heteroge-

neous dynamic systems can be used to compile 4D maps

(“3D movies”) of these systems. Key to this approach is

the availability of a sufficient number of snapshots to

define the manifold to the required granularity. The

unique characteristics of NGLS, particularly the combina-

tion of high flux per pulse and its high repetition rate, are

essential to this capability.NGLS, combined with mani-

fold-based methods, offers unprecedented access to

structural heterogeneity in complex dynamic systems.

4 .1 .2 .2 Conformational Heterogeneity and Dynamics:

Molecular Machines

Molecularmachinesundergodiscreteand/orcontinu-

ousconformationalexcursionsandinducesimilarchanges

inthesubstrate.21Usingultralow-signalsnapshotswith

no orientational or timing information, these can be

mappedbymanifold-basedanalyticaltechniques.29

Thedatamanifoldcontainstheentireinformationcon-

tentofthedataset.Often,thismanifoldmustbedeter-

mined in the presence of overwhelming noise.

Noise-robustmanifoldmappingalgorithmshavebeen

demonstrated with simulated and experimental data

downto~10-2scatteredphotons/pixelinthepresenceof

backgroundscattering,withsignals-to-noiseratiosaslow

as~-20dB.19,26,29

Comprehendingtheinformationcontentofthedataset

istantamounttobeingableto“navigateonthemanifold”

soastoreachanydesiredpoint.Forexample,onemay

wishtoreconstructthe3Dstructureofanenzymeata

particularpoint in itsconformational landscape.A3D

modelisequivalenttotheabilitytoproduceany2Dpro-

jectionatwill.Onthedatamanifold,thisinvolvesnavigat-

ingfromacertainpoint(agivenconformation)along

directionsofpureorientationalchangeonthemanifold,

and thus producing any desired 2D projection of the

givenconformation(Figure65).Similarly,onemaywish

toextracttheconformationalchangesobservedfroma

certaindirection—equivalenttomovingalongthemani-

foldinadirectionofpureconformationchange.

Graph-theoreticmeansforidentifyingdatamanifolds

arewell-established,30-34but ithasproveddifficult to

assignphysicalmeaningtothedimensionsofso-called

embeddedmanifolds35andthuspurposefullynavigate

onthem.Recentworkhasshownthatdatamanifoldspro-

ducedbyanyscatteringprocess—elastic,inelastic,kine-

matic(single)ordynamical(multiple)scattering—possess

200

400

600

800

1000

1200

1400

1600

1800 Figure65 Manifold traced out by simu-lated diffraction snapshots of a molecule of adenylate kinase as it unfolds due to heating.27 Insets show the molecular structure at points on the manifold. Advanced analytical techniques offer a route to reconstructing the 3D structure of the molecule during it evolution — essentially a 3D movie of the unfolding process.29

96


whicheachclassoriginatedidentified.Thisisequivalentto

attachinganaddresslabeltoeachclass.Thenumberofbits

requiredforeachaddressisdeterminedbythesizeofthe

object,thenumberofdistinguishablestatesaccessibleto

thesystem,andtheresolutionofthemethodofobserva-

tion.7,19,27,38Whenthesignalissolowthattheinforma-

tioncontentofthedatasetissmallerthanthenumberof

bitsrequiredtolabelitsclasses,theprocessbreaksdown.

Downtothisverylowlimit,38onecantradetotalaccumu-

latedsnapshotsagainstthedoseperpulse,i.e.,compen-

sate forextremely lowscatteredsignalsbycollecting

moresnapshots.

Thepracticallimitissetbythenumberofsnapshotsin

thecollection,which,inturn,dependsonthesourcerepeti-

tionand/ordetectorread-outrates.Asaspecificexample,

recoveringthestructureofarigidobjectto~1nmresolu-

tiontypicallyrequires~104snapshots.19Distinguishing

between100differentconformationsofamoleculewitha

singleconformationaldegreeoffreedomincreasesthe

numberofsnapshotsto~106(Reference27).TheLCLS

produces~107snapshotsperday,limitingthesize,com-

plexity,andresolutiontowhichtheconformations,con-

figurations,anddynamicsofasystemcanbestudied.

NGLS will be capable of generating more than

1010 snapshots per day. Assuming a one-day experiment

and 104 snapshots to recover the 3D structure of each

configuration to ~1 nm resolution, one can study systems

with ~106 conformational states. The largest number of

conformational states mapped so far is ~ 50 (Reference

21).NGLS represents a spectacular advance in the study

of molecular machines and processes at the nano-atto

scale.

The projected capabilities outlined above must be

comparedwith thoseexpected fromalternative tech-

Whenamolecularmachinepossessesdiscreteconfor-

mationalstates,suchastheclosedandopenconforma-

tionsoftheenergy-relevantenzymeadenylatekinase,

simulationsshowthatsuitablealgorithmsareabletosort

thesnapshotsintotwodifferentmanifoldsanddetermine

theorientationofeachsnapshot.Thesortingconfidence

canbeextremelyhigh(~8.5σ)evenatverylowsignal

(~4x10-2scatteredphotonsperShannonpixelinthepres-

enceofPoissonnoise)27asshowninFigure65.Theability

toseparatediscreteconformationswithhighconfidence

anddeterminetheorientationfromwhicheachsnapshot

emanatesisanindicationoftheefficientusemanifold-

basedapproachesmakeoftheinformationcontentofthe

entiredataset.

Inmanyinstances,molecularmachinesundergocon-

tinuousconformationalchanges,sweepingoutamani-

foldwithdimensionscorrespondingtoorientationaland

conformationalchanges(Figure65).Suchmanifoldsare

bestregardedfromthepointofviewofanantcrawling

onthemanifold,withacompassprovidedbythesymme-

tries underlying image formation by scattering.29

Specifically,theisometrieshencethenaturaleigenfunc-

tionsof thescatteringmanifoldallowone to identify

directionscorrespondingtoorientationalandconforma-

tionalchanges.Inthisframework,onecancompile4D

maps(3Dmoviesintime)ofcontinuousconformational

changes in molecules and their assemblies. Such

approachesarecurrentlybeingusedtomapconforma-

tionsofmolecularmachinesusingcryo-EMimages,21,26

andX-rayFELdiffractionsnapshots.26

Thefundamentallimittothisapproachissetbyinfor-

mation-theoreticconsiderations.Attheconclusionofthe

analysis,theexperimentalsnapshotshavebeen“sorted”

intoclasses(“bins”),andthestateofthesystemfrom

CLOSED

OPEN

LID 4πSignal: 4x10-2ph/pixel

2π

2π0 Correct orientation

Dedu

ced

orie

ntat

ion

LID

NMP

NMP

CORE

CORE

AP5A

Manifold 1

Manifold 2

Figure66Manifold mapping separates snapshots from different conformations and determines the snapshot orientations within each set with no a priori knowledge. When a mixture of diffraction snapshots from the molecule adenylate kinase in its open and closed conformations is presented to noise-robust versions of graph-theoretic techniques at sig-nal levels corresponding to 0.04 ph/pixel at 0.18 nm, the snapshots are automatically sorted into different manifolds and their orientations determined.27 The 8.5-σ separation between the two manifolds implies extreme fidelity in separating different conformations.

97


torwith~109pixels,a30xincreaseinlineardimensions

comparedwithdetectorscurrentlyinuse.Theobjectis

firstreconstructedinto83=512voxels,withthevoxelof

interestsuccessivelysubdividedinto512voxels.Starting

witha~10μmfieldofview,fourlevelsofzoomsufficeto

reach~1nmresolution(Figure67).3Dreconstructionat

eachlevelneedsonly~5x104snapshots.

Akeyissueconcernsthenumberoftopologicalstates

availabletosuchcomplexsystems.Thefollowingsum-

marizesthenumberofstatesavailabletoobjects,whose

topologiescanbedescribedintermsofgraphsoforder8

(graphswith8nodes,or“anchorpoints”).41,42Thelarge

numberoftopologicalstatesissubstantiallyreducedas

soonasasingletopologicalclass(familyofgraphs)is

considered.Representingthetopologyoftheregionof

interestateachlevelofzoomasagraphoforder8(i.e.,

assuming that the topology of 512 voxels can be

describedby8anchorpoints) reduces thenumberof

topologicalstatestobeexploredateachlevelofzoom

from~6x108to~2x105.Thislevelofheterogeneitycanbe

easilyexploredby the~1010snapshotsavailable.The

numberoftopologicalstatesexploredinfourlevelsof

zoom,however,exceedsAvogadro’snumber,indicatinga

toopermissiveestimateof thenumberof topological

statestobeconsideredateachlevel.Inpractice,itmaybe

niqueswhenNGLScommencesoperation.Thereisno

doubtthatcryo-EMwillcontinuetomakeprogressin

investigatingmolecularmachines.Cryo-EMdatasetscur-

rentlyspan~106snapshots.Whilefurtherprogressispos-

sible in key steps such as sample preparation and

microscopeoperation,itispresentlydifficulttoenvisage

increasesofmorethan100xinthesizeofcryo-EMdatas-

ets, which will still be 100x smaller than capabilities

offeredbyNGLS.Moreimportantly,ithasnotbeenpos-

sibletointroducetimeresolutionincryo-EMotherthan

byslowingreactionschemically.

The high repetition rate and the time-resolved capabil-

ities of NGLS represent an inherent advantage in study-

ing dynamic systems on the nano-atto scale.

4 .1 .2 .3 Cellular Imaging

Topological heterogeneity: Molecular factories

Beyondconformationalheterogeneity,complexsys-

temsofagivenclasscandifferinconfigurationandinthe

numberofcomponentstheycontain.Molecularfactories,

suchasthoseinvolvedincarbonsequestrationandbio-

logicalcellsthusdisplayconformational,compositional,

andtopologicalheterogeneity.

A10μm-diameterobjectcontains~1012nanometer-

sizevoxels.3Dreconstructionofsuchanobjecttonano-

meter level is computationally intractable, and

unnecessary.Inpractice,onebeginswithinitialscrutiny

atlowmagnification,successively“zooming”intoselected

regionsofinterest.Zoomingcanbeachievedbynear-

fielddiffraction,39orviaholography.40Bothapproaches,

which have been demonstrated experimentally for

X-rays,encodepositionalinformationinthesnapshot.

The largenumberofsnapshotsprovidedbyNGLS

enableadecisiveadvance.Insteadofrelyingontheinfor-

mationfromasingleobject,whichisquicklydestroyed

byradiationdamage,onesuccessivelyintersectseachof

~1010membersofaheterogeneousensemblewithan

intensepulse.Dataarecollectedbeforeonsetofdamage

andsubsequentsampledestruction.Asoutlinedbelow,

manifold-basedapproachesallowonetousetheinfor-

mationcontentofthedatasetfromtheentireensembleto

reconstructeachmemberof theensemble,providing

unprecedentedandstatisticallyvalidatedinformationon

suchheterogeneoussystems.

Onebeginsbyrecordingsnapshotswithsufficiently

largemomentumtransfertoallowreconstructionto~1nm

level.Fora~10μmdiameterobject,thisrequiresadetec-

y

Root cell0 1 2 3 4 5 6 7

x z

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0 1 2 3 4 5 6 7

0 1

131 1 1 1 10 1 2 3 4

130 1 1 1 0130

zyx 1 1 1 0

7 7 7 7 4

2 3 4 5 6 7

Figure67Taming voxels by repeated “divide and conquer”. Reconstruct the macroscopic (e.g. 10 μmØ) object from the coher-ent diffraction patterns, initially with 83=512 voxels. Zoom in to a single voxel, and then reconstruct again with 83=512 voxels (8x zoom) — repeat. Four zooms span from μm to nm scale.

98


4.1.3 Diffract and Destroy at High Repetition Rates

An important consideration fordiffractive imaging

applicationsdescribedinSections4.1.1and4.1.2isthat

theywillrelyonfundamentallynewimplementationsof

the“diffractanddestroy”method thathasbeenpio-

neeredatFLASHandLCLS.BothfXSandbillion-shotdif-

fractiveimagingdealwiththeproblemofrandomsample

orientationinnewwayswhichcapitalizeonthecapabili-

tiesofNGLSX-raylasers.Bothoftheseapproachesrely

onsamplereplacementbetweenX-rayshots,withthe

expectationthat thesample isdamagedordestroyed

eachtime.However,theX-rayexposurepermolecule

is limited (distributed among many molecules either

seriallyinthesingle-moleculecase,orinparallelinthe

fXScase),insuringthatscatteringoccursbeforedisrup-

tionofthesamplestructure.Importantly,thehigh-repeti-

tion-rate of NGLS (and advanced computational

algorithms19,27,29,37,44) are exploited to both address

theissueofrandomsampleorientation,andtoprovide

sufficienttorestrictthegraphordertoalowervalue,per-

hapsaslowasfive.Theseconsiderationsleadtotheview

thatthezoom/octalsearchapproachismorethanade-

quate tomapthe topologicalheterogeneity in typical

objectsofinterest.

Thedatamanifoldnowreflectsconformationaland

configurational (“topological”) degrees of freedom

(Figure68).Navigatingsuchamanifoldwillallowoneto

exploretheconformationalandconfigurationalspace

availabletoanyregionofinterestinamolecularfactory.

Specifically,~1010differentsamplesofagivenregioncan

be investigated.As~104snapshotsareneededfor3D

reconstruction,thismeansthe3Dstructureofselected

regionsfrom~106differentmolecularfactoriescanbe

explored.

Cryo-EMandtraditionalX-raytomographyarealterna-

tivetechniquesforstudyingmolecularfactories.Cryo-EM

requiresthinsectionsbecauseofstrongmultiplescattering

ofelectronsinmicron-thickobjects.X-raytomographyis

limitedbyradiationdamage.43Theweakscatteringof

X-rays,thepossibilitytocollectinformationbeforeonset

ofradiationdamage,18andtheabilitytoexplorealarge

numberofheterogeneousobjectsmakeNGLSanunri-

valedinstrumentforinvestigatingmolecularfactories.

Giga-shot digital cellular microscopy with NGLS offers an

unprecedented capability to reconstruct selected regions of

a large number of objects with orientational, conforma-

tional, and configurational degrees of freedom, providing

a route to statistically validated examination of molecular

factories beyond the limits set by radiation damage.

V

Figure68The manifold (in multi-dimensional data space) repre-sents the information from the entire ensemble of ~1010 particles, spanning all orientations, conformations and configurations (topologies). To reconstruct the 3D structure of all conformations and configurations, one zooms in to a region of interest. Navigating along the manifold corresponding to the selected region allows one to image all such regions sampled during the experiment, and thus reach statistically validated conclusions on heterogeneous systems.

Figure69Hydrated bio-particle jet for interaction with the FEL beam. The jet provides a controlled chemical environment, e.g. for living cells or membrane proteins. A coaxial high-pressure gas sheath focuses the entrained liquid from a nozzle large enough to avoid clogging. The concentration of the particles is arranged to ensure 100% hit rate — with each FEL pulse strikes one particle, or several particles in the case of fXS. (From DePonte et al.13)

99


10. Howells, M.R., et al., Attempt to demonstrate collection of biological

solution SAXS data suitable for 3D structure determination by spatial

correlation averaging. 2009, ESRF: Grenoble.

11. Saldin, D., et al., Structure of isolated biomolecules obtained from ultra-

short x-ray pulses:exploiting the symmetry of random orientations. J.

Phys.: Condens. Matter, 2009. 21: p. 134014.

12. Cho, H.S., et al., Protein structural dynamics in solution unveiled via 100-

ps time-resolved x-ray scattering. Proceedings of the National Academy

of Sciences, 2010. 107(16): p. 7281-7286.

13. DePonte, D.P., et al., Gas dynamic virtual nozzle for generation of micro-

scopic droplet streams. Journal of Physics D: Applied Physics, 2008.

41(19): p. 195505.

14. Chapman, H.N., X-ray imaging beyond the limits. Nat Mater, 2009. 8(4): p.

299-301.

15. Frank, J., Three-Dimensional Electron Microscopy of Macromolecular

Assemblies, in Three-Dimensional Electron Microscopy of

Macromolecular Assemblies. 2006, Oxford University Press: New York.

16. Solem, J.C. and G.C. Baldwin, Microholography of living organisms.

Science, 1982. 218(4569): p. 229-235.

17. Neutze, R., et al., Potential for biomolecular imaging with femtosecond

X-ray pulses. Nature, 2000. 406(6797): p. 752-757.

18. Gaffney, K.J. and H.N. Chapman, Imaging atomic structure and dynamics

with ultrafast X-ray scattering. Science, 2007. 316(5830): p. 1444-1448.

19. Fung, R., et al., Structure from fleeting illumination of faint spinning

objects in flight. Nature Phys., 2009. 5: p. 64-67.

20. Ludtke, S.J., et al., De Novo backbone trace of GroEL from single particle

electron cryomicroscopy structure. Structure, 2008. 16(3): p. 441-448.

21. Fischer, N., et al., Ribosome dynamics and tRNA movement by time-

resolved electron cryomicroscopy. Nature, 2010. 466: p. 329-333.

22. Scheres, S.H.W., et al., Disentangling conformational states of macro-

molecules in 3D-EM through likelihood optimization. Nature Methods,

2007. 4: p. 27-29.

23. Brink, J., et al., Experimental verification of conformational variation of

human fatty acid synthase as predicted by normal mode analysis.

Structure, 2004. 12(2): p. 185-191.

24. Yu, I.M., et al., Structure of the immature dengue virus at low pH primes

proteolytic maturation. Science, 2008. 319: p. 1834-1837.

25. Levin, E.J., et al., Ensemble refinement of protein crystal structures:

Validation and application. Structure, 2007. 15(9): p. 1040-1052.

26. Shaw, D.E. et al. Atomic-level characterization of the structural dynam-

ics of proteins, Science 2010. 330: p.341-346

27. Schwander, P., et al., Mapping the conformations of biological assem-

blies. New J. Phys., 2010. 12: p. 1-15.

28. Nadler, B., et al., Diffusion maps, spectral clustering and reaction coordi-

nates of dynamical systems. Appl. Comput. Harmon. Anal. , 2006. 21: p.

113-127.

unprecedentednewinformationonsampleheterogene-

ity.27Thisisincontrasttosingle-molecule“diffractand

destroy”asoriginallyproposed,17,18inwhichtheorienta-

tionofthesamplemustbedeterminedonashot-by-shot

basis(settingalowerlimitontherequiredfluxperpulse),

whileatthesametimetryingtoavoiddisruptingthemolec-

ularstructureduringthepulseduration(settingastrict

upperlimitonthefluxperpulseand/orpulseduration).

DiffractiveimagingstudiesatNGLSwillexploitthetre-

mendousadvancesinexperimentalcapabilitiesthathave

beenbroughaboutbyfirst-generationX-rayFELs.Inpar-

ticular,asshowninFigure69,DoakandSpenceetal.,

havedevelopedaliquid“aerojet”injectorforgenerating

a1MHzstreamof~1micronsizeliquiddropletsatveloci-

tiesof~10m/s.13Thishasbeensuccessfullydemonstrat-

ed in initial experiments at LCLS (using protein

nano-crystalsinsolution).Thetemporalstabilityanduni-

formpulsespacingderivedfromNGLSsuperconducting

linacoperatinginCWmodewillbeessentialforsynchro-

nizationwithfuturehigh-speedaerojetinjectors.

References:

1. M. Andersson, et al., Structure, 2009. 17: p. 1265-1275.

2. Putnam, C.D., et al., X-ray solution scattering (SAXS) combined with

crystallography and computation: defining accurate macromolecular

structures, conformations and assemblies in solution. Quarterly Reviews

of Biophysics, 2007. 40(3): p. 191-285.

3. Kam, Z., Determination of Macromolecular Structure in Solution by

Spatial Correlation Averaging. Macromolecules, 1977. 10(5): p. 927-934.

4. Kam, Z., M.H. Koch, and J. Bordas, Fluctuation x-ray scattering from bio-

logical particles in frozen solution by using synchrotron radiation. Proc.

Natl. Acad, Sci. USA, 1981. 78(6): p. 3559-3562.

5. Saldin, D.K., et al., Beyond small-angle x-ray scattering: Exploiting angular

correlations. Phys. Rev. B, 2010. 81: p. 174105.

6. Saldin, D., et al., Crystallography without crystals: Structure from diffrac-

tion patterns of randomly oriented molecules, in Coherence 2007. 2007:

Asilomar, USA.

7. Shneerson, V.L., A. Ourmazd, and D.K. Saldin, Crystallography without

crystals. I. The common-line method for assembling a three-dimensional

diffraction volume from single-particle scattering. Acta Cryst. A, 2008. 64:

p. 303-315.

8. Saldin, D.K., et al., Structure of a single particle from scattering by many

particles randomly oriented about an axis: toward structure solution

without crystallization. New Journal of Physics, 2010. 12: p. 14.

9. Saldin, D.K., et al., New light on disordered ensembles: Ab-initio struc-

ture determination of one particle from scattering fluctuations of many

copies. Phys. Rev. Lett., 2010: p. inpress.

100


37. Moths, B. and A. Ourmazd, Bayesian algorithms for recovering structure

from single-particle diffraction snapshots of unknown orientation: a

comparison. http://arxiv.org/abs/1005.0640, 2009.

38. Elser, V., Noise limits on reconstructing diffraction signals from random

tomographs. IEEE Trans Information Theory, 2009. 55(10): p. 4715 - 4722

39. Nugent, K.A., Coherent methods in the X-ray sciences. Advances in

Physics, 2010. 59(1): p. 1 - 99.

40. Marchesini, S., et al., Massively parallel X-ray holography. Nat Photon,

2008. 2: p. 560-563.

41. Harary, F. and E. Palmer, G., Graphical Enumeration. 1973, New York:

Academic Press. 271.

42. Sloane, N.A.J., Online encyclopedia of integer sequences, http://oeis.

org/.

43. Le Gros, M.A., G. McDermott, and C.A. Larabell, X-ray tomography of

whole cells. Current Opinion in Structural Biology 2005. 15(5): p. 593-600.

44. Loh, D.N. et al, Cryptotomography: reconstructing 3D Fourier intensities

from randomly oriented single-shot diffraction patterns, (2010) Phys. Rev.

Lett. 104: 225501

29. Giannakis, D., et al., The symmetries of image formation by scattering.

http://arxiv.org/abs/1009.5035, 2010.

30. Tenenbaum, J.B., V.d. Silva, and J.C. Langford, A global geometric frame-

work for nonlinear dimensionality reduction. Science, 2000. 290(5500): p.

2319-2323.

31. Roweis, S.T. and L.K. Saul, Nonlinear dimensionality reduction by locally

linear embedding. Science, 2000. 290(5500): p. 2323-2326.

32. Coifman, R.R., et al., Geometric diffusions as a tool for harmonic analysis

and structure definition of data: Diffusion maps PNAS, 2005. 102(21): p.

7426-7431.

33. Belkin, M. and P. Niyogi, Laplacian eigenmaps for dimensionality reduc-

tion and data representation. Neural Computation, 2003. 15(6): p. 1373-

1396.

34. Coifman, R.R. and S.e. Lafon, Diffusion Maps. Appl. Comput. Harmon.

Anal., 2006. 21: p. 5-30.

35. Coifman, R.R., et al., Reference free structure determination through

eigenvectors of center of mass operators. Applied and Computational

Harmonic Analysis, 2010 28(3): p. 296-312.

36. Hu, B.L., Scalar waves in the mixmaster universe. I. The Helmholtz

Equation in a fixed background. Phys. Rev. D, 1973.8: p. 1048-1060.

101

4 . NEW TECHNIQUES ENABLED NY NGLSX-RAY IMAGING: FROM HIGH RESOLUTION TO HIGH SPEED

andparallelworkinvisiblelightopticsbyJimFeinup2in

phasereconstructionalgorithms,thefirstpracticaldem-

onstrationofreconstruction,appliedtoanano-fabricated

2Dpatternofgolddots,wasmadejustoveradecade

ago.3Thesubjectoflenslessimaginghasdevelopedrap-

idlyoverthelasttenyearsandareviewofthecurrent

stateofthearthasrecentlybeenpublished.4

TheoriginalformofcoherentX-raydiffractionmicros-

copyhasbeenappliedtomanysystems,andherewe

illustratethetechniquewithapplicationtoimagingatan-

talum oxide nanofoam.5The general arrangement is

showninFigure70.Thesampleisplacedonathintrans-

mitting membrane and monochromatic transversely

coherentX-raysdiffractfromthesampleandtheresult-

ingdiffractionpatternisrecordedonaCCDcamera.The

realexperimentalsituiationismorecomplexasthedirect

unscatteredradiationhastobeblockedwithastopand

usuallymultipleexposureshavetobetakenatdifferent

intensitylevelstoextendthedynamicrangeofthedetec-

tortocoverthefullrangenecessary,ie.fromtheintense

lowqtoweakhighqranges.

Compared to lens-based transmission microscopy

suchasTXM,thetechniqueeliminatesthelowefficiency

ofaprojectorzoneplateandisnotlimitedinresolution

bythecharacteristicofanX-raylens.Furthermore,unlike

alens-basedsystem,thereisnointrinsiclimitonresolu-

tionsetbydepthoffieldin3Dimaging.Havingobtained

thediffractionpattern,inversiontorealspaceisaccom-

plishedbasedonaniterativephasereconstructionmethod,

whichusesasaconstraintsomeknowledgeoftheobject,

suchasitsphysicalextent.Thissupportfunctioninthe

reconstructioncanbesetbyphysicalpriorknowledgeor

dynamicallyduringreconstructionfromathresholded

autocorrelation.6Thesetechniqueswereappliedinthe

caseofthenanofoamexamplegivenaboveforarangeof

samplerotationangles,toaccomplishafull3Drecon-

struction.Arenderingofthe3Dreconstructionandalocal

regionofthesampleareshowninFigure71.Theresolu-

tioninthestructureshownextendstoaround15nm.

Thisformofdiffractionmicroscopysuffersfromsev-

eralproblems,themostseverebeingthattheobjectmust

beisolated.Toavoidthisissue,anewformoflensless

microscopy,ptychography,hasrecentlybeendeveloped.7

AdefininingcharacteristicofNGLSisthecombination

ofhighpeakpowerandhighaveragecoherentX-ray

power.Thesefeatureswilldramaticallyadvancemany

formsofX-rayimagingoverthatpossiblewithconven-

tional3rdgenerationsynchrotronsources.Hereweout-

line two general areas: first, diffractive imaging of

structures at the nanoscale; and second, continuous

imaging of chemically evolving systems at the mac-

roscale.Manyotheradvancesbetweentheseextremes

willalsobeenabledbyNGLS.

4.2.1 Chemically Specific Nanoscale Imaging

NGLSwillprovideapproximatelyfiveordersofmagni-

tudehigheraveragebrightnessand10ordersofmagntitude

higherpeakbringhtnessthana3rdgenerationundulator

sourcesofsynchrotronradiation.Thiswillenableanew

generationofnanoscaleimagingbasedontheenormous

increaseincoherentpowerwewillhaveavailable.

HighresolutionX-rayimagingwasuntilrecentlybased

onzoneplateoptics,either inScanningTransmission

X-ray Microscopes (STXM) or inTransmission X-ray

Microscopes(TXM).Formsofthesetypesofmicroscopes

mostlikelywillbeusedattheNGLSdependingontheir

matchtoparticularexperimentalneeds.Forexamplethe

STXMhasmanyuniquefeatures—suchastheabilityto

use fluorescence detection, valuable in speciation of

dilute components using fluorescence NEXAFS.

However,oneofthemostexcitingprospectsistheuseof

lenslessformsofimaging,basedoncollectionandinver-

sionofcoherentdiffractionpatterns.Fromthepioneering

workofDavidSayre,whofirstrecognziedthepossibility

forphasingdiffractionpatternsofcontinuousobjects,1

Figure70Geometry for coherent X-ray diffractive imaging. (From Barty et al.5)

4.2 X-rayImaging:FromHighResolutiontoHighSpeed

102


4.2.2 Cinematic Chemically Specific Macroscale Imaging

Aparalleldevelopmentenabledbytheveryhighaver-

agepowerofNGLSistoperform3Dimagingwithchemi-

calspecificityoverarangeofobjectsizestypicallyupto

themacroscopicsizescaleofmm,withcontinuousobser-

vationofachemicalorstructuralprocess.Applications

range from analysis of combustion chemistry (as

describedinSection3.3),appliedtofueljetsandflames,

analysisofflowthroughporousmediatopolymerpro-

cessing.Thedistinguishingchallengeinthisareaisthat

theobjectisreactive,andisconstantlychangingin3D,

i.e.chemicalreactionsaretakingplacethroughoutavol-

ume.Herewedescribetwomethodsforattackingthis

problem using relatively conventional tomographic

methods,combinedwiththechemicalsensitivityafford-

edbyX-rayabsorptionandfluorescence,andtheunprec-

edentedaveragebrightnessavailablefromNGLS.

X-ray3Dimagingisnormallyachievedthrough

projectionmethods,whereinthesimplestform,direct

transmissionofabeamisimagedontoanX-raysensitive

detector,anddifferentviewsareachievedbysimplerota-

tionoftheobjectaboutoneaxis.Fordynamicallyevolving

objects,onenormallycannotusethissamemethod,due

totheproblemofcoveringanadequatenumberofviews

inatimeshortcomparedtothecharacteristictimescale

fortheevolutionofthedynamicprocess.Inopticalimag-

ing,theproblemhasbeensolvedinavarietyofways,for

exampleconfocalimaging,two-photonconfocal,struc-

turedilluminationimaging,butallthesemethodsrelyin

somewayonveryhighnumericalapertureopticsthat

unfortunatelyarenotavailableintheX-raydomain.

The simplest method of fixed object tomographic

imagingisshownschematicallyinFigure72.Inthismeth-

od,lightiscollimatedintoathinsheet.Itintersectswith

theobjectandlightisgeneratedbyscatteringorfluores-

cenceina2Dplane.Thisplaneisthenimagedontoa2D

detector.Thesheetisthenscannedthroughtheobjectin

adirectionperpendiculartothesheet.Theopticalequiva-

lentof this iswell knownasSelectivePlane Imaging

Microscopy(SPIM).11-13 Inthiscasethe imagingoptic

(imagingthefluorescenceontothedetector)issimplya

highNAlens,andthedetector isaCCD. Ithasmajor

advantagesoverconventional laser-scannedconfocal

microscopyinthatitisfast,andcanbeusedtooptically

sectionverythickobjects.DynamicimagingwithX-rays

Inthiscase,aprobebeamisformedintoarelativelylarge

spotbyanapertureorzoneplatelens,andscannedin

overlappingregionsacrossasamplesothatateachpoint

adiffractionpatterniscollected.

Theoverlapturnsout togiveapowerfulnewcon-

straint in the iterativephase reconstruction,with the

practicalresultthatuniquenessisguaranteed,conver-

gencecanbethousandsoftimesfasterthanconventional

diffraction microscopy, and extended objects can be

imaged.TheoriginaldemonstrationbyRodenburgand

colleagues7hasbeenrefinedtoincludeanintermediate

stepintheiterativereconstructionthatdeterminesthe

probebeamdistributionandfurther improvesresolu-

tion.8LikeconventionalX-raydiffractionmicroscopy,this

morerefinedversionhasrecentlybeendevelopedintoa

3Dimagingmethod,byadditionofsamplerotationso

thattomographicdatasetscanberecordedandrecon-

structed.9

Diffractionmicroscopyasdescribedabove is in its

infancy,buthasalreadyshownthatithasrevolutionary

advantages over conventional X-ray microscopy.

HoweveroneofthemainlimitationsofallX-raymicros-

copywithcurrent3rdgenerationsourcesisthelimited

spatiallycoherentflux.Thisiscombinedwiththefactthat

therequiredfluxscalesastheinverseofthe4thpowerof

the resolution.10This makes high resolution imaging

techniquesslowandsetsapracticallimitofaround10nm

formostimagingmethods.Theenormousaveragecoher-

entfluxadvantagesoftheNGLSmeansthatwewillbe

abletoobtainresolutionclosetothefundamentallimit

setbythewavelength.Inaddition,thehugeincreasein

peakcoherentfluxopensupmanyofthesemethodsin

imagingdynamicprocessesatthenanoscale.

Figure71 Reconstruction of a tantalum oxide nanofoam material; the left panelshows a rendering of the whole 3 micron object and the right panel shows a 0.5 μm segment of the structure. (From Barty et. al.5)

103


bon,nitrogenandoxygenK-edgeregions,stateoftheart

multilayercoatedmirrorsnowachieveareflectivityof

~10%.15Inthesimplestarrangement,theincidentphoton

energywouldbetunedtoaparticularX-rayabsorption

featureintheNEXAFSregion,forexample,aparticular

molecularorbital feature foradefinedspecies in the

productsfromaflame,andthemultilayerimagingdevice

wouldberequiredtointegrateovertheX-rayfluores-

cencespectrumtolowerenergy.Inordertodothis,the

multilayermightbemadeslightlyaperiodictowidenits

naturalbandwidth.Inthehigherenergyregime,imaging

insuchafashionwillbedifficult,andforimagingthicker

objectswewouldneedtoresorttocomputationalimag-

ingmethods,suchascodedaperturemethods.

Thepromisingapproachofcodedaperatureimaging

iswelldevelopedandhasbeenappliedtoawiderangeof

X-ray imaging applications from X-ray astronomy to

medicalimaging.16-18Codedaperatureshavealsobeen

usedasmultipleobjectreferencesinX-rayholography.19

InsteadofadevicethatfocusesX-raystoanimageplane,

thecodedaperturesimplyconsistsofaseriesofpinholes

withknownlocation.Theimagefromeachpinholeinthis

multiplepinholecameraarrangementfallsonanX-ray

sensitivedetector.Asthenumberofpinholesincreases,

thedetectorsignalincreases,buttheimagesformedby

eachpinholeatsomepointcompletelyoverlap.However,

sincethelocationofeachpinholeisknown,theimage

canbecomputationallyrecovered.Thekeytodoingthis

inthemostefficientwayintermsofsignal-to-noiseratio

istouseaUniformlyRedundantArray(URA)whichhas

thefeaturethatit’scrosscorrelationisadeltafunction,

resultinginasinglepixeleffectivepointspreadfunction.

Togetfromthedetectorimagetotherealspaceimage,a

reconstructionmaskiscreateddirectlyfromtheURAand

thecyclicconvolutionofthemaskwiththedatayieldsthe

realspaceobject.

The stationary object tomographic reconstruction,

therefore,happensinseveralstages:(1)theexcitationis

confinedtoasheetatonetransverselocation,(2)theURL

transmitsanimagetothedetector,(3)theimageiscom-

putationally inverted, and (4) the sheet is translated

acrosstheobject.Thistranslationwouldbesynchronized

withthetimestructureoftheNGLS.Forchemicalimag-

ing,thephotonenergywouldbechosenbasedonfea-

turesintheNEXAFSdatathatprovideafingerprintofa

particularchemicalstate.Severalphotonenergieswould

havetobeusedforthispurpose.Aminimumwouldbe

presents two additional challenges: first, we need to

movetheX-raybeamrapidlyacrosstheobject;andsec-

ond,weneedtoimagetheX-raysoverrelativelylarge

objectsathighnumericalaperture.

InthecaseofNGLS,wecanaccomplishtheformerby

verysmallangularmodulationofaplanemirror.The

sheetisthencreatedasalowapertureconvergentbeam

focusedinoneplaneintothesample.Duetothediffrac-

tion-limitednatureofthebeam(formicron-thicksheets

scanningovermm-scaleobjects)thedeflectionmirror

canbemanymetersupstreamofthesample,implying

verysmalldeflectionangles.Duetotheextremecollima-

tionoftheNGLSX-raylasers,thebeamsizeonthemirror

willbesmall,henceitcouldbelight,andthereforecou-

pledtoahighfrequencyresonantdeflectionstructure.In

thisway,itwillbepossibletomovethebeambyaresolu-

tionelementperpulseoftheFEL(i.e.resolutionelement

per1-10μs).Othermethodsalsoexistforthisdetection

systemincludinguseofacousto-opticallygeneratedgrat-

ings;theperiodwouldbechangedenoughtosweepthe

beamovertheobject.

Toaddressthesecondissue,thetypeofimagingoptic

showninFigure72(labeledURA)needstobetailoredto

theparticularapplication.ForimagingsoftX-rayK-edge

fluorescence,forexample,thedevicecouldbeanormal

incidencemirror,orCassegrainmirrorpair.14Inthecar-

Fuel spray(reactive flow)

Detector

X-rays

URA

Translating sheet

Figure72Fixed object tomography using a transversely scanned sheet.

104


4. Chapman, H.N. and K.A. Nugent, Coherent lensless X-ray imaging.

Nature Photonics, 2010. 4(12): p. 833-839.

5. Barty, A., et al., Three-Dimensional Coherent X-Ray Diffraction Imaging

of a Ceramic Nanofoam: Determination of Structural Deformation

Mechanisms. Physical Review Letters, 2008. 101(5): p. 055501.

6. Marchesini, S., et al., X-ray image reconstruction from a diffraction pat-

tern alone. Physical Review B, 2003.68(14): p. 140101.

7. Rodenburg, J.M., et al., Hard-X-Ray Lensless Imaging of Extended

Objects. Physical Review Letters, 2007. 98(3): p. 034801.

8. Thibault, P., et al., High-Resolution Scanning X-ray Diffraction

Microscopy. Science, 2008. 321(5887): p. 379-382.

9. Dierolf, M., et al., Ptychographic X-ray computed tomography at the

nanoscale. Nature, 2010. 467(7314): p. 436-439.

10. Howells, M.R., et al., An assessment of the resolution limitation due to

radiation-damage in X-ray diffraction microscopy. Journal of Electron

Spectroscopy and Related Phenomena, 2009. 170(1-3): p. 4-12.

11. Huisken, J., et al., Optical Sectioning Deep Inside Live Embryos by

Selective Plane Illumination Microscopy. Science, 2004. 305(5686): p.

1007-1009.

12. Swoger, J., J. Huisken, and E.H.K. Stelzer, Multiple imaging axis micros-

copy improves resolution for thick-sample applications. Opt. Lett., 2003.

28(18): p. 1654-1656.

13. Verveer, P.J., et al., High-resolution three-dimensional imaging of large

specimens with light sheet-based microscopy. Nat Meth, 2007. 4(4): p.

311-313.

14. Walker, A.B.C., et al., Soft X-ray Images of the Solar Corona with a

Normal-Incidence Cassegrain Multilayer Telescope. Science, 1988.

241(4874): p. 1781-1787.

15. Eriksson, F., et al., 14.5% near-normal incidence reflectance of Cr Sc

x-ray multilayer mirrors for the water window. Opt. Lett., 2003. 28(24): p.

2494-2496.

16. Fenimore, E.E., Coded aperture imaging: the modulation transfer function

for uniformly redundant arrays. Appl. Opt., 1980. 19(14): p. 2465-2471.

17. Fenimore, E.E., Time-resolved and energy-resolved coded aperture

images with URA tagging. Appl. Opt., 1987. 26(14): p. 2760-2769.

18. Caroli, E., et al., Coded aperture imaging in x and gamma ray astronomy.

Space Science Reviews, 1987. 45: p. 349-403.

19. Marchesini, S., et al., Massively parallel X-ray holography. Nat Photon,

2008. 2(9): p. 560-563.

20. Candes, E.J. and M.B. Wakin, An introduction to compressive sensing.

IEEE Signal Processing, 2008. 21.

two:oneonaparticularNEXAFSfeature,andonewell

abovetheedgetoprovideforlocalnormalizationofden-

sity.Thiswillallowchemicalspeciationatclosetothe

maximumrepetitionrateoftheNGLS.Ifthephotonenergy

canbescannedthroughtheNEXAFSregionforeachobject

slice,theoverall3Dchemicalvolumemappingwillbe

increasedbyafactoroftypically20.Thiswillbesetbythe

rateatwhichanNGLSX-raybeamenergycanbemodu-

lated.Akeypointisthattheoverallprocesscanberela-

tivelyefficient,andcombinedwiththeveryhighflux/

pulseandthehighrepetitionrate,highspeed3Dchemical

imagingwillbecomepossibleforthefirsttime.

Theabovesectiondescribesfastacquisitionoftomo-

graphicdatausingthehighrepetitionrateofNGLSto

advantageforobjectsthatmustremainstationary.The

spatialresolutionofsuchasystemismodest,~5μmfor

largeobjects,setbythe2Dsheetwidthandthecoded

aperture.To go beyond this, a form of multiple-view

tomographywillbeemployedthatinvolveseitherdirect

transmission and projection, possibly with an X-ray

imagingsystemthatmagnifies,asinnormaltransmis-

sionX-raymicroscopy,ora formofmulti-view-angle

lenslessmicroscopy.Thetechnicalchallengeistosplitthe

incidentX-raybeamintomanybeamlets,deflectthem

throughthesampleatdifferentangles,anddetecteach

beamlet separately.The number of views required

depends strongly on the sparseness of the object.

Methodsbasedoncompressivesensingmaybeusefulin

reducing thenumberofviews required toapractical

value.20Workisunderwaytodesignapracticalimple-

mentationofamulti-viewsystemthatcouldbeusedwith

NGLSX-raylasers.

References:

1. Sayre, D., Imaging processes and coherence in physics, A.e.a.

Schlenker, Editor. 1980, Springer. p. 229-235.

2. Fienup, J.R., Reconstruction of an object from the modulus of its Fourier

transform. Opt. Lett., 1978. 3(1): p. 27-29.

3. Miao, J., et al., Extending the methodology of X-ray crystallography to

allow imaging of micrometre-sized non-crystalline specimens. Nature,

1999. 400(6742): p. 342-344.

105

4 . NEW TECHNIQUES ENABLED NY NGLSMULTIDIMENSIONAL X-RAY SPECTOGRAPHY

complexconsistsofsevencoherently-coupledexciton

states.Themulti-dimensionalspectroscopymap(Figure

73,lowerright)showsthecouplingbetweentheseexci-

tonstates—asnapshotofthecoherentandincoherent

transferofchargepopulationbetweenthesevenstatesat

adelayof1000fsdelayfollowingtheinitialexcitation.

4.3.1 X-ray Multi-Dimensional Spectroscopy

IntheX-rayregion,thetremendouspromiseofmulti-

dimensionalspectroscopyliesinthecapabilitytofollow

coherentchargeflowandenergyrelaxationonfunda-

mental (attosecond to femtosecond) timescaleswith

accesstothefullrangeofvalencestates(unrestrictedby

dipoleselectionrules).Importantly,theelementspecifici-

typrovidedbyX-rays(tunedtocore-levelabsorptions)

willenableusforthefirsttimetofollowchargeandenergy

flow between constituent atoms in materials.These

essentialcapabilitiesarenotattainableusinginfraredor

visiblelaserpulses,andwillprovidecriticalinsighttocor-

relatedelectronsystemsandmolecularcomplexeswith

strongcouplingbetweenelectronicandnucleardynam-

ics. Figure74 illustrates twomulti-dimensionalX-ray

spectroscopyschemes:core-holecorrelationspectroscopy,

andstimulatedX-rayRamanspectroscopy.

4 .3 .1 .1 Multi-Dimensional Spectroscopy —

X-ray Core-hole Correlation

Multi-dimensionalcore-holecorrelationspectroscopy

(Figure74b)3,6isessentiallytwo-dimensionalelectronic

spectroscopyperformedinthesoftX-rayregime.Itexploits

nonlinearinteractionswithcoherent,attosecondX-raypuls-

estoprobecorrelationeffectsbetweenpairsofvalence

electronsexcitedatdifferentatomicsitesinamolecule.

Figure75presentsanexampleandoutlinesthetheo-

reticalbasisforcore-holecorrelationspectroscopy.Here,

anaminophenolmoleculeinteractswithtwoattosecond

pulses,onecenteredat400eV(ωN)andtheotherat535

eV(ωO),inresonancewiththe1scoreexcitationsinNand

Oatoms,respectively.Inthecoherentfour-wavemixing

process,thetargetmoleculeinteractswiththreeX-ray

pulsesseparatedbytimest12andt13andemitsafourth

Overthepastseveraldecades,2ndand3rdgeneration

synchrotronsourceshaveenabledthedevelopmentofa

widerangeof incisive linearprobesof theelectronic,

atomic, and chemical structure of matter. Examples

includeX-rayemissionspectroscopy,X-rayabsorption

spectroscopy,andinelasticX-rayscattering,tonamejust

afew.AsaMHzX-raylaser,theuniquecapabilitiesof

NGLSwillopentheentirelynewfieldsofnonlinearX-ray

scienceandmulti-dimensionalX-rayspectroscopy.

Multi-dimensionalX-rayspectroscopyincorporates

time-orderedsequencesofX-raypulsestogeneratea

signalthatisafunctionofmultipletimedelaysand/or

photonenergies.Thesearenonlinearcoherentwavemix-

ingtechniquesinwhichX-raypulsesareusedasbotha

pump,topreparespecificnear-equilibriumstatesofmat-

ter,andasaprobeoftheseevolvingstates.Thesenew

toolsrelyonsimultaneouscombinationsof:highpeak

power,highaveragepower(highrepetitionrate),spatial

coherence,temporalcoherence,andtunability.

Radio Waves (NMR) ⇒ Infrared ⇒ Visible ⇒ X-rays

Theanalogoustechniqueofnuclearmagneticreso-

nance(NMR)illustratesthetremendouspotentialimpact

ofmulti-dimensionalX-rayspectroscopy.NMRincorpo-

ratessequencesofradio-frequencypulsestogeneratea

two-dimensionalsignal-map(Fouriertransformofthe

timeintervalsbetweenpulses)thatisafingerprintofspe-

cificchemicalstructures,andtheirrelationshipwithina

molecularcomplex.Overthepastdecade,multi-dimen-

sionalspectroscopy(enabledbyultrafastlasersources)

hasbeenextendedtotheinfrared1-3(toprovideafinger-

printofthecouplingbetweendifferentvibrationalmodes

inamolecule)andtothevisibleregime3-5(tomapthe

dynamiccouplingbetweenelectronicstates).Thesetech-

niqueshavebecomeinvaluableforfollowingquantum

coherencesandcharge relaxationbetweenelectronic

statesinsystemsrangingfromchlorophyll(responsible

forlightharvestinginphotosynthesis)toexcitonicstates

insemiconductors(forareview,seeReference3).

Figure73 illustratesamulti-dimensionalelectronic

spectroscopymeasurementofthebacteriochlorophyll

photosynthetic reactioncenter.5 This light-harvesting

4.3 MultidimensionalX-raySpectroscopy

106

4 . �NEW�TECHNIQUES�ENABLED�NY�NGLSMULTIDIMENSIONAL�X-RAY�SPECTOGRAPHY

An important criterion for core-level correlation spec-

troscopy is that the X-ray pulse durations must be faster

than the Auger decay time (~5 fs), since Auger decay sup-

presses the correlation signal of interest.

4.3.1.2 Multi-DimensionalSpectroscopy—Stimulated

X-rayRaman

A complement to core-hole correlation spectroscopy

is stimulated or Coherent X-ray Raman Spectroscopy

(CXRS).7 Whereas conventional optical Raman spectros-

copy techniques exploit visible or infra-red laser fields to

probe lower-frequency vibrational resonances in matter,

pulse with temporal profile S(t, t13, t12). The two-dimen-

sional Fourier transform of this signal with respect to t12

and t13 yields a two-dimensional electronic spectrum in

frequency space. Off-diagonal features in this 2D spec-

trum are present only when there is correlation between

the two excited valence electrons on the N and O atoms;

no signal should be seen in the Hartree-Fock limit of inde-

pendent orbitals. Calculations show that the extent of this

correlation depends not only on molecular structure (i.e.,

it differs in ortho- and para- aminophenol), but also on the

nature of the molecular orbitals excited within the energy

envelopes (∼10 eV) of ωN and ωO.6

0.4

0.3

0.2

0.1

0.0

1

12,000

Frequency (cm-1)

Abso

rptio

n (O

D)

12,300 12,600

2 3 4 5 6 776

543

2

1

1 2 3 4 5 6 7

-12,000 -12,300 -12,600

T = 1000 fsωτ(

cm-1

)

ωτ(cm-1)

Signalamplitude

0.8

0.4

0

-0.4

-0.8

12,600

12,300

12,000

C

B

A

D

Figure73�Top:�Generalized�schematic�of�multi-dimensional�electronic�spec-troscopy�using�a�four-wave�mixing�geometry�with�a�three-pulse�sequence�(k1,�k2,�k3).�The�signal�of�interest�is�the�nonlinear�polarization P(3)

sig�—�shown�here�resolved�in�phase�and�amplitude�via�heterodyne�detection�with�a�local-oscillator�field�ELO.�Bottom:�2D�elec-tronic�spectra�snapshot�(at�1000�fs�delay,�t13)�of�bacteriochlorophyll�pho-tosynthetic�reaction�center5�which�consists�of�seven�coherently-coupled�exciton�states�(lower�right).�The�two�(energy/frequency)�axes�of�the�2D�spectrogram�are�the�Fourier�variables�corresponding�to�the�delay�t12,�and�the�delay�between�k3�and�ELO.

a b c

11

Valance-excitedstates

2 Core-excitedstates

Core levels(e.g. 1s or 2p)Atom 1 Atom 2

Atom 1 Atom 2

Coupledvalencestates

2

1

1

2

2

Figure74Multi-dimensional�spectroscopy�schemes�using�sequences�of�two-color�puls-es�(A).�(B)�Illustrates�core-hole�correlation�spectroscopy�in�which�resonant�core-level�excitation�of�two�atoms�is�used�to�probe�the�coupling�between�their�respective�valence�states�f1�and�f2.�(C)�Illustration�of�stimulated�or�Coherent�X-ray�Raman�Spectroscopy�(CXRS)�in�which�localized�valence�excita-tions�<f1|g1>�and�<f2|g2>�are�created�and�probed�via�resonant�Raman�processes�at�specific�atoms.�This�approach�creates�a�local�valence�excitation,�and�enables�ele-ment-specific�probing�of�charge�flow.�

107


Thepowerofthismulti-dimensionaltechniqueisthat

itcreatesalocalizedvalenceexcitation,orcoherentelec-

tronicwavepacket,viaastimulatedRamanscattering

processoriginatingfromacorelevel(seeFigure74c).

ThusultrafastX-raypulses,tunedtocoreleveltransi-

tions,provideelementspecificity.Thetimeevolutionof

thewavepackets,andtheflowofvalencechargebetween

differentatomicsites,canthenbefollowedonfundamental

timescales.Inamulti-dimensionalimplementation,the

initialextitationiscreatedbyapulse-pair,andathird

pulse(inaphase-matchedgeometry)readsoutthescat-

teredRamansignal.TheFouriertransformofthesignal

withrespecttothetimedelaysofthepulsescreatesatwo-

dimensionalmapof thevalenceelectronicstatesand

theirevolution.Importantly,sincethefinalstateisnotcore-

excited,butonlyvalence-excited,multidimensionalsig-

nal-mapscanbemeasuredovermuchlongertimescales

thanarepossiblewithcore-holecorrelationtechniques.

4 .3 .1 .3 Multi-Dimensional Spectroscopy — NGLS

Multi-dimensionalX-rayspectroscopyandnonlinear

X-raysciencewillbehallmarksofNGLSastheyrequire

capabilitiesthatarenotavailablefromanyotherX-ray

source.Highpeak-powerX-raypulsesarejustoneofsev-

eralessentialrequirements.Equallyimportantistheabil-

ity to control the degree of X-ray nonlinearity while

resolvingsmallsignalswithhighfidelity.Highrepetition

rate isabsolutelyessential toachieve this inorder to

avoiddisruptingtheelectronicstates(orothersample

attributes) that are being investigated.An important

benchmarktorecognizeisthatthescientificimpactof

multi-dimensional laser techniqueswas realizedonly

after thedevelopmentofmulti-kHzandMHzultrafast

laser sources.These laserscombinedbothhighpeak

powerandhighaveragepowertoenableextremelysen-

sitivity measurements of controlled near-equilibrium

interactionsoflaserpulsesequenceswithmatter.

Followingisabroaderdescriptionofsomeofthecom-

pelling advantages of investigating valence electron

dynamicsandcorrelatedphenomenaviamulti-dimen-

sionalX-rayspectroscopy:

• Temporal (or phase) information —unavailablefrom

conventionalRIXSmeasurements,whichprobethe

spectraldensity-densitycorrelationfunctionS(q,ω)in

thefrequencydomainbutwithoutphaseinforma-

tion.Powerfulcapabilitiesoftime/phasemeasure-

ments include: (1) distinguishing different

CXRSusesX-raystoprobevalenceexcitationsinmatter

(Figure 74c). One may consider CXRS as a powerful

extension (stimulated version) of spontaneous X-ray

RamanprocessessuchaRIXS(asdiscussedinSections

3.1and3.7).AsastimulatedRamanscatteringprocess,

CXRSmeasuresathird-order,χ(3),four-wavemixingpro-

cesswherebyasequenceofthreeincidentpulses(three

fields):En(kn,ωn)|n=1,2,3,generateastimulatedsignal,e.g.,

Esig(-ω1+ω2+ω3),inthemomentum-matcheddirection,

ksig=-k1+k2+k3.

0

3

2

0 1

1

0

-12 3

1 2 3

O1s XANES

HO

OH

NH2

NH2

ortho

para

3

2

0 1

1

0

-12 3

f

gg

O

gO

t3

t2

t1

eN

eN

eO e

N

f

Figure75Top left: para and ortho isomers of aminophenol, and the predicted corresponding 2D X-ray core-hole correlation maps. Top right: Valence and core-excited states of aminophenols. Bottom: double-side Feynman diagram representing one of the contribu-tions to the predicted cross-peak of the 2D X-ray signal map. (From Reference 6)

108


• Quantum selectivity —pulsesequencesandmomen-

tummatchingallowonetoeffectivelyisolatespecific

termsofthecontributingLiouville-spacepathwaysthat

comprisethetheoreticaldescriptionofcoupledquan-

tumsystemsbasedonatime-dependentperturba-

tionapproach.Thisselectivitymakesitpossibleto

distinguish forexamplecoherentchargecoupling

fromincoherentchargetransfer,electronicrelaxation

fromexcited-stateabsorptionetc.Incombination

withelementspecificityandultrafasttimeresolution,

thiscapabilitywillconstituteamajorbreakthrough

forunderstandingcorrelatedsystems.

References:

1. Hybl, J.D., et al., Two-Dimensional Electronic Spectroscopy. Chem. Phys.

Lett. , 1998. 297: p. 307-313.

2. Asplund, M.C., M.T. Zanni, and R.M. Hochstrasser, Two-dimensional

infrared spectroscopy of peptides by phase-controlled femtosecond

vibrational photon echoes. PNAS, 2000. 97: p. 8219-8224.

3. Mukamel, S., et al., Coherent Multidimensional Optical Probes for

Electron Correlations and Exciton Dynamics: From NMR to X-rays.

Accounts of Chem. Res., 2009. 42: p. 553-562.

4. Li, X., et al., Many-body interactions in semiconductors probed by optical

two-dimensional fourier transform spectroscopy. Phys Rev. Lett., 2006.

96(5): p. 057406.

5. Brixner, T., et al., Two-dimensional spectroscopy of electronic couplings

in photosynthesis. 2005. 434(7033): p. 625-628.

6. Schweigert, I.V. and S. Mukamel, Coherent ultrafast core-hole correla-

tion spectroscopy: X-Ray analogues of multidimensional NMR. Phys Rev.

Lett., 2007. 99(16): p. 163001.


linear local probe for electronic excitations. Phys Rev. Lett., 2002. 89(4):

p. 043001.

contributionstothedensity-correlationspectraldis-

tribution,e.g.homogeneousversusinhomogeneous

distributionsofcorrelatedstates,and(2)following

emergentpropertiesastheyevolvefromperturbative

non-equilibriumconditionscreatedbytailoredelec-

tronicorvibrationalexcitationsrangingfromtheTHz

totheX-rayrange(e.g.,modulationormanipulation

ofcorrelatedstatesviacoherentvibrationalmodesor

charge-transferexcitations).

• Element and chemical state specificity —essentialfor

understanding the correlation between valence

statesassociatedwithparticularatomicormolecular

orbitals.Forthefirsttimeitwillbepossibletodirectly

followthecoherentflowofvalencechargebetween

differentatomicsitesintime,energy,andspace.This

abilitywillbeextremelypowerfulforunderstanding

mixed-valencemolecularcomplexes,dilute-magnet-

ic semiconductors, multiferroics, charge-transfer

complexes,cuprates(electronicstatescoupledtoCu

orbitalsversusOorbitals),andmuchmore.Although

conventionalRIXSiselementspecific,itisnotableto

distinguishcoherencesacrossdifferentatoms.

• Symmetry selectivity —sensitivitytospecificvalence

states(e.g.,3dversus2p)andthecapabilitytodistin-

guishspinandorbitalmomentsviapowerfulsoft

X-raydichroismeffects.

• Access to the entire manifold of valence momentum

sates —sincethesoftX-rayexcitationwavelengthis

comparabletotheunit-cell/molecularsize(kvector

largecomparedtotheBrillouinzone),thestrictdipole

selectionrulesthatmediateopticaltransitionsare

substantiallyrelaxed.Themomentumspacespan-

ningtheentireBrilliounzonecanbesampledwith

exquisiteresolution.SoftX-raytransitionsfromcore

levelsdirectlyprobeimportantd-dexcitationsthat

areopticallyforbidden.

5 Proposed facility

InthissectionwedescribetheNGLSfacility.TheNGLS

isenvisionedasahigh-powerX-rayfree-electronlaser

(FEL)facilitythatwillbeamachineofunrivaledinitialper-

formanceandoutstandingfuturecapabilities,preemi-

nentinX-raysciencefordecadestocome.InSection5.1

weoutlinetherequiredcharacteristicsandcapabilitiesof

theNGLS,andcomparetocurrentorunder-construction

light sources.Section5.2presents thecapabilitiesof

potentialalternateapproaches,includingothertypesof

accelerator-driven lightsources, laser-basedhigh-har-

monicgeneration,aswellaswhatperformancemaybe

expectedofthosetypesofsourcesinthefuture;weargue

foracontinuouswave(CW)superconductinglinac-based

arrayofFELsastheoptimalmeanstomeetthescience

needsdescribedinSection3.AnoverviewoftheNGLSis

giveninSection5.3,summarizingFELperformance,facil-

itylayoutandacceleratorparameters.Todeliveraworld-

class lightsourcerequiresanaggressivedesign,and

Section5.4beginswithashortreviewofthechallenges

foranX-raylasersuchastheNGLS,andthendescribesin

somedetailourpre-conceptualNGLSdesignthatenables

thesenewcapabilities.

5.1 CapabilityRequirements

5.1.1 Requirements for the NGLS

ThescientificrequirementsdescribedinSection3,can

beoptimallymetbyanarrayoflinac-driven,high-repeti-

tion-rate,X-rayFELs.Thehighpower,coherence,and

thusbrightness,ofX-rayFELsmakethemuniquetools

fortheexplorationofthestructureanddynamicsofmat-

teratfundamentalscalesoflength,time,momentum,

andenergy.

The NGLS approach combines significant recent

advancesinhighbrightnessphotocathodebeamgenera-

tion,accelerationandtransportwithstate-of-the-artsuper-

conductingRF(SCRF)technologyandundulatordesigns

aswellasrevolutionaryconceptsforseededFELopera-

tion.Theuniformpulsespacingathighrepetitionratewill

provideunprecedentedcapabilitiesatstart-up,accommo-

datingmorediverseorchallengingexperimentsthancur-

rentorplannedsourcesand thepotential to leverage

advancesinavarietyoftechnologiesandnewconcepts,

includingseedlasers,superconductingundulators,X-ray

Injector

Linac 0 Linac 1

Harmoniclinearizer

Linac 2Beam spreader Array of

configurableFELs

X-raybeamline

Endstations

High brightness,high repetitionrate electron gun

Laserheater Bunch

compressor

Figure76 Schematic layout of the main components of the NGLS (not to scale).

110

5 . PROPOSED FACILITYCAPABILITY REQUIREMENTS

• Capabilityforrapidpolarizationcontrol

• Multipleindependentbeamlinessupportingalarge

usercommunity

Inthefollowingsections,wepresenttheNGLSbaseline

parametersforonepreliminarydesignpoint,acknowledg-

ingthesignificantflexibilityaroundthesepointparame-

ters.Nootherexistingorproposedfacilitycandeliverthe

combinationofultrafastcapability,highlongitudinalcoher-

ence,andhighaveragepower,togetherwithmulti-user

operability,flexibilityintimestructure(bothrepetitionrate

andpulseduration),andupgradepotentialtoaddboth

additionalcapacity(uptosevenadditionalFELs),andnew

capability(e.g.higherrepetitionrates,higheraverageand

higherpeakX-raypower,longerpulses,higherresolving

power,shorterwavelengthandlongerwavelengthFELs,

tightersynchronization,X-raypulsefeedbackandshaping

possibilities.)Thedesignoptionspresentedherearemeant

toillustratetheexcitingscientificopportunitiespresented

bytheconvergenceofrapidlyadvancingFELtechnology,

superconductingacceleratortechnology,andsophisticated

micro-manipulationofhigh-energy,high-brightnesselec-

tronbeams.Adetailedoptimizationofcost,performance,

andriskhasyettobeperformed,andwillbuildonthebase-

linepre-conceptualdesignpresentedhere.

5.1.2 Capabilities of Present Facilities

HerewebrieflycomparethecapabilitiesofUVtosoft

X-rayresearchtoolsutilizingring-basedsources,High

HarmonicGeneration (HHG)sources,andFELs.Ring-

basedsources, inparticularstorageringsandenergy

recoverylinacs(ERLs),providemodestaveragepower

withlowpeakpower.Theysupporttunabledevicesthat

providephotonpulsesatveryhighrepetitionrates,and

may be effectively considered CW sources for many

applications. Storage ring-based sources are proven

technology,havewell-establishedusercommunities,and

willremainessentialtoabroadrangeofX-rayscience.

ERLsareanemergingtechnologynotyetoperatinginthe

X-rayrange.HHGsourcesprovidemodestaveragepower

inalmosttable-top-sizedsources,andarerapidlydevel-

opingnewcapabilities,althoughtheyarenotyetavail-

ableintheX-rayrange.FELscanprovideveryhighpeak

power,aswellashighaveragepower,andarenowoper-

atingintheX-rayrangeatlowerrepetitionrates.

Existing storage-ring-based, spontaneous X-ray

sourcesproduceamaximumdegeneracyparameter,or

opticsandFELoscillators.Thedistributedmulti-beam

approachallowsforcapacityincreasethroughongoing

growthinend-stations.Thereisalsotremendousoppor-

tunityfortranslatingadvancesinmachinecontroland

operationintoflexibilityandadditionalcapabilityofX-ray

pulsegeneration.

Figure76showsthemajorcomponentsoftheNGLS;

theinjector,laserheater,CWSCRFlinacsections,linear-

izerandbunchcompressionsystems,beamdistribution,

an array of independent FELs, and X-ray beamlines

willeachbedescribedinmoredetailinlatersectionsof

thisdocument.

The NGLS facility will provide many benefits and

advantagesoverexistingandplannedlightsources,and

willultimatelyfeaturethefollowingcapabilities:

• Highpulserepetitionrates(100kHzorhigherateach

experimental endstation), ultimately 100 MHz at

specificendstations,approachingstorageringrepe-

titionrates

• Very high average flux and brightness (several

orders-of-magnitudegreaterthanthird-generation

ringsandfirst-generationFELs,withpeakpowers

of gigawatts and average powers of up to about

100Wperbeamline)

• Pulsedurationsrangingfromhundredsofattosec-

ondstohundredsoffemtoseconds

• HightemporalcoherenceoftheFELoutputpulses

(closetotheFourier-transformlimit)

• Hightransversecoherence(approachingdiffraction

limits)

• Controlofthetimeduration,bandwidth,andotherlon-

gitudinalfeaturesofthepulses(i.e.,thepossibilityof

envelopeshaping,modulation,orstructuring,andulti-

matelyfeedback-basedcontroloftheseparameters)

• Capability for excellent spectral resolving power,

withouttheneedformonochromators

• SynchronizationoftheFELpulsestoaseedlaserorto

other IRor terahertzsources (with jitteror timing

errorsontheorderof1–10fs)

• FEL output wavelengths (including harmonics)

ultimatelyextendingovermorethantwoorders-of-

magnitude,from~10nmto~1.2Å

• Capability forprecision2-colorX-raypump/X-ray

probeexperiments

• Capabilityforprecisionpump/probeexperimentswith

combinedprobesinthesoftX-rayorEUVandpumps

intheUV,optical,IR,THz,orotherbands

111

5 . PROPOSED FACILITYCAPABILITY REQUIREMENTS

overeachindividualX-raypulse.Forexample,10fspulses

with a large number of X-ray photons can only be

achievedwithFELsources.

Normal-conducting,linac-drivenFELsunderrealistic

operatingconditionsarequitelimitedinrepetitionrate,

andassuchtheiraverageX-raypoweriscomparableto

storagerings.Incomparison,thehighrepetitionrateof

theNGLSwillprovidethreeorders-of-magnitudehigher

averagepowerthannormal-conductinglinacfacilitiesof

comparablebeamenergy, threeorders-of-magnitude

greaterpowerthanstoragerings,andsixorders-of-mag-

nitudegreaterpowerthanHHGsources.TheEuropean

XFELwillbebasedoncryogenicsuperconductingaccel-

erator technology, which will provide average X-ray

powersimilartoNGLS,butbyoperatinginaburstmode

with3000microbunchesspacedby200ns,repeatingat

10Hz.Figure77showsacomparisonofthetimestructure

ofring-basedsources,HHGlasers,andFELs.Acompari-

sonofNGLSparameterswiththoseofotherexistingand

plannedFELfacilitiesisshowninTable1.

HHGsourcescurrentlyprovidenJpulsesuptoabout

100eV,with10kHzrepetitionrate.FutureHHGsources

areexpectedtoimproveinrepetitionrateandenergyper

pulse,and reachsoftX-raywavelengths (seeSection

5.2.4).NGLScapabilitiesinprovidinghighaveragepower

ultrafastpulseswillalsodevelop,bothastheseedlaser

technologyadvances(potentially includingtheuseof

HHGforseeding),andaslow-charge,highrepetitionrate

self-amplifiedspontaneousemission(SASE)operationis

implemented(withbunchchargeofafewpC,atpoten-

tially100MHzrateinadedicatedoperatingconfigura-

tion). In this mode of operation, the short electron

bunchesradiateinasingleorfewopticalmodes,produc-

ing intensecoherentradiationofafewfemtoseconds

duration,andatextremelyhighrepetitionratelimitedby

thetotalelectronbeampower(installed1.8MWcapabili-

ty).Producing~108photonsperpulseat1keV,anaver-

agepowerof~2Wwouldbeproduced(threeordersof

magnitudegreaterthanthatprojectedforHHGsources).

5.2 AlternateApproaches

Inthissectionwediscussfouralternateapproachesto

generatinghighaverage-powerultrafastsoftX-raypuls-

esthateitherexistpresently(storageringsandpulsed

linacs),orareunderdevelopment(energyrecoverylinacs

photon number emitted per“coherence volume,” of

about10-2photonsinasix-dimensionalphasespacecell

whosesizeissetbytheHeisenberg-Fourieruncertainty

principle.Thisdegeneracyparameteris,ineffect,simply

amorefundamentaldescriptionofwhatisoftencalled

brightness,andreflectshowfarthepulseisfromthe

ultimate transformanddiffraction limits inwhichall

photonswouldoccupy thesamemode.FutureERLs

mayproducemorephotonsinsideasmallertotalphase-

spacevolumethanthird-generationsynchrotronsources,

andmayachievedegeneraciesoftheorderof102or10;3

FELsources, throughthecoherentamplificationpro-

cess,producehighlydegenerateX-raypulses,with1010

ormorephotonspercoherencevolume,andwithfur-

ther orders-of-magnitude increase in degeneracy

obtainablefromseededoroscillator-basedFELs.

TheNGLSFELcomplexwillsurpassthecapabilities

ofexistingX-rayfacilitiesinnumerousways:increased

averagephotonfluxandmultiple,simultaneouslyoper-

ableX-raybeamlineswillbeprovided;amoderately

highpeakfluxwillhelptoavoidundesireddamagetoor

perturbationofsensitivetargetmaterials,therebymaxi-

mizingthefractionofusefulphotonsineachpulsein

manyexperiments;thecombinationofveryhighrepeti-

tionrateswithhighphotonfluxesinshortpulseswill

openupentirelynewexperimentalvistas;themultiple

X-raybeamlineswillgivetheflexibilitytoservemany

differenttypesofexperimentsandprovideprobeswith

awidearrayofphotonpulsestructures;thepotentialfor

precisesynchronizationwillallowformultidimensional

spectroscopyandpump-probeexperimentswithmulti-

colorprobesintheX-rayrangeandpumpsintheTHz,IR,

visible,UV,orX-raybands;theuseofseedingschemes

foranFELwillallowforexcellenttunabilityandulti-

matelywillenablefeedback-basedcontroloftheX-ray

pulsestabilityandcharacteristics.

TheFELadvantageinpeakflux(uptosixorders-of-

magnitudegreaterthanstoragerings)andbrightnessor

degeneracy(uptotenorders-of-magnitudegreaterthan

storagerings)arisesbecausetheFELamplificationpro-

cessproducesaverylargenumberofphotonsineach

pulse,andpacksthesephotonsintoasmallopticalphase

space.FELshaveanotheradvantageover ring-based

sources,inthattheirelectronpulsesarepreparedbya

linacandthenusedonlyoncetocreateX-rays.Thisulti-

matelyenablesatransferofbrightnessfromtheelectron

bunchestothephotonpulses,andallowsprecisecontrol

112

5 . PROPOSED FACILITYALTERNATE APPROACHES

structurestorepetitionratesbelowaboutakilohertz.For

example,theSPring-8XFELdesignconsidersanoperat-

ingscenarioinwhichtheirC-bandlinacispulsedatabout

1kHz,atsignificantlyreducedacceleratinggradient,in

ordertoprovidehigherrepetition-ratesoftX-raycapabil-

ity(versus60Hzoperationathighergradientintheircur-

rent design for a hard X-ray range).The maximum

repetitionrateattheLCLS-IIwilllikelybe360Hz.

Supporting a uniform, one-MHz bunch rate with a

SLAC-typelinacwouldrequireCWoperationofthefinite-

conductivity accelerating structures.The 3 m S-band

structureshaveafill-timeofabout700ns(roughlyequal

tothepulseduration)andaninputpowerofabout25MW

andlaser-basedhigh-harmonicgeneration).Weconsider

thesealternativesandtheirabilitiestomeetthescientific

needsdescribedinSection3.Wecontendthatafacility

designbasedonanarrayofFELsdrivenbyaCWsuper-

conductinglinacprovidesthebestchoicetomeetthe

identifiedneeds,andtoprovideaflexibleandupgrade-

ablesourceforthefuture.

5.2.1 Conventional Pulsed Linacs

Warm (finite-conductivity) linacs operating at high

acceleratinggradient (typicallyontheorderof tensof

MV/m)arelimitedbypowerdepositionintheaccelerating

Table1 Comparison of NGLS FEL baseline parameters and technical features to other FEL facilities worldwide.

Wavelength (nm)

Photon Energy (keV)

Pulse duration

(fs, FWHM)

Effective x-ray pulse repetition rate (Hz)

Photons per pulse

Bandwidth (approxi-

mate)

Energy per pulse

(µJ)

Photons per

second

Average photon beam power

(W)

NGLSbaselineparameters

High-power 1 1.2 250 106 1011 10-3 20 1017 19

SASE 3.3 0.38 250 106 1012 10-3 60 1018 61

Seeded,narrow 1.2 1 150 105 1011 10-4 20 1016 2

bandwidth 4.5 0.28 150 105 1012 10-4 40 1017 4

Attosecond 1.2 1 0.25 105 108 10-2 2x10-2 1013 0.002

4.5 0.28 0.25 105 109 10-2 4x10-2 1014 0.004

LCLS&LCLS-II 0.15 8.2 10-100 102 2x1012 10-3 2x103 2x1014 0.24

5 0.25 10-300 102 7x1013 10-3 3x103 8x1015 0.34

FLASH 6.8 0.18 10–50 2x104 2x1012 10-2 60 4x1016 1.15

47 0.026 10–50 2x104 2x1012 10-2 8 4x1016 0.17

XFEL 0.1 12.4 100 3x104 1012 10-3 2x103 4x1016 71

6.4 0.2 100 3x104 4x1014 10-3 104 1019 413

FERMI@elettra 3 0.41 ~40 50 1011 10-4 7 5x1012 0.0003

10 0.12 ~40 50 1012 10-4 20 5x1013 0.001

SPring8XFEL 0.1 12.4 50 60 1011 10-3 2x102 6x1012 0.01

SwissFEL 0.1 12 0.6–28 102 1011 10-3 2x102 1013 0.02

7 0.18 ~1–28 102 1013 10-2 7x102 2x1015 0.07

PohangFEL 0.1 12 ~50 60 1012 10-3 2x103 6x1013 0.12

1 1.2 ~50 60 4x1012 10-3 8x102 2x1014 0.05

ShanghaiFEL 0.1 12 ~75 50 7x1010 10-3 102 3x1012 0.01

9 0.13 ~200 10 5x1012 10-3 102 5x1013 0.001

Note: FLASH and XFEL are based on pulsed superconducting linacs, and utilize trains of bunches (see Figure 2). Effective pulse rate for FLASH is based on 4000 bunches spaced by 333 ns and repeating at 5 Hz; effective pulse rate for XFEL is based on 3000 bunches spaced by 200 ns and repeating at 10 Hz.

113


resultsina2–3kmlinac.Inthesecases,thepowerdissi-

patedintheacceleratingstructureswouldbeapproxi-

mately30kW/m,andwouldrequirewall-plugpoweron

theorder100MW—significantlylessefficientthanthe

CWSCRFproposedfortheNGLS,whichistooperateat

about 10 MW. Operation of a room-temperature RF

machineinpulse-trainmodecouldallowforimprove-

mentinefficiency.However,thisoperatingmodedoes

nothavethebenefitsofCWfeedback,consistentX-ray

pulsespacing,orhighreproducibilitythatcanbeobtained

withanSCRFCWmachine.

Weconcludethatpowerlimitationsofpulsedconven-

tionallinacsprohibitthemfromdeliveringthehighaver-

agepowerX-raypulsesthatarerequiredbythescience

casedescribedinSection3.Reference1summarizesthe

statusandfuturecapabilitiesofFELs,includinguseof

conventionallinacs.

inordertoachieve17MV/mgradientsatarateof60Hz.

TomaintainroughlythesameaverageRFpowerdissi-

pationinthestructureswhenoperatinginCWmode,the

gradientwouldneedtobereducedtoabout0.1MV/m,

sothelinacwouldneedtobeover15kmlongtopro-

duce1.8GeVelectrons(asintheNGLSbaselinedesign).

Otherroom-temperaturestructuresoperatedintrueCW

modewillhavesimilarlimitsonaveragepoweroron

totallength.Onecouldprobablyallowtheheatdissipa-

tion to increase significantlywithadditional cooling

capacity;assumingafactorof100maybeachieved,the

maximumgradientcouldbeincreasedbyafactorof10

comparedtotheaboveestimate,andthusanormal-

conducting,CWS-bandlinacmightberequiredtobe

2–3kmlong.Thepowerconsumptionwouldincrease

proportionally. Similar analysis for an X-band linac

basedonNextLinearCollider (NLC) technologyalso

Ring-basedStorage ringNSLS-II

~10 ps ~ns~nJ, 0.1% BW

ERL JLAB FEL ~100 fs ~0.1 µs

~10 µJ, 0.1% BW

HHG Tabletop ~0.1–10 fs ~100 µs

~nJ, 1% BW

FELXFEL

~100 ms

600 μs

10 to 100 fs

200 ns

~mJ, 0.1% BW

~mJ, 0.1% BW

10 to 300 fs

~1 msLCLS

NGLS

1 μs250 as to 500 fs

~0.1 mJ, 0.1% BW

Figure77 Comparison of X-ray pulse structure of different light source types, based on current capability or near-term capability of facilities under construction. Storage ring and FEL performance is for soft X-rays (around 1 nm). ERLs and HHG sources are not currently operating at soft X-ray wavelengths, and thus perfomance is shown for UV and EUV wavelengths. Pulse energy is in the central cone of undulator radiation, and NGLS values reflect the baseline design SASE FEL repetition rate (the seeded FELs operate at up to 100 kHz, and may produce similar pulse energy in some operating modes; the NGLS may operate in SASE mode at even higher repetition rate with reduced per-pulse energy).

114


opticalcavitybuiltaroundtheFELundulator;theradia-

tionisout-coupledthroughonewindow,andveryhigh

averagepowershavebeenachievedintheIR.Asingle-

pass,high-gainFELcouldbepossible,howeverthedeg-

radationinbeambrightnessfollowinganFELintroduces

significant challenges in implementing multiple FEL

sourcesinanERLconfiguration.Thissingle-FELarrange-

mentofferslimitedflexibilityandpermitsfewerusers.

AparallelarrangementofFELswouldpresenttechnical

challengesandcoststhatareprohibitive.

Weconclude thatERLsarenot flexible,multi-user

sourcesofthehighaveragepower,coherent,ultrafast,

X-ray pulses that are required by the science case

describedinSection3.Reference2summarizesthestate-

of-the-artofERLsaslightsources,andReference1gives

anoverviewofFELperformanceinERLs.

5.2.3 Third- and Fourth-Generation Storage Rings

Storageringshavebeenhighlyproductivefordecades

andarefinelyhonedlightsources.Theyofferreliability,

stability,moderateaverageflux,tuningrange,andpolar-

izationcontrol.Beyondtheexistingthird-generationrings,

the“ultimate”storageringsofferthepossibilityofreduc-

ingelectronbeamemittance(andtherebyraisingX-ray

beambrightness)byafactorof100to1000overexisting

storagerings.Thenewlyapproved,3GeVMAX-IVringin

Swedenwillgoalongwaytowardsachievingthebest

foreseeablestorageringelectronbeamemittance.

StorageringsproduceX-rayfluxestypicallyinthe

range of ~106–108 photons per pulse. Photon pulse

lengthsaretypicallygreaterthantenpicosecondsRMS

induration,butshorterpulselengths,ontheorderof

1ps,canbereachedat lowercharge-per-bunchwith

latticetuning(a“low-alpha”lattice),orathighercharge-

per-bunch but in limited sections of the ring with

RFdeflectionsystems(“crab”cavityschemes).X-ray

pulsedurationsof~0.1–1pshavealsobeenachievedin

storageringsby“laser-slicing”techniquesatrepetition

ratesintothetensofkHz,butwithfluxperpulselimited

bythefractionofthebunchchargeinvolvedinthepro-

cess.FutureringsmayincorporatesoftX-rayFELsina

“partiallasing”mode,orinaswitchedbypass,butwith

limitedgainand/orlimitedrepetitionrate.

5.2.2 Energy Recovery Linacs

ERLs are potential future X-ray synchrotron light

sourcesthatcombinesomeofthequalitiesofstorage

ringswiththoseoflinac-basedlightsources.Ahigh-repe-

tition-rate(uptoGHz)andhigh-current(upto100mAfor

someoperatingmodes)injectorandCWSCRFlinacpro-

videsveryhighbeampower.TheERLconfigurationhas

theadvantageofprovidinganaffordableRFpowersys-

tembyrecoveringmostof theenergyof theelectron

beam.Thisemerging technologypromises toprovide

veryhighaveragebrightnesswithhighspatialcoherence

(~50%ormore)bypreservingthevery lowemittance

(about~0.1µmnormalizedRMStransverseemittance)

andlowrelativeenergyspread(about10-4)achievable

fromafull-energy,high-currentsuperconductinglinac.A

singleturnaroundaring-liketransportlatticeaccommo-

dating several sequential insertion devices produces

spontaneousradiationwithflux-per-pulsesimilartothat

ofstoragerings.Followingtheinsertiondevices,theelec-

tronbeamwouldthenbereturnedtothelinacwhereitis

deceleratedtorecovertheenergyinthesuperconducting

RFstructure.

Energyrecovery linacsoffer thepotential toreach

high spectral brightness (exceeding 1022 photons/s/

mm2/mrad2/0.1%bandwidth)withhighspatialcoher-

ence,andcontrolofpulsedurationdowntotheorderof

1psinahigh-current(~100mA),high-brightnessmode,

anddown to theorderof tensof femtoseconds ina

lower repetition-rate (~1MHz)mode. Fluxperpulse

wouldbesimilartostoragerings,upto~107photons

perpulsedependingonmodeofoperation.Bandwidth

islimitedbythebeamenergyspreadinlonginsertion

devices,andtheX-raypulsesgeneratedbyspontaneous

emission in insertion devices have limited temporal

coherence.ERLshavedemonstratedenergyrecoveryof

over1MW,aswellastwo-passrecirculation,however

withbeamcurrentandwithbeamenergymuchlower

thandesiredandwithtransverseemittancesonetotwo

orders-of-magnitudegreaterthanrequiredforanultra-

brightX-raysource.

AnERLmayincludeanFEL,asinthecurrentstate-of-

the-artJLABIR/UVDemoFEL.1Inthiscase,thehigh-rep-

etition-rate of the JLAB ERL enables an oscillator

configuration,inwhichthelaserspulseiscontainedinan

115

5 . PROPOSED FACILITYNGLS: A TRANSFORMATIVE TOOL FOR X-RAY SCIENCE

5.3 NGLS:ATransformativeTool forX-RayScience

Over the last decade, theoretical innovations and

experimentaladvanceshaveledtoarenaissanceinthe

physicsofX-rayfreeelectronlasers.InGermany,FLASH

usingSCRFlinacs,andatSLAC,theLCLS,usingroom

temperatureRFtechnology,operateroutinelyandreli-

ably,andhaveconclusivelydemonstratedthetechnology

requiredtoproduceanddeliverhigh-brightnessbeams

essentialforX-rayFELs.These,andotherfacilitiesinclud-

ingtheSCSSFELatSPring8inJapan,theFERMI@elettra

FELinTriesteandtheSPARCfacilityinFrascati,arecur-

rently developing seeded FEL capabilities. Looking a

decadeahead,thenextgenerationofX-rayFELsmust

buildontheoutstandingsuccessesofthesepioneering

machines.Inthissectionweprovideanoverviewofthe

NGLSdesignfeaturesandlayout,FELperformance,and

acceleratorparameters.Amoredetaileddescriptionof

thetechnicalfeaturesfollowsinSection5.4.

5.3.1 Machine Overview and Performance

TheNGLSwilloperateinanovelparameterregime,

providingasuiteofuniquefeaturescomparedtoexisting

orplannedX-raylightsources,includingmostnotablya

high-repetition-rate (1MHz),high-brightnesselectron

source,andaSCRFelectronlinacoperatinginCWmode

whichwillprovidebunchesathighaveragebeampower

withuniformbunchspacing.Thesebuncheswillbedis-

tributedviaaspreadersystemtoanarrayofindepen-

dentlyconfigurableFELs,eachoperatingatthreeormore

orders-of-magnitudehigherpulserepetitionratesthan

existingX-rayFELs,andeachwithadjustablephoton

pulsepower,centralwavelength,polarization,andultra-

fast temporal resolution down into the attosecond

regime.Themajorcomponentsofthefacilityareshown

inFigure76.Ourdesignisbasedonmeetingthemost

criticaloftheanticipatedscienceneedsandprovidinga

largeusercapacitywhilerealizingthephysicsandengi-

neeringconstraintsoftheacceleratorandFELs,allthe

whilecognizantofthefacilityandoperatingcosts.Our

baselinedesignforasetofthreesimultaneouslyopera-

bleX-raybeamlineswillservealargenumberofexperi-

mentsperyear,withthecapabilityofprovidingupto

Inconclusion,currentstorageringsdonot,andfuture

storage rings will not, simultaneously deliver the

requiredhighaveragepower,coherent,ultrafast,X-ray

pulsesthataredemandedbythesciencecasedescribed

inSection3.Reference3summarizesthepotentialof

futurering-basedsources.

5.2.4 HHG Laser Systems

Wavelengthsinthehardultraviolet,orpossiblyin

thefuturethesoftX-rayspectralregion,areattainable

intheveryhighharmonicsproducedwhenanintense

infraredlaserpulseisfocusedintoagas.High-harmonic

generationcanbeproducedwithtemporalandspatial

coherenceproperties similar to thoseof thedriving

laserfield.Theyhaveahighdegreeofpolarization,and

sub-femtosecond pulse duration. Such sources have

beengeneratedusingcommercialdrivelasersatthesev-

eral-wattlevel,withrepetitionratesrangingfrom10Hzto

10kHz.Thecut-offoftheharmonicspectrumextendsto

shorter wavelengths as the drive laser intensity is

increased,uptoasaturationintensitywhereharmonic

generationdecreases.IncurrentHHGsystems,theout-

putfluxisroughlyconstantbetween200-500eV,with

about105photonsperpulseinafractionalbandwidth

ofΔλ/λ ≈10-2.Byusinggasspecieswithahigherioniza-

tionpotentials,andahigh-power,longer-wavelength

drivelaser,togetherwithphase-matchingtechniquesin

theharmonic-generationmedium,thespectralcut-off

ofHHGmaybeextendedupto~1keVwithaconversion

efficiencyoftheorderof10-5.Aper-pulseenergyofup

to20nJisprojectedforadvancedlasersoperatingat

100kHzinthefuture.

FutureHHGsourceswillrequiresignificantdevelop-

mentsinlasertechnologytoprovidethehighaverage

power, coherent, ultrafast, X-ray pulses that are

requiredbythesciencecasedescribedinSection3,

andareunlikelytoreachtheaveragepowerlevelsof

100Wobtainablewithhighrepetition-rateFELs.HHG

sourceswill,however,havedirectapplicationsasseed

sourcesforEUV/XUVorsoftX-rayFELs.TheFELspro-

videseveralordersofmagnitudegainactingastun-

ablenarrow-bandX-rayamplifiersfortheHHGsource.

Reference4summarizesthecapabilitiesofpresentand

futureHHGsystems.

116


secondprecision.Oneof the twoseededFELswillbe

capableofproducing“two-color”X-raypulses,whilethe

otherseededFELwillprovidebetterenergyresolution

withlongerpulsesandhightemporalcoherence.Thethird

FELwillbeanon-seededSASEdevicecapableofoperat-

ingatthefullrepetitionrateofthelinac,therebyproviding

veryhighaveragepowerX-raybeams.Atapproximately

constantaverageelectronbeampower, theNGLScan

operateatahigherpulserepetitionrateusingbunchesof

lowercharge, shorterduration,buthigherbrightness.

These bunches might enable lasing at shorter wave-

lengths,orpossiblytheoperationofaSASEbeamlinein

so-called“single-spike”configuration,witheachshort

electronpulsenaturallyradiatingintoatmostaveryfew

longitudinalmodes.Table2summarizesmajorFELperfor-

manceparametersforthebaselinemachine,assuminga

pointdesignof300pCbunchcharge(andwhichmayvary

fromafewpCtopotentially1nC).Figure78showsthe

nominalnumberofphotonsperpulseproducedineach

ofthethreebaselineFELs,asafunctionofwavelength

tuning.Furtherdetailsof theFELdesignaregiven in

Section5.4.5.

AsdescribedinSection5.4.2,buncheswiththerequired

highbrightnesswillbegeneratedatthedesiredhighrepe-

titionratebyastate-of-the-artVHFelectronphoto-gun,and

willundergoemittancecompensationandcompression

~100Wofaveragepowertoeachofsixend-stations(two

perFEL),withtunabilityspanningtheimportantabsorp-

tionedgesofcarbon,oxygen,nitrogenandtheL-edgesof

thefirst-rowtransitionmetals(i.e.,to1.2keVinthefunda-

mental,andultimatelyto10keVinthe3rdharmonic).

Whilethefirst-generation,low-repetition-rateX-rayFELs

provideorders-of-magnitudeimprovement,primarilyin

peakpowerandtemporalresolution,comparedtothird-

generationsynchrotronsources,peakpowerisnotasub-

stituteforthelevelofaveragepowerand/orcoherent

powerthatwillbeprovidedbyNGLS.

TheprimaryspectralrangeoftheNGLSbaselinedesign

willextendfrom280eVto1.2keVatthefundamentalof

theundulatoremission(usingundulatorswithdifferent

periods)anduptoapproximately3keVatmuchreduced

intensitywiththegenerationofharmonics.Lowerphoton

energiesmightbereachedbyextractingsomeelectron

bunchesat lowerenergy.Fluxmaybecontrolledfrom

about108toabout1012photonsperpulseinthefunda-

mental,dependingonthedesiredwavelength,pulsedura-

tion,andrepetitionrate.Laserseedingwillbeimplemented

toproducepulseswithdurationasshortas250attosec-

onds,withtemporalcoherenceapproachingfundamental

transformlimits,withthepossibilityofsomecontrolover

chirporlongitudinalpulse-shape,andwithsynchroniza-

tionoftheX-raypulsestoend-stationlaserswithfemto-

Table2 Baseline performance parameters for the three FEL designs. Details are given for example design points of pulse duration and wavelength, and for the baseline 300 pC bunch charge. NGLS will be capable of a broad range of operating configurations, potentially extending the range of pulse lengths, photons per pulse, and repetition rate.

Beamline 1 Beamline 2 Beamline 3

Type Seeded,time-bandwidth-limited 2–colorseeded SASE

Feature Shortcoherentpulses 2-colorX-raypump/probewithadjust-abledelayandattosecondpulses

Highaveragefluxandbrightness

Pulselength(fs,FWHM) 5–150 0.25–25 ~5–250

Wavelengthrange(fundamental,nm)

1.2–4.5(1.0–0.28keV)

1.2–4.5(1.0–0.28keV)

1.0–3.3(1.2–0.38keV)

Maximumrepetitionrate(kHz) 100 100 1,000

Totalphotons/pulse ~1011(150fs,1.2nm)~1012(150fs,4.5nm)

~108(sub-fs) ~1011(250fs,1nm)~1012(250fs,3.3nm)

Photonsper6Dcoherencevolume ~1011 ~108 ~1010

Peakpower(GW) ~0.1(1.2nm)–1(4.5nm) ~0.05(1.2nm)–0.1(4.5nm) ~0.1(1nm)–1(3.3nm)

Averagepower(W) ~1(150fs,1.2nm)–10(150fs,4.5nm)

~0.001(sub-fs)–0.1(fs) ~0.1(5fs)–100(250fs,3.3nm)

Powerin3rdharmonicrelativetofundamental(%)

~0.1(1.2nm)–1(4.5nm)

~1 ~0.1(1nm)–1(3.3nm)

Relativebandwidth(%,FWHM) ~0.005(150fs,1.2nm)–0.02(150fs,4.5nm)

≥1.4(sub-fs) ~0.2(1nm)–0.5(3.3nm)

Polarization Variable,linear/circular Variable,linear/circular Variable,linear/circular

117


Veryshort-period,superconductingundulatorswould

allowevenmoreoptions.Section5.4.5.3discussesundu-

latordesign.

Choicesforbeamenergyandpulserepetitionrates

necessitate theadoptionofSCRF technology for the

linac:Sections5.4.3.5and5.4.3.6outlineourcryomod-

ule and RF systems designs, based on the 1.3 GHz

TESLA-typemulticellcavities.Ourchoiceofanacceler-

atinggradientofapproximately14MV/misconserva-

tiveintermsofpresent-daycavitycapabilities;however

itiswithinabroadoptimumofacceleratinggradients

whenfullconstructionandoperatingcostsareconsid-

ered.Furtherstudieswilldetermineanoptimalsetof

operatingparametersfortheNGLSperformance,bal-

ancingriskamongtheinjector,linac,andFEL.Besides

offering the desired high pulse repetition rates, CW

operationoftheSCRFlinachasanothersignificantoper-

ationalbenefitinthatitallowsforautomatedhigh-fre-

quency feedback control to ensure quality and

uniformityoftheelectronbunches,withjitterinX-ray

pulseparametersperhapstentimessmallerthanthat

currentlyachieved.

Considerationofthetrade-offsaffectingtheelectron

brightnesshaveledustoselectforthehigh-qualitycore

ofthebeamthefollowingcharacteristicswhenentering

theFELundulators:0.6μmorsmallernormalizedslice

transverse emittance, 50–60 keV uncorrelated RMS

energyspread,and500Aorhighercurrent.Thelower

limit to the length of the high-quality beam core is

determinedbythetwo-colorFELbeamlineanddesired

levelofradiationoutputfromtheothertwobeamlines.

Inparticulartherequirementtohaveupto150fsdelay

betweenthetwoseedinglaserpulsesandasafetymar-

ginagainsttime-jitterestimatedtobe±50fsimpliesa

needforatleasta250fsdurationfortheusablebeam

core.Aconservativeallowanceforuptohalfofthetotal

bunchchargetoresidewithintheunusableportionof

thebeamthenimpliesatotalbunchchargeof250pCor

larger.Thebaselinemachinedesigndiscussedherepre-

supposes300pCbunches,consistentwithdeliveryby

theinjectorofbuncheswithRMSnormalizedtransverse

emittanceof0.6μmorsmaller.Table3liststhebaseline

electronbeamparametersforthehigh-qualitybunch

core;additionaldetailsaregiveninSection5.4.3.Not

includedinTable3areparametersforlow-chargeSASE

operation,althoughduetoitssimplicitythismaybethe

appropriateconfigurationforcommissioningandinitial

byballisticandvelocitybunchingthroughtheinjector.

Furthercompressionwilloccurthroughamagneticchi-

caneinthelinacbeforeaccelerationtothefinalbeamener-

gy.The machine is designed for an average current

capabilityupto1mA,beyondourinitialparametersof

300pCand1MHzbutconsistentwithawiderangeof

bunchchargeandtimestructures.Ourbaselinedesignhas

beendevelopedassumingabunchchargeof300pC,and

allowsflexibilityto increaseversatility inperformance.

Higherchargeoperationisanticipatedfor longerpulse

durationswhichweexpectmayreach500fs,orforhigher

peakcurrenttoimproveefficiencyofphotonproduction.

Furtherstudieswillberequiredtodelimittheexactbound-

ariesofthebeamparameter-spaceaccessiblebytheNGLS.

Themaximumelectronbeamenergyof1.8GeVhas

beenchoseninourbaselinedesignsoastobeableto

produce1.2keV(1nm)photonswithreadilyavailable

undulator technology (periodsofabout18mm),but

withaminimalacceleratorfootprintandcost.Beamlines

utilizingdifferentundulatorparametersandtechnolo-

giescouldachievedifferentperformanceorcostgoals.

Forexample,anundulatorwitha26mmperiodwould

coverwavelengthsfrom13.4nm(93eV)to1.8nm(688

eV),andanAPPLE-typeundulatorwithperiodof38mm

andamagneticgapof5.5mm(providingabeamclear-

anceof4mm),wouldcoverwavelengthsfrom12.5nm

(99eV)to2.6nm(476eV)andwitharbitrarypolarization.

107

108

109

1010

1011

1012

1013

1 1.5 4 4.53.52Wavelength (nm)2.5 3

Phot

ons

per p

ulse

Beamline 1Beamline 2Beamline 3

150 fs pulses250 as pulses250 fs pulses

Figure78 Projected baseline output at the NGLS: Beamline 1 is a seeded FEL shown here for 150 fs pulse duration; Beamline 2 is a 2-color attosecond beamline, here with 250 as pulses; Beamline 3 is a SASE FEL here with 250 fs pulses. Section 5.4.5 describes FEL design and performance.

118


5.3.2 Layout, Conventional Facilities, and Utilities

TheinitialNGLSmachinewillconsistprimarilyofa

straightsection(forelectronaccelerationandtransport)

ofapproximately450m,whichwill then fanoutover

180 m into multiple beamlines and end-stations, as

showninFigure79.

Themajorcivilconstructionstructuresarethelinac

vaultandklystrongallery,spreaderhall,FELvault,and

experimentalhall.Additionalspacewillberequiredforthe

cryogenicsplantincludingassociatedgasstorage,acryo-

moduleacceptancetestfacility,andmachinemaintenance

activities.Conventionalfacilities,includingcoolingtowers,

low-conductivitywatersystems,chilledwatersystems,

electricalswitchingandtransformerstations,alsoneedto

be housed.These construction elements have been

includedinthecostestimategiveninSection8.1.

Anexceptionallystablefoundationwillbeneededto

supporttheentireNGLSmachine.Long-termsettlement

andvibrationmustbeminimizedforefficientmachine

operationandoptimumperformance.

Roughlytenmetersofcombinedconcreteandearth

shieldingwillenclosetheNGLS.Beamdumps,locatedat

theendofthespreaderandattheendofeachFEL,willbe

positioned below floor level and inclined downward.

Concretevaultssurroundingthebeamdumpswillfurther

isolatetheseunitsfromthemainportionofthemachine

andfromthesoilinwhichtheyareburied.

Thewidthandheightoftheinjectorandlinacenclo-

surewillbesufficienttohousethebeamlinecomponents,

supportequipmentandutilitieswhilemaintainingawalk-

wayforinstallationandremovalofafullcryomodule.The

shieldingenclosureinthespreaderregionwillhavean

increasedwidthnecessitatedbytheshallowinitialangle

betweenthebranchlinesandmainbeamaxis.Thiswide

hallwilltransitiontoindividualbranchenclosuresforthe

FELs downstream of the final bend magnet on each

branch.TheFELvaultsextendanadditional~120meters

totheshieldingend-wallandthebeginningoftheexperi-

mentalhall.

SpacingbetweenFELsistobeabout6meters,ade-

quatefortwoormorephotonbranchlines.X-raybeam-

lineswillextendabout50metersfromthefirstoptic,

housedwithintheshieldwall,andreachend-stations

nearthefarwalloftheexperimentalhall.Anapproxi-

operationoftheNGLS.Inthisconfiguration,electron

bunchesofapproximately10pCchargeand10fsbunch

lengthwouldbedeliveredtotheSASEFEL,producing

~108photonsperpulseat1nminpulsesofafewfemto-

seconds duration, and at the full repetition rate of

theinjector.

Distributingtheelectronbeamtoanarrayofbeamlines

throughaspreaderutilizingpulsedkickersoperatingat

100kHzpulserepetitionratedeliversaregularstreamof

bunchessimultaneouslytoeachseededFEL(withtherate

ultimatelylimitedbyseedlaserpowerandspreaderper-

formance),andbunchesforthefinaldownstreamSASE

FEL(notusingaseedlaser)atuptothefullrepetitionrate

of the injector. Section 5.4.4 describes the technical

detailsofthespreader. Table3 Electron beam parameters for baseline operation with 300 pC bunches. With the flexibility offered by a photocathode gun and CW superconducting linac, NGLS will offer a range of other modes of operation, delivering beams required for specific FEL configurations and experimental needs.

Parameter

Bunchcharge(pC) 300

Repetition rate (MHz)

Outoflinac 1

IntoFEL 0.1-1

Average current (mA) 0 .3

Bunchlength(fs)

Outofinjector(FWHM) ~5000

IntoFEL(inusablebunchcore) 250

Peak current (A)

Outofinjector >40

IntoFEL(inusablebunchcore) >500

Emittance (slice, normalized, µm)

Outofinjector <0.6

IntoFEL 0.6

Energy spread (slice, rms, keV)

Outofinjector <4

IntoFEL(inusablebunchcore) 50

WeareengagedinR&Dtofurtherreducetechnical

risk,allowincreaseddefinitionofthemachineconfigura-

tionandperformancedeliverables,andreliablyunder-

standcostsandbenefits:Section5.4.1describestechnical

andphysicschallenges,andSection8.3summarizesour

riskmanagementandR&Dplans.Obviously,muchofthe

physicsofseededFELoperationatX-raywavelengthsis

stillrapidlydevelopingacrossanumberofU.S.andinter-

nationallaboratories;riskmanagementwillinvolvecoor-

dinatedresearchovermanyinstitutions.

119

5 . PROPOSED FACILITYDESIGN CONSIDERATIONS AND CHALLENGES

designsformajoracceleratorsystemsoftheNGLS.We

startwith theelectronsourceand injector,andmove

downstreamhighlightingdesignchoicesandfeaturesof

theacceleratorsystemsandFELbeamlines.

5.4.1 Overview of FEL Physics and Technology Challenges

LasingatX-raywavelengthsrequiresveryhighbright-

nesselectronbeams.Topreservebrightness,wenaturally

endeavortobendtheelectronsaslittleaspossibleasthe

bunchesaregenerated,manipulated,accelerated,and

transportedtotheradiatingsections.Thereforeasingle-

passlinac-baseddesignisanaturalchoice.

AssumingtypicalFELconversionefficienciesofthe

electronbeampowertoX-raypowerontheorderof10-4,

NGLSwillachieveanoverallfacilitypowerefficiencyfor

theproductionofradiationontheorderof10-5.Bycom-

parison,thisisseveralorders-of-magnitudegreaterin

efficiencythancanbeachievedtoproducefew-nm-wave-

lengthHHGradiationdrivenbyopticallasers.

Thetargetof1MHzorgreaterbunchrepetitionrate

enables,withrealisticoverallpowerrequirementsand

capitalcosts,anarrayofmultipleX-raybeamlineswith

flexibleandcutting-edgeperformance.Fromanaccelera-

torperspective,onceoneacceptstheutilityofbunchrates

inexcessofabout1kHzorperhaps10kHz,thetechnology

ofchoiceclearlyshiftsfromwarmtosuperconductingRF

linacs.Atthesehighrepetitionrates,CWoperationofthe

SCRF linac enjoys significant advantages over pulsed

operationwithoutagreatpenaltyinpowerrequirements.

Forexample,CWoperationnaturallyallowsforhigh-band-

matelyten-meterwalkwayattheendofthephotonbeam-

linewillprovideaccessfortheinstallationandremovalof

experimentalequipment.Electronicracks,utilitiesand

supportequipmentwillbelocatedinasecondfloorabove

theexperimentalfloor,leavingthegroundfloorclearof

heatandnoisesources.Lasersystemsforthephotocath-

odesource,FELseeding,andendstationexperiments

willbehousedinseparate,temperature-controlledrooms

outsideoftheradiation-shieldedenclosure.

Themostcostlyancillarysystemwillbethetwo-kelvin

liquid helium refrigeration system for cooling of the

superconductingRFstructures.Thecompressorsforthe

refrigeratorwillbehousedonaseparatefoundation,suf-

ficiently removed to limit transmission of vibration

throughthegroundtothemachinebutnotsofarasto

incur unreasonably high cryogenic transmission line

costs.

TheRFpowersupplysystemwillbethelargestpower

drawat~7.2MW,followedbythecryogenicssystems

witha3MWpowerdraw.Magnets,vacuumsystem,

experimentalequipmentandancillarysupportsystems

willcontributeapproximately2.5MWtotheload.Intotal

theutilitypowerloadwillbeapproximately13MW.The

installedcapacitywillbegreater.

5.4 DesignConsiderations andChallenges

Inthefollowingsectionwefirstoutlinethechallenges

ofbuildinganFELthatexceedsthecurrentstate-of-the-

art,andwethendescribeinsomedetailthepre-conceptual

GUNLINAC 2 SPREADER

FEL-TWO COLOR

PHOTONBEAMLINES

1st M

IRROR

SHIELD W

ALL

BEAM DUMP

FEL-SASEFEL-SEEDED

DUMP MAGNET

BEAM DUMP

INJECTO

R

LINAC 0

LASER

HEATE

R

LINAC 1

LINEA

RIZER

BUNCH COMPRESSOR

31

58

107

137

328

366

463

633

120

3

48.5

Figure79 Layout of the NGLS, with dimensions in meters.

120


edtheelectronbeambrightnessrequiredforX-rayFEL

operation,andourdesignincludesstate-of-the-artbeam

brightness, theNGLSrequiresthreetofourordersof

magnitudegreater repetition rate than theLCLS.The

NGLSinjectorrequiresdifferenttechnologythanLCLSfor

theelectrongunacceleratingcavityandthephotocath-

ode/lasersystems,aswellasdifferentbeambunchcom-

pressionandemittancecompensationschemes.

ThesaturatedFELoutputpowerisproportionaltothe

electronbeampower.FortheNGLS,theenergyperpulse

mayreachhundredsofmicrojoules,andultimatelymilli-

joules forsomebeamlineswhenoperatingwithhigh-

chargebunches.Ourdualgoalsforlongpulses(250fs

FWHM in the baseline design, and ultimately

longer—whichnaturallyrequireshigherchargeandthere-

forehigher-emittancebunches)togetherwitharelatively

lowbeamenergy(forlowercost),requirecarefuloptimiza-

tionforarobustfacility.AsSCRFtechnologydevelops,the

linacmayallowforincreasedbeamenergywithoutsignifi-

cantlygreaterrisk,therebyreducingtherequirementon

beambrightness.Physicsandtechnologychallengesofan

X-rayFELarediscussedmoregenerallyinReference5.

Thecurrentpreferenceforseedingschemesistouse

echo-enabledharmonicgeneration(EEHG),whichcanbe

implementedat100kHzusingmodestdevelopmentsof

currentlyavailablelasers.Thesecondchoiceistorelyon

HHGseedingatwavelengthsinthe~30nmrange,which

canalsobeeffectedbycurrentlyavailablelasertechnology

operatingat10kHz,andsourcesareanticipatedtoreach

100kHzrepetitionratesatthetimeofcommissioning.

High-gainharmonicgeneration(HGHG)isanalternative

option,andsystemswithpowersuitablefor10–100kHz

operationalreadyexist.Severalfacilitiesworldwideare

developingandevaluatingFELseedingtechniques,and

wehaveincludedseedingexperimentsandtechnology

developmentsintheNGLSR&Dneeds.

Othertechniqueshavebeenproposedtoincreasethe

coherenceorbrightnessoftheFELoutputwithoutseed-

ingtheFELusingexternallasers.Eliminatingseedlasers

has thepotential forenablingmuchhigher repetition

rateswhilestillofferingsomecontroloverthetimingand

durationoftheradiationpulse,butatthecostoflosing

theshot-to-shotconsistency,tightlycontrolledsynchroni-

zation,andtheexquisitecontroloverpulsesofwhich

externalseedingviaanopticallasersystemsiscapable.

OnemethodavoidingseedinglasersisFELself-seeding,

width,automatedfeedbackandcontrolsystemsthatcan

helpensureveryhighstabilityandreliabilityofelectron

bunchpropertiesandconcomitantphotonattributes.The

choiceofCWSCRFtechnologybeyond1–10kHzrepetition

rateopensupallthescientificbenefitsofevenhigherpulse

repetitionrateswithoutsignificantlyfurtherimpactingthe

acceleratorconstructioncosts.Thefinalmachineparame-

terswillbechosentoachieverelativelylowcapitalcostper

experimentallyusablephoton,withpulsesofshortlength

andhighspatialandtemporalcoherence,andmoderately

highbutvariablephotonfluxprovidingintensitiesbelow

thedamageordisruptionthresholdsrequiredformany

sensitiveexperiments.

Ourchoiceofabout14MeV/macceleratinggradient

favorsreducedtechnicalriskinthelinac.Weexpectto

continuetodevelopplansforcryomodulesbasedoncost

andperformanceoptimizationsofthecurrent ILCand

XFEL-baseddesigns—weseethisasoneofseveralareas

whereapartnerlaboratorywithrelevantexperienceand

expertisecouldassumeavaluablerole.Thelinacdesign

isnotyetoptimized,andwedeferfinaljudgmentonthe

exactchoiceofgradienttotheengineeringdesignphase

aftercompletionofthoroughparametriccostandperfor-

manceoptimizationoftheentireFELsystem.

TheFELoperationrequiresthat:εn/γ≤λ/4π,whereεn

theelectronbunch’snormalizedtransverseRMSemit-

tance,γistheLorentzfactor,λistheresonantX-raywave-

lengthgivenbyλ = λu(1+K2/2)/(2γ2),λuistheundulator

period,andKistheundulatorparameter.Additionally,

theenergyspreadmustbesufficientlysmallthatparticles

donotlongitudinallyde-phaseoveranFELgainlength.

Thegainisafunctionofthepeakcurrent,whichscales

proportionallytotheenergyspreadstartingfromagiven

initialbunch.Thus,compressingabunchtoincreasethe

peakcurrentandthusthegainwillunavoidablyincrease

theenergyspread,andbeyondsomepointfurthercom-

pression will become ineffective; conversely, a long

bunchwillhavealowgainowingtolowpeakcurrent.

Thesix-dimensionalelectronbrightness,definedtobe

proportionaltothedensityofelectronsintheirsix-dimen-

sionalphasespace,i.e.,B~Ne/(εnxεnyεnz),istheprimary

beamparameterthatshouldbemaximizedtooptimize

FELperformance.Thisquantityisinvariantalongtheelec-

tronbeamlineunderidealcircumstances,butinpractice

its(near)-conservationcanonlyoccurwithcarefullycon-

sideredmachinedesign.WhiletheLCLShasdemonstrat-

121


generatingtheelectronbunchesoftherequiredqualityat

highrepetitionrateusingpresentlyavailablelasertech-

nology.Thefollowingsectionsprovideanoverviewofthe

injectorandofitssimulatedperformance,followedbya

descriptionofitsmainhardwarecomponents.

5 .4 .2 .1 Injector Overview

Aconceptualdesignoftheinjectorlayoutisshown

inFigure80,andaCADrepresentationisprovidedin

Figure81.Theinjectorchainbeginswithaphotocathode

installedina187MHzRFelectrongunoperatinginCW

mode,andadrivelaser.A“bucking”solenoidintegrated

intotheguncontrolsthemagneticfieldatthecathode

surface.Thenextelementsareasolenoidfollowedbya

bunchercavityandthenbyasecondsolenoid.Theseele-

mentsinitiateemittancecompensation8,9whilesimulta-

neously performing“ballistic” bunch compression.10

Thebuncher isanormal-conducting,Cornell-designed

cavity11operatinginCWmodeat1.3GHz.Thenextele-

mentalongthelineisacryostatcontainingasingle1.3

GHz,CW,TESLA-like,superconducting9-cellcavity.

This superconducting cavity accelerates the beam

from750keVatthegunexitandperformsvelocitybunch-

ing12byde-phasingtheRFwithrespecttothemaximum

acceleration phase. Downstream from this cryostat,

anotherroom-temperaturesolenoidcontinuestheemit-

tancecompensationprocessandallowsforthecontrolof

thetransversebeamsizeintheremainingsectionsofthe

injector.Thelastelementintheinjectorisasecondcryo-

statcontainingfive1.3GHzCWTESLA-likesupercon-

ducting9-cellcavities.Withtheexceptionofthefirst

cavityinthislastcryostat,whichisde-phasedforcon-

tinuingthevelocitybunching,alltheothersarephased

formaximumacceleration.Theenergyattheexitofthe

injectorisdesignedtobeabout70MeV.

Figure81showsaCADmodeloftheassembledbeam-

lineincludingbeamdiagnosticsystems,suchasbeam

positionmonitors,currentmonitors, transverseemit-

tancemeasurementsystems,andbeamprofilemonitors,

aswellassteeringmagnetsfororbitcorrections.Inthe

mainlinac(notshowninthefigure,seeSection5.4.3.1)

downstreamfromtheinjector,atransversedeflecting

cavitysystemjointlywithaspectrometersystemwill

allowforbunchlengthmeasurements,“slice”emittance

measurements,andfullcharacterizationofthelongitudi-

nalphasespace.

whereundulatorX-rayoutputgeneratedfromthebeam

itself ispassed throughamonochromator toensure

coherenceandthenmadetooverlapthesameelectron

bunch,suitablydelayed.Whileself-seedingmethods

may have important applications, especially in hard

X-raymachines,forsoftX-raystheenormousadvantag-

esofexternalseedingusingshort-pulselasers,asinthe

EEHG, HHG and HGHG schemes, are major perfor-

mance-enhancingcapabilitiesthatareexpectedtoplay

criticalrolesinfutureX-rayFELfacilities.Atrepetition

ratesofafewMHzorhigher,oscillatorsmightbeusedin

placeofexternal lasersforseeding,openingupnew

regimesofoperation.However,thispotentiallybreak-

through technique is in its infancy at the moment.

AnotherpossibilityistogenerateshortX-raypulsesof

highcoherencewhileavoidingseedingaltogether,by

producingFELradiationin“single-spike”SASEmode,

using low-charge, low-emittance electron bunches

whoselengthisontheorderofafewSASEcooperation

lengths.Thismodeofoperationoffersalmostfulltem-

poralandtransversecoherenceaswellasfemtosecond

timingcharacteristics,butsuffersfromlargepulse-to-

pulsevariationsinX-raypower,andlesscapabilityto

synchronize timingprecisely toexternal lasers.Note

thateachalternativetechniquemightrequireadistinct

beamlineconfiguration,andinsomecasessubstantially

different electron beam parameters, but the relative

conceptualsimplicityoftheseideassuggeststhateach

maywarrantfurtherexploration.

5.4.2 Injector

TheelectronbeamqualityandthustheFELperformance

dependfundamentallyontheinjector.Theremainingsys-

tems of the accelerator can at best preserve, but not

improve,the6Dbrightnessoftheelectronbunchfromthe

injector.WhiletheexcellentresultsofLCLSandPITZ/FLASH

injectors6,7havealreadyproventhecapabilityofgenerat-

ingthebrightbeamsasneededforNGLSatrelativelylow

repetitionrates(~100Hz),noneofthepresentguntechnol-

ogieshaveyetdemonstratedacomparableperformanceat

highrepetitionrates(i.e.,tensofkHztoMHzorgreater).

TheinjectorforNGLSisbasedonanovelelectron

photo-gun design presently being pursued at LBNL.

Thisgun,inconjunctionwiththeuseofhighquantum

efficiency(QE)photo-cathodes,ispotentiallycapableof

122


fromthecathode,ischosentobetrapezoidal,withaflat

topoftheorderof50psandroughly10%riseandfall

times.Inthelow-energyregiondownstreamfromthe

gun,space-chargerepulsionfurtherexpandsthebeam

longitudinally, and, in order to achieve the required

~250fsbunchcorelengthattheFELundulatorentrance,

anappropriatecompression isrequired.Bunchcom-

pressionstartsintheinjector,wherethebunchlengthis

reducedtoatypicalFWHMvalueof~5ps,anditissub-

sequentlycompletedfurtherdownstreaminthelinac.

Atdifferentbunchcharge,bunchlengthsandcompres-

sionfactorscandifferduetodifferentspacechargecon-

ditions,wakefieldandCSReffects,etc.,buttheoverall

pictureissimilar.

IntheNGLSinjector,beamcompressionisrealizedwith

a“conventional” ballistic buncher stage, followed by

velocitybunchingintheacceleratingcavities.Theballistic

Table3containsthebaselineinjectorbeamrequire-

ments.Inaddition,preliminarysimulationsofdifferent

modesofoperationwithbunchchargefrom~10pCto

~1nCindicatetheabilityoftheinjectortosuccessfully

operateoverawiderangeofconditions.

InadditiontotheparametersindicatedinTable3,the

controlofenergy-timecorrelations(chirp)inthelongitu-

dinalphasespaceattheinjectorexitwillbecriticalto

effectiveFELperformance.Thechirpcanbepartiallycom-

pensatedbydephasingtheRFinspecificlinacsections,

andtosecond-orderbyusingahigher-harmoniccavity

linearizerupstreamfromthelinacmagneticcompressor.

Higher-ordercorrelationtermsmustbeminimizedtoallow

forsmoothcompressioninthelinacbunchcompressor.

Inordertocontrolspace-chargeemittancedilutionfor

thebaselineNGLSinjectorconfiguration,theshapeof

thelaserpulseand,hence,oftheelectronbunchemitted

Gun

~15 m

Bunc

her

Room temperature solenoid magnet Room temperature RF cavity Superconducting RF cavity and cryomodule

Exit of Gun0.75 MeV

Exit of First Cavity

~10 MeVExit of Injector

70 MeV

Figure80 Schematic layout showing the main components of the NGLS injector. RF systems operate in CW mode, and repetition rate is 1 MHz (upgradeable to ≥100 MHz at low bunch charge).

Figure81 CAD view of the NGLS injector, showing assembled systems including gun, diagnostics, buncher cavity, solenoids, and cryomodules. The injector delivers stable, high-brightness bunches at 1 MHz initially (upgradeable), for further acceleration, compression, and transport to the FELs.

123


5 .4 .2 .2 Photocathode Materials

ThephotoinjectorforNGLSisdesignedtooperateat

1MHzrepetitionrateandupto1nCpulsecharge,orpossibly

athigherrepetitionratebutcorrespondinglylowercharge.

Theoptimalwaytoachievethisdesigngoalistousemuch

higherQEcathodematerials.ConsideringthattheLCLS

coppercathodehasaQEoftypically2×10-5,andrequiresa

largelasersystem,weneedtooperateat~104timeshigher

repetitionrate,sowerequireacathodematerialwithan

overallefficiencythatishigherbyapproximatelythisfactor

tomaintainreasonabledrivelaserparameters.

Weare thereforeassessingpositive-electron-affinity

semiconductorphotocathodessuchascesiumtelluride

(Cs2Te)asusedatFLASH,anddi-potassiumcesiumantimo-

nide(K2CsSb).BothcathodesofferinitialQEssignificantly

higherthan5%withphotoemissionintheUVforCs2Teand

inthevisiblefortheK2CsSb.Becauseofitslowerelectron

affinity,thelatterisaparticularlyappealingcandidatefor

theNGLSgun.Ininitialworkinourlaboratory(seeFigure

84),wehaveshownthatthismaterialhasaQEofaround

7%underilluminationwithgreenlightat532nm,givingan

effectiveQE,includingSchottkybarrierloweringduetothe

gun’sacceleratinggradient,of~15%.K2CsSbhashighreac-

tivityandrequiresvacuumoperatingpressuresinthelow

10-11Torr,andinparticularverylowpartialpressuresfor

reactivegassessuchasO2,H2O,andCO2.

buncher,operatedin“zerocrossing”mode10doesnoton

averageacceleratethebeam. Incontrast, thevelocity

bunching,whichinourcaseisperformedbyoperating

thefirsttwosuperconductingcavitiessignificantlyoff-

crest,allowsforasimultaneousaccelerationandcom-

pressionofthebunch.12

Intheballisticbuncher,theenergychirpinducedon

thebeamisnearlylinear,andtheresultingcompression

usuallygeneratesmoresymmetricdistributionsthanin

thecaseofvelocitybunching,wheretheoff-crestopera-

tionandtherelativisticvelocitycompressionintroduce

non-lineareffectsandhenceasymmetricdistributions

withlongertails.Ontheotherhand,theadvantageof

velocitybunchingisthatitacceleratesthebeamasquick-

lyaspossible,allowingforbettercontroloverthetrans-

verseemittancegrowthduetospacecharge.

Theinjectoroptimizationprocessconsistsinfinding

anappropriatetrade-offbetweenthesetwocompression

schemeswhilesimultaneouslycontrollingspace-charge

inducedemittancegrowthbyuseofemittancecompen-

sationtechniques.Longitudinalcompressionincreases

thepeakcurrentandhenceincreasestransversespace

charge,couplinglongitudinalandtransversedynamics.

A largenumberofvariablesaffect the injectorperfor-

mance.ForNGLS,weapproachedtheinjectordesignby

usingamulti-objectivegeneticoptimizationalgorithm,

introducedinthecontextofacceleratorphysicsandpho-

to-injectorsbytheCornellgroup.13TheASTRAtracking

codewasusedforthesimulations.14

Anexampleoftheoptimizationforthebaseline300pC

caseisgiveninFigure82,whereasetofpossiblesolutions

isshowntohighlightthetrade-offbetweenRMSbunch

length(y-axis)andnormalizedtransverseprojectedemit-

tance(x-axis)attheendoftheinjector.Figure83shows

beamphase-spaceforaparticularsolutionthatrepresents

a possible match to the requirements ofTable 3.The

simulationshavebeenperformedassuminganintrinsic

(thermal) emittance for cesium telluride cathodes, as

experimentallydeterminedatPITZ.15Additionally,auni-

form“hard-edge” transversedistribution for the laser

pulseatthecathodewasusedtominimizespace-charge

effects.ForthesolutionsshowninFigure83,themaxi-

mumacceleratinggradientintheTESLA-likecavitieswas

~14MV/m,theCornellbuncherwasusedwithinitsdesign

limits,andthemaximumfieldinthe20cmlongsolenoids

was~0.1T.

0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

7

6

5

4

3

2

1

0

RMS

Bunc

h Le

ngth

(mm

)

Normalized Transverse Emittance (mm-mrad)

Figure82 Results of a multi-objective optimization study of injector configurations showing trade-off between normalized transverse projected emittance and RMS bunch length at the exit of the injec-tor for a 300 pC bunch.

124


emitsinunderUVillumination,sothattheradiationfrom

theIRlasermustbefrequencyquadrupled,withadditional

lossesinefficiency,andhenceamorepowerfulIRdrive

laserisrequiredthanisthecaseforK2CsSbwheretheIRis

onlyfrequencydoubled(seeSection5.4.2.3).

Withthisdualapproach,wearedevelopingtheideal

materialaswellasalowerriskalternativeincaseofunan-

ticipatedproblemswiththeantimonidecathodes.LBNL

hasaphotocathodelaboratorydedicatedtothiswork.We

cangrowthesematerialsbyMBEtechniquesaswellas

characterizetheirpropertiesusingwavelength-dependent

yield,angle-resolvedphotoelectronspectroscopy,PEEM

andmanyotherstandardtoolsformaterialsdevelopment

andanalysis.Wearealsousingmaterials-sciencebeam-

linesatALSandatNSLSincollaborationwithBNLtomea-

AlthoughK2CsSbhasbeentestedbeforeinphotoin-

jectors,16weareengagedinanR&Dprogramtoassess

performanceanddevelopmaterialsforoperationinour

photocathodegun,includingunderstandingoflifetime

under high repetition-rate laser shock loading and

vacuumconditionsinthegun,andcharacterizingtrans-

versemomentumspectra,surfaceroughness,andmany

otherissues.

InparallelandincollaborationwithINFN-MilanoLASA,

wearedevelopinganalternativepathofferedbytheCs2Te

cathode technology.This material does not require

vacuumpressurestobeaslowasK2CsSbdoes,andhas

beenextensivelyusedinFLASHathighrepetitionratesin

burstmode.WewillextendthisworktoexplorethefullCW

operatingmodeoftheNGLS.Thiskindofcathodephoto-

Emitt

ance

(mm

-mra

d)

-12 -10 -8 -6 -4 -2 0 2 4 6

100% projectednormalized emittance:

0.66 μm

t–t0 (ps)-12 -10 -8 -6 -4 -2 0 2 4 6

t–t0 (ps)

RMS

Ener

gy S

prea

d (k

eV)

-12 -10 -8 -6 -4 -2 0 2 4 6

Pz0 = 70 MeV/c

t–t0 (ps)

-12 -10 -8 -6 -4 -2 0 2 4 6

Current Profile

t–t0 (ps)

I (A)

0.6

0.5

0.4

0.3

0.2

0.1

0.0

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

-0.8

60

50

40

30

20

10

0

P z _ P z

0 (M

eV/c

)

6

5

4

3

2

1

0

Normalized Slice Transfer Emittance Slice RMS Energy Spread

Longitudinal Phase Space

Figure83 Example of phase-space parameters at the exit of the injector for a single optimization point in Figure 82, and for the baseline beam parameters of Table 3. The bunch head is at t-t0 > 0. The bunch has the characteristics to allow the linac systems to deliver beam of required pararameters to the FELs.

125


theinfraredregion,andlaserswitha1064nmcentral

wavelengtharereadilyavailable.Startingfromthiswave-

length,frequencydoublingandquadruplingcangener-

atethe532nmandthe266nmpulsesrequiredbythe

K2CsSbandCs2Tecathodes,respectively.Frequencyup-

conversionefficiencyvaluesare30%orbetterforsecond-

harmonic generation and ~5% for fourth-harmonic

generation.Therequirementsforper-pulseenergyatthe

cathode,given inTable4,havebeencalculated fora

chargeperbunchofupto1nCandaQEof1%.InitialQE

valuesforthesecathodematerialscanbeasmuchasone

order-of-magnitudehigher,but,becauseoffinitelifetime,

theQEprogressivelydecreasesduringoperation.Inour

assumptions, such degradation will be compensated

byincreasingthelaserenergyuntilthelowerQEbound

of 1% is reached, at which point the cathode may

bereplaced.

Thetotalper-pulseenergybudgetmustalsotakeinto

account losses in thebeamshapingoptics, transport

opticstothecathode,theUHVvacuumwindowreflectiv-

ity, and beam sampling for diagnostic purposes.

Accountingfortheselosses,anoverall15%energyeffi-

ciencyforthe532nmlaserisestimatedfromtheIRampli-

fieroutputtothecathodeplane,whichimpliesarequired

outputIRenergyfromthelaseramplifierofabout1μJ

perpulse(or1Wonaverageat1MHz).Inthecaseof

266nm,theoverallenergyefficiencygoesdownto~2%,

duetothephotonenergyquadrupling.ThisleadstoanIR

outputpowerrequirementofupto10μJperpulseafter

surethegrowthandpropertiesofthesematerials,using

X-rayprobes.

5 .4 .2 .3 Photocathode Laser

Thechoiceofthephotocathodelasersystemiscriti-

calinthedesignofamachinedevotedtosupportauser

facility. In addition to the technical specifications in

termsofpower,energyandpulseduration,otherquan-

titiessuchasrobustreliability,stability,andreproduc-

ibility,areimportanttechnicalcharacteristicsthatmust

beaddressed.

FortheNGLS,fiberlasersrepresentagoodmatchfor

thefollowingreasons:

• Theyhavebeen in industrialproductionforsome

time,andhighlyengineeredlaserlayoutswithhigh

reliabilityarenowcommerciallyavailable17

• Fiberlasershaveoptimumperformanceintermsof

timingandenergyjitter

• Theycanbeefficientlypumpedbydiodelasers

• Theyhavethecapabilityofdeliveringhighaverage

power—fiberlaserswithoutputpowerofupto~10W

ataMHzrepetitionrateareavailable

• TheuseofanactivemediumsuchasYb3+assuresa

gainbandwidthlargeenoughtosupportsub-picosec-

ond pulse durations and rise-times as may be

demandedinsomeoperatingmodesofNGLS

InTable4thelaserparametersrequiredatthecathode

planefortheNGLSareshownforthetwocathodematerials

alsounderdevelopmentanddescribedinSection5.4.2.2.

Dependingonthechoiceofphotocathodematerial,

thelaserwillberequiredtooperateatdifferentwave-

lengths.Mostofthecommercialfiberlasersoperatein

25

20

15

10

5

0200 300 400 500 600 700

Wavelength (nm)

Quan

tum

effi

cien

cy (%

)

Figure84 Quantum efficiency of K2CsSb measured in the photo-cathode laboratory at LBNL, potentially offering a very efficient photocathode design.

Table4 Laser requirements, at the cathode, for the two photo-cathodes under development to provide high brightness bunches at high repetition rate and with conventional laser systems.

LaserParameters Valueatthecathodeplane(K2CsSb)

Valueatthecathodeplane(Cs2Te)

Wavelength(nm) 532 266

Energyperpulse(nJ) upto100 upto200

Transversedistribution Quasi-uniformhard-edge

RMStransversesize*(mm) from~0.1to~1

Longitudinaldistribution Trapezoidalwith~10%riseandfalltimes

Flat-topwidth*(ps) From~1to~60

Repetitionrate(MHz) upto1(higherchargeperbunch)[Goal of 100+ at low charge per bunch, with future developments]

*Dependingonthechargeperbunch.ThechargemaybeintherangefromafewpCuptoonenC.

126


5 .4 .2 .4 Photocathode Gun

TheRFphoto-gunproposedfortheNGLSisbeing

developedatLBNLintheAPEXR&Dproject.Thegunis

basedonreliableandmaturemechanicalandRFtechnol-

ogies21,22,23—characteristicsthatareimportanttopro-

videthenecessaryreliabilityfortheoperationofauser

facility.Theprincipalcomponentofthegunisanormal-

conducting,copperRFcavityresonatingat~187MHz

(intheVHFband).Figure85showsacrosssectionofthe

VHFcavityidentifyingthemaincomponents,whileTable5

listsitsbaselinedesignparameters.

As described in Section 5.4.2.2, semiconductor

cathodescanoffertherequiredQEbutaretypicallyvery

sensitivetoionback-bombardmentdamageandcontam-

ination.Extremelylowvacuumpressures,inthe10-11torr

range,arehencenecessaryinordertomaximizethelife-

timeofthecathodes.

Two major goals are targeted in the gun design:

CWoperationcapabilitytoallowoperationathighrepeti-

tionrateofMHzorgreater,and lowvacuumpressure

underoperatingconditions.Becauseofthelowresonant

frequency,thecavitydimensionsarelargeandhencethe

powerdensityonthecavityinnerwalls,producedbyRF

currents,issufficientlysmalltobecontrolledbyconven-

tionalwater-coolingtechniques.Thus,thecavitycanwith-

standtheheatloadwhenoperatinginCWmodeatthe

requiredgradients.Thisdesigncanrealizeasignificantly

highergradientandbeamenergythancanbeachievedin

DCguns.Furthermore,thelongRFwavelengthallowsfor

thelargehigh-conductancevacuumports(thenumerous

finalamplification,or~10Wofaveragepowerat1MHz

operation.CommercialYbfiber lasersdelivering2μJ

perpulseat1MHz,andupto10μJperpulsewithsub-ps

pulsedurationarecurrentlyavailable.

Uniform,“hard-edge”transversedistributionscanbe

achievedbymeansofpulse-conditioningopticalsys-

tems.Anexampleofapracticalandflexiblelayoutcon-

sistsofatelescopesystemfollowedbyanaperture.The

telescopeexpandsthebeamtransversely,overfillingthe

aperture,therebyselectingthepulse’scentralquasi-uni-

formregion.Relayopticsthenimagetheapertureonthe

cathodeplane,withthedesiredmagnification,andwith

thebeneficialeffectofimprovingthelaserpointingsta-

bility.Thisisaneffectivebutinefficientscheme,withfor

exampleanorder-of-magnitudelossintransmissionfor

a10%flattopintensitydistribution.Alternativeschemes

basedonasphericlensescanalsobeused.18Thesesys-

temsbenefit fromahigherefficiency,but requirean

accurateandstablealignmentandrelyontheGaussian-

liketransversebeamprofilesaffordedbyfiberlasersin

ordertooperatecorrectly.Bothschemesareundercon-

siderationfortheNGLS.

Therequiredtemporallasershapingcanbeachieved

byvariousschemes.Amongthese,pulse-stackingusing

birefringentcrystalshavebeendemonstratedtobeboth

reliableandefficient.19Insuchsystems,asinglepulseis

splitintoapairofpulseswithorthogonalpolarizationby

abirefringentcrystal; thispairofpulsesthenpasses

throughanotherbirefringentcrystal,witheachpulse

formingapairofdaughterpulses,forming2npulsesfor

ncrystals;thesepulsereplicascanbearrangedtopar-

tiallyoverlapintime,formingaquasi-flat-topbeam.The

riseandfalltimesoftheshapedpulsedependonthe

originalpulselength.Theappealingcharacteristicsof

thisschemearesimplicityandefficiency (more than

90%whenusinganti-reflectioncoatingsonthecrystals).

Thedrawbackisalackofflexibilityinpulsetimedura-

tion.Otherpulse-shapingschemescanbeusedifthe

pulselengthneedstobecontinuouslytuned.Forexam-

ple,shapersbasedongratingpairsaregoodcandidates

insuchacase,20allowinganextendedrangeofcontinu-

oustuningofthefinalpulselength.Ontheotherhand,

suchflexibilityisprovidedattheexpenseofsystemeffi-

ciency,whichtypicallydropsto~50%.FortheNGLS,we

envisionthatultimatelyacombinationofsystemswill

beusedtoaccommodatethedifferentmodesofopera-

tionofthefacility.

Table5 The main parameters for the CW photocathode gun built and undergoing tests at LBNL. The nominal bunch rate is 1 MHz, with bunches up to 1 nC, upgradable to ≥100 MHz with significantly lower bunch charge.

Totallength(m) 0.35

Acceleratinggap(mm) 40

Q0(idealcopperconductor) 30900

Electricfieldatthecathode(MV/m) 19.5

Storedenergy(J) 2.3

Maximumwallpowerdensity(at0.75MVgapvoltage)(W/cm2) 25.0

Cavityinternaldiameter(m) 0.694

Cavityresonantfrequency(MHz) ~187

Gapvoltage(MV) 0.75

Peaksurfaceelectricfield(MV/m) 24.1

RFpowerfor0.75MVatQ0(kW) 87.5

Operationpressure(Torr) ~10-11

127


The resonant copper structure is surrounded by a

stainlesssteelshellthatensurestherequiredmechanical

rigidity,andprovidesthepumpingplenumandvacuum

sealing.Toavoidmechanicalmovingpartsinsidethecav-

ity,thefrequencytuningisachievedbyaremotelyoper-

atedmechanicalsystemthatslightlydeformsthecavity

wallatthebeamexitplane.TheRFpowerissupplied

through two magnetic loop couplers diametrically

opposedonthecathodebackwall.The~187MHzfrequen-

cychoiceiscompatiblewithboth1.3and1.5GHz,thefre-

quenciesofthetwodominantSCRFlinactechnologies

(theILCandXFELaredesignedwith1.3GHzstructures

operatinginpulsedmode,theTJNAFCEBAFandits12GeV

upgradecavitiesandtheJLABFELcavitiesare1.5GHz

structuresandoperateinCWmode.)

TheVHFcavityfabricationhasbeencompleted,and

thefirstRFtestsatlowpowerhavebeensuccessfullyper-

formed; the firstvacuumtestshavealsosuccessfully

completed.Figure11showsthecompletedVHFcavity

duringcalibrationoftheRFcouplers.

5.4.3 Linac

5 .4 .3 .1 Overview of the Linac

Thelinacisthecentralpartofthebeam-deliverysys-

tem,performingthebasicfunctionsofacceleratingand

manipulatingtheelectronbeamasrequiredforlasing.

Designedtoacceptelectronbunchesatabout70MeV

energyfromtheinjector,itprovidesaccelerationupto

1.8GeVbeforedirecting thebeamto thespreader for

distributionintotheseparateFELundulatorlines.

Theproposedlayout,basedonthepreliminarychoice

of TESLA-like superconducting cavity technology,

includescomponentsthathavebecomeconventionalin

existingorproposed4th-generationlightsources25,26,27,28

(seeFigure76).Thelinacconsistsofsixmainsections.

Thefirstsection,Linac0, interfacesthelinacwiththe

injector,providesabout90MeVacceleration,andaccom-

modatesthediagnosticsstationsneededtomonitorthe

beam phase space (see Section 5.4.2.1) before its

entranceintothe“laserheater.”

Thelaserheaterisintendedforcontrolofthebeam’s

uncorrelatedenergyspreadandforstabilizationofthe

beamdynamics.Thebeamisthenfurtheracceleratedin

Linac1(with225MeVenergygain),conditionedbypas-

sagethrougha3.9GHzthird-harmonicRFstructure,com-

pressedthroughasingle-chicanebunchcompressorat

slotsvisiblealongthe“equator”ofthecavityinFigure85)

thatarenecessaryforachievingthedesiredvacuumpres-

surewhencoupledtoapumpingplenumsurroundingthe

cavityequator,andwithnegligibleRFfielddistortion.The

useofa largenumberofnon-evaporablegetter (NEG)

modules as the main pumping system will efficiently

removethosemolecules(H2O,O2,CO2,etc.)thatarepar-

ticularlydamagingtothecathodeandreducelifetime.

Avacuumload-locksystem,basedonadesignusedat

FLASH24willallowforthereplacementofandtheinsitu

conditioningofphotocathodes.

IncontrasttoSCRFguns,whichareaffectedbyfield

exclusionandmagneticfieldquenchinglimits,thenor-

mal-conducting structure allows for straightforward

applicationofmagnetic fields, required foremittance

compensationandexchangetechniques.

NEG modules

Tuner plate

Cathode

Beam exitportSolenoid

RF Couplers

Cathodeinjection/extraction

channel

Figure86 Assembly of the coaxial power input lines on the CW VHF cavity.

Figure85 A cross-section through the diameter of the CW VHF cavity, showing the main components. The cavity design allows for ≥1 MHz bunch rate, with a relatively high gradient at the cath-ode, and low vacuum pressure.

128


emittance,andenergyspread)thatmatchthosefromthe

injector(seeSection5.4.2).Furtherstudieswillincludefull

start-to-endbeamdynamicssimulationsanddesignopti-

mizationoftheintegratedinjectorandlinacsystems.

5 .4 .3 .2 Collective Effects

Thebeamdynamicsthroughthelinacareprimarily

determinedbytheelectrons’interactionwiththeexter-

nallyappliedelectromagneticfields,withthebeamself-

fieldsplayinganon-negligibleandmostlydisruptiverole.

Indeed,considerationofcollectiveeffects,andmeansto

mitigatetheirimpactonthebeam,isanimportantaspect

ofasuccessfullinacdesign.Thesignificantsourcesofcol-

lectiveeffectsincludeRFwakefields,space-chargeforces,

andcoherentsynchrotronradiation(CSR).

RFwakefields impactthesingle-bunchlongitudinal

phasespacebydirectlyinfluencingthebeam’slinearand

nonlinearenergychirpandby indirectlyaffecting the

bunchcurrentprofileasthebeamtravelsthroughthe

magneticcompressor.TheRFwakefieldcontributionto

thelinearchirpisgenerallybenignand,infact,itturns

outtobebeneficialasitassistswiththeremovalofthe

beam energy chirp beyond the bunch compressor.

QuadraticcontributionsbytheRFwakestotheenergy

chirpbeforethebunchcompressorareusuallymodestand

can,inprinciple,beoffsetbyappropriatetuningofthelin-

earizer,whilethethird-ordercontributionscanbelarge

andarenoteasilycompensated.Tosomeextenttheirpres-

encecanalsobebeneficial;forexample,theymaycause

thepeakedbunchprofilesnaturallyemergingfromthe

injectortobecomeflatterasthebeamexitsthemagnetic

compressor,whichisnotundesirable.However,strong

cubicandhigher-ordercomponentsintheenergychirp

haveatendencytocausefoldingofthebeamdistribution

inphasespaceandleadtocurrentspikesattheedgesof

thebunches.Thiscanlimitthemaximumachievablecom-

pressionorcompromisethepreservationofbeamquality.

about350MeVenergy,andthenacceleratedtothefinal

energybyLinac2,thelastlinacsection.Giventhe30–50

Arangeforthebeampeakcurrentoutoftheinjector,a

10–17compressionfactorisrequiredinthelinac.Figure

87showsthebeamenergyatdifferentsectionsofthe

machine.Thebetatronanddispersionfunctionsthrough

thespreaderareplottedinFigure88.Thelatticedesign

includesprovisionsforbeamcollimatorsplacedinthe

bunchcompressorandatvariouslocationsalongLinac2

incorrespondencetothelocalmaximaofthebetatron

functions,andadditionalshieldingwillbeemployedin

theseareas.

Thelatticeisavariantofourearlierconceptofa2.4GeV

linacdriverdiscussed inReference29,andhasbeen

modifiedtodeliver1.8GeVenergybeams.Thestudieswe

havecarriedouttodatetosimulatethemachineperfor-

mancehaveusedidealized(i.e.Gaussian)electronbunch-

es at injection with basic properties (peak current,

Exit of Injector70 MeV

Laser Heater160 MeV

Exit of Linac 0160 MeV

Exit of Linac 1385 MeV

Exit of Harmonic Linearizer350 MeV

Bunch Compressor350 MeV

Exit of Linac 21.8 GeV

Figure87 Schematic layout showing beam energy at major sub-systems along the linac for the NGLS baseline design.

Dz

300.

150.

200.

150.

100.

50.

0.0

Beta

tron

func

tions

(m)

Disp

ersi

on fu

nctio

n (m

)

0.0 100. 200. 300. 400. 500.

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

s (m)

Figure88 Lattice functions from Linac 0 through the beam spread-er. The expansion of the betatron functions along Linac 2 allows for some compensation of the geometric emittance reduction due to acceleration, as a means to ease collimation. Collimators may be located at the maxima of the betatron functions in the linac.

129


Thechoiceofbeamenergyatwhichthemagneticcom-

pressionoccursaimsatreducingtheimpactofCSRonthe

horizontalemittance(whichscalesinverselywiththebeam

energy),butfacesothertrade-offs.Alowerbeamenergy

wouldbefavored,amongotherreasons,byconsideration

ofthemicrobunchinginstability(alowerenergywould

increaselongitudinalphase-spacemixingandreducethe

impactofLSCforces,asalower-energychicanewould

reducethebeamtime-of-flightbetweentheinjectorand

thechicane).Thevalueadoptedforthisproposal,350MeV,

appearstostrikeanadequatebalancebetweenthesecom-

petingrequirements.Furthercontainmentofthecollective

effects can be accomplished by careful lattice design

aimedatminimizingthedispersioninvariantfunctionin

theendregionofthechicane.Theadopteddesignisacon-

ventionalfour-bend,C-shaped,12.64mlongchicanewith

nominalR56=–0.135m.Thelayoutincludestwosmalltrim

quadrupolesfollowingthefirstandprecedingthefourth

dipoleandacollimatorplacedbetweenthesecondand

thirddipoles.

Adegreeofcontroloverthemicrobunchinginstability

isofferedbytheuseofalaserheater.38Thelaserheateris

essentiallyaninverseFELconsistingofawigglerinserted

inasmall,dedicatedchicane.Aconventionallaserpulse

interactswiththebeaminthewigglerandinducesan

energymodulation(atawavelengthequaltothatofthe

laser),whichbytheexitofthechicaneiseffectivelycon-

vertedintoanuncorrelatedenergyspread.Asthedevelop-

mentofmicrobunchingissensitivetotheuncorrelated

energyspread,propertuningofthelaserpulsepower

allowsforaneffectivecontroloftheinstability.Thepro-

posedlaserheaterlocatedataboutthepointof160MeV

beamenergyissimilartotheLCLSdesign39andisbased

ona800nmlaseranda9-period,3cmwavelengthwig-

gler.Alaserpulse,sufficientlylongtoaccommodatethe

electronbunchandcarryingafewμJ’sofenergy,willsuf-

ficetoinducethefew-keVenergyspreadthatourstudies

indicateareneededtostabilizethebeam.29Commercial

laserswithsuchcharacteristicsarereadilyavailable.

Anessentialcomponentofthemachinelayoutisahigh-

erharmonicRFstructure40,41neededtolinearizethelongi-

tudinalphasespacebycorrectingthequadraticterminthe

beamenergychirpbeforethebeamentersthebunchcom-

pressor.Theseenergy/positioncorrelationsarecausedby

theRFwaveformintheacceleratingstructures,thenonlin-

eartermsinthemomentumcompactioninthebunchcom-

pressionchicaneand,possibly,acontributionfromtheRF

Transversespace-chargeeffectsaremostlyconfined

tolowbeamenergyandaregenerallynotofconcernin

thelinac,havingbeensuccessfullydealtwithintheinjec-

tor. Longitudinal space-charge (LSC) effects are also

strongeratlowerenergy,butcontinuetohaveanimpact

onbeamdynamicsathigherbeamenergy,particularlyon

shortlength-scales,andarethemaindriveroftheso-

called“microbunching instability”.30,31This instability

developsfromenergymodulationsalongabunchcaused

byLSCorothercollectiveeffectsandbytheunavoidable

smallchargedensityfluctuations(i.e.,shotnoise),pres-

entinthebeam.Asthebeamtravelsthroughthedisper-

siveregioninthebunchcompressorandthesubsequent

acceleratingand transport sections, theamplitudeof

thesedensityandenergyfluctuationscangrowandspoil

thebeamquality,unlesssuppressedbythelaserheater,

tobedescribedbelow.

Coherentsynchrotronradiationemittedinthebending

magnetsbyalongitudinallysmoothbeamcausesener-

gy/position correlations along a bunch analogous to

thosegeneratedbyRFwakefields,32,33andhorizontal

emittance growth via longitudinal/ transverse motion

coupling.Inaddition,CSRcanaggravatethepresenceof

smallcharge-densityfluctuationsandfurtherenhance

themicrobunchingphenomenoncausedbyLSC.34,35

5 .4 .3 .3 Bunch Compressor, Laser Heater, and

RF Cavity Linearizer

Considerationofthemicrobunchinginstabilityisthe

mainmotivationforourpreferencetohaveasingle-chi-

canebunchcompressorinthelattice.Themicrobunching

instabilitycancauseanincreaseinuncorrelatedenergy

spreadbeyondaleveltolerableforefficientapplicationof

laserseeding,andpreviousstudies36,37haveshownthat

the instability is substantially amplified by passage

throughmultiplechicanes.Themagnitudeoftheamplifi-

cation,however,isalsocriticallydependentonthebeam

current,andourlatticedesignchoiceswillberevisited,to

furtherweighthebenefitsofamultiple-chicanecompres-

sor(suchasreducedsensitivitytobeamtimingjitterand

morecontroloverthebeamenergychirp)againstthe

consequences of the microbunching instability.

Moreover,additionalbunchcompressorsmaybeneces-

saryiffuturedesignoptimizationstudiesindicateaneed

to modify the balance in favor of more compression

occurring in the linacversus thatperformedat lower

energyintheinjector.

130


TheELEGANTsimulationsincludedthelongitudinal

RFwakefields,employingtheavailablemodelsforthe

TESLA-likecavities,46,47,29CSRasimplementedbythe1D

modeldescribed inReference48butnot longitudinal

spacechargeeffects(toavoidamplificationofartificial

instabilitiescausedbythelimitednumberofmacroparti-

cles),whereastheIMPACTsimulationsincludedfulllon-

gitudinalandtransversespace-chargemodelingaswell.

TheIMPACTsimulationswerecarriedoutwithonebillion

macroparticles(onlyaboutafactoroftwosmallerthan

theactualelectronbunchpopulation).

wakefields.Athirdharmonic(3.9GHz),RFstructuredevel-

opedfromonerecentlyinstalledatFLASH,42with5MeV

maximalenergypercavitybutwith7orperhaps9cavities

insteadofthe4cavitiesoftheFLASHlinearizer,representsa

natural choice. Beam dynamics simulations point to a

requirementforthelinearizervoltageontheorderof35MV.

5 .4 .3 .4 Simulated Beam Dynamics and

Expected Performance

Theperformanceof theproposed linacdesignhas

beeninvestigatedwithmacroparticlesimulations.These

numericalstudiesfocussedonthreeimportantaspectsof

beamdynamics:theevolutionofthelong-scalefeatures

ofthelongitudinalphasespace,CSR-inducedemittance

growth,andthemicrobunchinginstability.Weevaluated

thefirsttwoeffectswiththecodeELEGANT 43usinga

relativelysmall(butforthispurposeadequate)number

of macroparticles (2×105), whereas we employed the

IMPACTcode’s44capabilitiesforhighresolution,billion-

macroparticlesimulationstoaddressthemicrobunching

instability,45whichisnotoriouslysensitivetospurious

noiseinducedbyasmallpopulationofmacroparticles.

ResultsofthesesimulationsareshowninFigure89and

Figure90.Inbothcasesweassumedatthelinacinjection

a300pCbeamwithGaussiandensitytruncatedatabout

3σinthefull6Dphasespace(1.8σinzintheIMPACTsim-

ulations),40Apeakcurrent,and0.6μmnormalizedtrans-

verseRMSemittance(i.e.abeammatchingthebasic

propertiesoftheinjectorbeamasrevealedbytheASTRA

simulationspresentedinSection5.4.2.1).Thelinacwas

tunedtogenerateabouta15-foldcompressiontoachieve

apeakcurrentbetween550–600Aatextraction.

1804

1802

1800

1798

1796

1200

1000

800

600

400

200

0

E (M

eV)

Curr

ent (

A)

z (mm) z (mm)-0.05 0.00 0.05 0.10 -0.05 0.00 0.05 0.10

dE/E

(%)

z (mm)

–0.18

–0.08 –0.06 –0.04 –0.02 0 0.02 0.04 0.06 0.08 0.1

–0.16

–0.14

–0.12

–0.1

–0.08

–0.06

Figure89 Density plot of the beam longitudinal phase space (left) and current profile (right) at exit of the spreader starting from a 40 A peak current Gaussian bunch at injection to the main linac. Note a small residual energy chirp (left) and relatively flat charge density (right) in the beam core. The bunch head is at z<0 (ELEGANT simulation).

Figure90 Density plot of the beam longitudinal phase space at exit of the linac showing evidence of modest microbunch-ing instability growth. The slice rms energy spread averaged over the 300 fs (or 100 μm) long beam core is about 62 keV, having started with a 4 keV uncorrelated RMS energy spread beam at injection (high-resolution IMPACT simulation using a billion macroparticles).

131


structuresmustoperateatatemperatureof~2K,there-

forerequiringalargecryogenicsystem.

TheoriginalTESLAmodulesdevelopedatDESYwere

intendedtorunatalowdutycycle(1%),andhencethe

cryostatdesignisnotsuitableforCWoperation.However,

there already exist successful examples of cryostat

designs51supportingTESLA-likecavitiesthathavebeen

modifiedandconvertedtoallowforCWoperationata

reducedaccelerationgradient,and theTJNAF12GeV

upgrademodulesweredesignedtoruninCWmodeat

highgradientof~20MV/m;seeFigure91.

FortheNGLSlinacwehaveassumedanacceleration

gradientofabout~14MV/m,asanestimatedoptimaltrad-

eoffbetweencapitalcostsscalingwiththenumberofcryo-

modulesandsizeofcryogenicssystems,andoperating

costs.Furtherstudieswillrefinethisvalue,andwenote

thatwithahigherQ-valuewemayuseahigheraccelerat-

inggradientforafixedcryosystemcapacity,increasingthe

electronbeamenergyandtherebythemachineperfor-

mance,withoutsignificantincreaseincosts.

Thecryomoduleconfigurationmustprovideadequate

operationalflexibilityandeasyaccessforfuturemainte-

nanceandrepair.Itshouldalsohaveamanageablesizethat

candelivertherequiredacceleratingvoltage,acceptable

higher-order-mode(HOM)damping,capabilitytoabsorb

beam-inducedpower,andnecessarycryogeniccooling.

Thefinalcryomoduledesignwillberefinedtoreliablymeet

therequirementsfortheNGLS.ExperienceatTJNAFwith

both the12GeVupgradecryomoduledesign,and the

operationoftheIRFELinwhichsignificantlygreaterthan

1mAaveragebeamcurrenthasbeencirculatedsuccess-

fully(albeitatlowerenergy),indicatethatoptimalengineer-

ingsolutionscanbefoundfortheNGLSparameters.

ThecryomodulesandSCRFcavitiesattherelevantfre-

quencyfortheS-band(3.9GHz)harmoniccavitylinearizer

BoththeELEGANTandIMPACTsimulationsshowthat

theinitialGaussianbunchtransformsalongthelinacinto

abunchwitharelativelyflatenergydistributioninthe

coreasaresultofthelongitudinalRFwakefieldsgenerat-

edintheRFstructuresbeforethebunchcompressor.The

beamenergychirpbeyondthebunchcompressorispar-

tiallyoffsetby theRFwakes inLinac2,butcomplete

removalofthechirprequiresoperatingLinac2offcrest

byabout25degrees.TheELEGANTsimulationsshowa

CSRinducedprojectedemittancegrowthto0.73µmover

theentirebunchandto0.65µmovertheusefulcoreof

the beam (with the slice emittance in the beam core

remainingaboutunchangedat0.6µm).

IMPACTruns,takingintofullaccountLSCeffects,were

repeatedforvariousvaluesoftheuncorrelatedenergy

spreadoftheinputbunch,meanttomodeldifferentset-

tingsofthelaserheater.Wefoundthatabeamwithan

initial4keVuncorrelatedRMSenergyspreadisscarcely

affectedbythemicrobunchinginstability(seeFigure90),

andthattheresultingenergyspreadattheendofthe

linacremainsclosetothevalueexpectedfromidealcom-

pression(i.e.,60keV).

5 .4 .3 .5 CW SCRF Cryomodules

Thehighenergybeamrequiredforlasinginthesoft

X-rayrangecanonlybeattainedattheintendedhighrep-

etitionrateof1MHzbymeansofSCRFcavitiesoperating

intheCWmode.SuperconductingRFlinacstructures

havebeendevelopedovermanydecades,withsignifi-

cantprogresshavingbeenmadeonL-Band(1.3-1.5GHz)

operation.Thebaselineoptionadoptedforthisproposal

assumesthe1.3GHzTESLA-typecryomoduleswith9-cell

cavitystructures,49,50with the1.5GHzTJNAF12GeV

upgrademoduleswith7-cellcavitystructuresobviously

representingaviablealternative.Thelinacaccelerating

Figure91 TJNAF concept for a CW SCRF cryomodule for the 12 GeV upgrade of CEBAF. SCRF cryomodules operating in CW mode with accelerating gradient exceeding the NGLS specifications have already been developed at TJNAF.

132


The RF system will be distributed along the path of the

linac and in close proximity to it in order to limit the length

of the RF transmission lines. Each RF station will be

equipped with an RF power distribution system including

circulators to protect amplifiers from reflected power, and

a digital RF control system (see Section 5.4.3.7). The RF

controller design derives directly from experience both

with controlling the SNS superconducting linac53 as well

as with the FERMI@elettra FEL.54 This system will be close-

ly integrated with the RF timing and pulse distribution sys-

tem such as the one LBNL delivered to the LCLS.55, 56, 57

The RF system for the S-band linearizer will follow the

same approach as that of the main linac. A 3.9 GHz CW klys-

tron is commercially available. The RF controller for the lin-

earizer will be analogous to the controller for the main linac

for a configuration in which all clocks and reference pulses

are commonly derived and fully synchronized.

5.4.3.7 RFControl

A digital RF controller will constitute the signal coordi-

nation hub of each RF station. The information derived

from cavity RF vector measurements will be used to con-

trol the RF power source, correct for cable drift, detect

faults, operate the tuner(s), and set the cryogenic balanc-

ing heater; the controller’s digital nature also will allow

the recording of all key variables to aid in first-fault identi-

fication. All these functions will run autonomously under

the global control system and in coordination with the

logic embedded in the RF power system. At the lowest

level, a digital self-excited-loop, as pioneered by Jefferson

Lab, is the appropriate model for turning on and stabiliz-

ing a narrow-band CW superconducting cavity. The RF

controller is a key element in managing cavity frequency

perturbations caused in particular by mechanical vibra-

tion (i.e., microphonics).

Drift and jitter of the phase and amplitude of the accel-

erating sections of the linac would cause undesired fluc-

tuations of the electron beam energy, correlated energy

spread, peak current, slice emittance, and arrival-time of

the beam at the undulator. This could include drift and jit-

ter of an individual RF station as well as drift of the rela-

tive phase and amplitude of different linac sections. The

control of the vector-sum of the accelerating fields will be

accomplished by a combination of local and global feed-

back and feed-forward.

The requirements of the RF control system are derived

from the desired beam parameters such as bunch-

to-bunch energy spread, the bunch compression in the

have already been developed at Fermilab.52 The design will

be developed to accommodate CW operation.

LBNL plans to engage partner laboratories and institu-

tions in providing the required SCRF expertise for NGLS.

Several DOE Laboratories and NSF institutions have the

required experience and infrastructure.

5.4.3.6 RFPowerSystems

The RF power system must allow for delivery of beam of

up to 1 mA at 1.8 GeV energy (i.e. a beam power of 1.8 MW),

with sufficient margin for stability. We assume for the base-

line design an unloaded Q0 for the cavities of 1x1010 and

further design studies will refine the performance

capabilities,with potential improvements to be gained with

higher Q0. Table 6 shows major RF system parameters.

AQ-value of 1.4x1010 would allow a 20% increase in gradient

(see Section 5.4.3.6 and Table 6), and improve performance

of the FELs both in the power output in the fundamental and

in the harmonics, as well as in the photon energy reach.

Each multicell cavity in the linac will be powered by a ded-

icated RF power source, of approximately 21 kW output

power. Individual sources offer robustness and beam avail-

ability, and optimal control of the high-Q superconducting

cavities. Both suitable CW klystrons and Inductive Output

Tubes (IOTs) are commercially available. The latter offer high-

er efficiency and lower operating cost, although at a lower

gain. One of the main advantages of IOTs is their low group

delay, which allows for the design of more effective feedback

control loops around the power source for gain, phase, and

amplitude control. In either case, we plan to develop a solu-

tion in close collaboration with industry and, whenever pos-

sible, with multiple sources to ensure competition as well as

to reduce risk. A 21 kW IOT requires a 40–50 kW DC supply,

and ~80 kW installed capacity, drawing ~53 kW wall-plug

power for the conditions in Table 6.

Table 6 Parameters for the NGLS linac RF structures. Mainlinac Linearizer

RF frequency (GHz) 1.3 3.9

Qo 1×1010 5×109

Qext 1x107 3.3×106

R/Q (Ω) 1036 750

Ibeam (mA) 1 1

Cavity voltage (MV) 14 5

Cavity gradient (MV/m) 13.5 14

Beam phase (degrees) variable –180

Cavity type 9 cell 9 cell

Number of cavities 144 7

RF tube power rating per cavity (kW) 21 3.1

Total installed RF power capability (MW) 3 0.02

133


on the distribution system. Practical limits on heat

exchangersizesforthe5Kto2Kstageareconsistentwith

incorporatingsuchaunitintoeachcryomodule.

Returngaspipesizeswithinthecryomodulesmustbe

sufficienttocontroltwo-phaseflowvelocitiesand,incon-

junctionwiththegasreturnpiping,mustlimitthepressure

droptoafewmbar.Cryomodulepipingsystemsmustalso

accommodatetherapidheliumvaporgenerationassoci-

atedwithaloss-of-vacuumaccident.

Detailedrequirementsforthecryogenicssystemswill

bedevelopedinparallelwiththeoveralllinacdesign.

5.4.4 Beam Spreader

Theelectronbeamspreaderdistributesthebunchesfrom

thelinacintotheindividualFELundulatorlines.Inthepro-

poseddesign,allthebeamlineslieononesideofthelinac

axis.Thebunchesareextractedintoallthebeamlines(except

thelastone)byfastkickersoperatingatupto100kHzrepeti-

tionrate,anddistributedalongaFODOtransportchannel,as

sketchedinFigure92.Thelastbeamlinereceivesbunches

diverted by a conventional electromagnet and is thus

enabledtooperateatthemaximumrepetitionrateallowed

bythemachine,representingtheobviouschoiceforhosting

aSASEFELlineunconstrainedbyseedlaserrequirements.

Thetwo-meterlongstriplinemagnetkickersarelocated

betweenfocusing(F)anddefocusing(D)quadrupoles,with

the downstream defocusing quadrupole adding

0.7mradtotheprimarykick.Theorbitpassesthroughthe

focusingquadrupolewitha15.8mmoff-setfromthemagnet

centerandisforcedtofollowalinealmostparalleltothe

FODOaxisbeforethebeamenterstheseptum.Thedefocus-

ingquadrupoledownstreamoftheseptumisalarge-bore

design,centeredontheFODOaxisandsupplyinganaddi-

tional17mradkick.Pastthisquadrupole,thetwobeamlines

continueinseparatedvacuumchambers.Thenextdown-

streamfocusingquadrupolealongtheFODObeamlineisa

small-boreseptumquadrupole:thelinebranchingoffpasses

injector,andthearrival-timeofthebeamattheundulators.

Thebeamparameterscanbetranslatedintotherequire-

mentsforphaseandamplitudestabilityoftheaccelerating

fieldofindividualcavities.Forexample,RFsystemsinthe

injectorwillrequiretightfieldcontrolontheorderof0.01%

fortheamplitudeand0.01°forthephase.29

Thedegreeofprecisionachievableforthecontrolof

each individualRFstation isultimately limitedbythe

noiseflooroftheRFsignalprocessingelectronics.For

furthercontroloftheacceleratingfields,itisenvisioned

thatthecontrolsateachRFstationwillincludeinputs

from beam measurements derived from diagnostic

stationsalongthelinac,allowingbeam-basedfeedback

controlofthelinac.Diagnosticsincludemeasurementof

the relativebeamenergy,bunch length,charge,and

arrivaltimerelativetothemasterclock.Asuitablelinear

combinationoftheseparameters,alongwithindividual

RFstationcontrol,willallowthebestpossiblestabiliza-

tionofthebeamparameterscriticalforstableoperation

of theFEL.Adiscussionof thepotentialadvantages

offeedbacksystemsonstabilityoftheX-raypulsescan

befoundinReference29.

5 .4 .3 .8 Cryogenic Refrigeration and Distribution

Thecryogenicheatload,scaledfromtheCEBAF12GeV

upgradecryomoduletestresultstoNGLSlinacoperating

conditions(Table6),isabout3kWat2K.Toallowfortran-

sients and non-optimal equipment performance, an

installedcapacityof4.5kWispreferred.Heliumrefrigera-

torsofcomparablescaleareinoperation,forexample,at

TJNAF’s12GeVrefrigerationsystem.Theratioofelectric

powertorefrigerationcapacityforsuchsystemsisabout

1kWinputpowerperWdissipatedat2K,andwethus

requireabout4.5MWinstalledcapacityand3MWtypical

powerdraw.

Distribution of liquid helium in vacuum-insulated

coaxiallinesiswell-establishedtechnology.Distributing

the5Kratherthan2Kliquidcaneasetherequirements

Kicker

F F10800

F F

F

D

F

D

D D DD

KickerKickerSeptum Septum

150 466

Figure92 Schematic layout of a section of the electron beam spreader showing the fast kickers and septa modularly inserted along the FODO channel. Dimensions are given in mm. Note that scales are different in the vertical and horizontal direction.

134


thoseofthebeamscontinuingstraightdownstreamofthe

take-offsection(justoutsidethemagnet)—seealsothe

insertpicture.Athinmagneticshieldiswrappedaroundthe

vacuumchamberinthislocationinordertoreducetheresid-

ualfieldexperiencedbythebeamtravelingoutsidethemag-

net.Calculationsshowanacceptableresidualfieldonthe

orderof0.05Gorsmaller(withthemagneticfieldinsidethe

magnetbeingabout1.1kG).To-datewehavemadea2D

modeloftheseptum:afull3Dmodelwillberequiredtofully

understandimportantendeffects.

5.4.5 FEL Beamlines

5 .4 .5 .1 Overview of the FELs

TheNGLSdesignincorporatesmultipleFELbeamlines

(assummarizedinTable2andinmoredetailinSection

5.4.5.2below),eachofwhichwilldeliverX-raybeamswith

distinctivephotonattributes,astoenergy,pulseduration,

bandwidth,polarization,photonflux,synchronization,and

pump-probecapabilities.Typicallyasinglebeamlinewill

spanafactorof3-5inphotonenergy,dependingonthe

undulatorparametersandtechnology(seeSection5.4.5.4).

Thethreeproposedbeamlinesare:

• Beamline1:Aseededbeamlineproducingcloseto

transform-limitedpulses

• Beamline2:Aseeded two-colorX-ray,ultrashort-

pulsebeamline

• Beamline3:ASASEbeamline

Thetwoseededbeamlinesarecapableofoperatingat

upto100kHzrepetitionrate,whereastheSASEbeamline

mayoperateat the fullmachine repetition rate.Each

beamlinewillcoverawavelengthrangefromaboutone

bythisquadrupoleata150mmdistancefromtheaxisofthe

FODObeamline.

Thelatticefunctionsthroughalineinthespreaderare

showninFigure88.Thespreaderlatticehastwodistinct

parts,namelythebeamtake-offsectionandtheFELfan-out

distributionsection.Eachpartisbuiltasatriple-bendachro-

mat.Inthebeamtake-offsectionthekicker,septumandoff-

setquadrupolesarefunctionallyequivalenttoonebending

magnet,whileanadditionalpairofbendingmagnetscom-

pletesthefirstachromatsupplyinga60mradangle.The

designforthesecondachromatprovidesfurtherbendingto

directthebeamlinesintotheexperimentalhallasdesired

withananglethatcanbeeasilyarrangedtobebetween

10–140mrad,accordingtotheoveralllayoutofthefacility.

Thedesignoftheachromatshastheflexibilitytoallowfor

tuningofthetransfermatrixelementR56(controllingthepar-

ticletime-of-flight)inordertoenforceisochronicity.Previous

beamdynamicsstudies58showedtheimportanceofthetwo

triple-bendachromatsbeingindependentlyisochronousso

astominimizetheeffectofmicrobunchingonthemicron

length-scale,whichcandevelopasthebeamtravelsthrough

thespreader.Simulationsshownosignificantdeterioration

inbeamqualityinpassingthroughthespreader.

5 .4 .4 .1 Kickers and Septa

The kickers and septa are key components of the

spreader,andaprototypekickerandpulserarebeingbuilt

atLBNLtodemonstrateperformancegoals.

Thekickersarerequiredtosupply3mradkicktothe

1.8GeVbeamoveramagneticlengthoftwometers,with

desiredriseandfalltimesofabout5ns(allowingforfuture

operationwithmuchhigherbunchrates),andapulserepeti-

tionfrequencyupto100kHz.Pulse-to-pulsefluctuations

andripplebetweenpulsesshouldbelessthan±0.01%of

thepulseamplitude.Thedesignassumesavacuum-core,

matched-impedance stripline kicker magnet.A solid-

state,transmission-lineaddertopologywasselectedas

thebaselinechoice,providingbipolar205Apulsesat10.2kV

with a 50 Ω termination. Each adder cell will require

sixMOSFETsinparallel,andtherewillbefifteencellsin

totalforeachpolarity.Anadvantageofthistopologyis

thatitiseasilyupgradablebyaddingmorecells.

InFigure93weshowacrosssectionoftheconceptual

designfortheseptummagnetstogetherwithrepresentative

magneticfluxlines.ThemagnetisaC-typewithasmallcon-

ductorminimizing theseparationbetween theadjacent

orbitsofthebeamsbeingkicked(justinsidethemagnet)and

Single turn coil

15 mm Beampipe

Magnetic shields

R = 3.5 mmHalf-gap 4 mm

Beam spacing

30 mm

Figure93 Cross section of the septum magnet located downstream from the fast kickers. The inset figure shows the 15 mm separation of unperturbed and deflected beams; the larger figure shows the magnetic field lines and shielding, clearly excluding the field from the unperturbed beam pipe. A length scale of 30 mm is indicated.

135


latorsarealsorequiredforresonantinteractionswithseed

lasersoperatingbetween200and800nm.

Beamlines1and3bothemploymultiple3mradiating

undulatorsectionsseparatedby1.5mbreaksforfocusing,

diagnostics, and required phase-shifting elements.

Thebreaksaresomewhatlongerthanthe1.3musedin

FERMI@elettra,27andwillincorporatesimilarcomponents.

Beamline2—the2-colorattosecondbeamline—requires

onlytwoindependentradiatingundulatorsections,each

about1minlength,oneforeachoftherequiredX-raypulses.

Theplanarundulatorswillnaturallyproduceradiation

withahighdegreeoflinearpolarization,estimatedtobe

about99%.Variablepolarizationcanbegeneratedusing

orthogonalplanarundulatorsseparatedbyaphase-shifter,

asdescribedinSection5.4.5.3.Theresultingoverlapofthe

radiationpulsesinthefarfieldyieldsadjustablepolarization.

Preliminaryinvestigations59suggestthatevenwithSASE

X-rayFELs,circularpolarizationof80%ormorecanbe

achieved with only moderate sacrifice of peak power.

Polarizationcontrolofattosecondpulsesmayprovemore

challengingandwarrantsfurtherstudy.

Withplanarundulators,eachbeamlinewillsimultane-

ouslyproduceharmonicsofthefundamentalFELradiation.

However,asthecurrentchoiceofelectronenergy(1.8GeV)is

neartheminimumforproductionof1nmradiation(inorder

tominimizecost),aroundthisresonancewavelengththepla-

narwigglerswillhaverelativelysmallK-values,resultingin

lowlevelsofharmoniccontent(approximately0.1%ofthe

fluxesachievableatthefundamental).Modestincreasesin

beamenergywouldgenerateuptoanorder-of-magnitude

moreharmonicenergyinBeamlines1and3. Increased

beamenergytoapproximately2GeVwouldimproveperfor-

manceandmaybeachievablewithoutsignificantimpacton

cost;furtherstudieswilldetermineanoptimalsetofparam-

etersfortheNGLSperformance,balancingriskintheinjec-

tor,linac,andFEL.

Synchronizationtoexternalpumporprobelasersinthe

UV,visible,IR,orTHzbandscanbeachievedbythetiming

andsynchronizationsystemsdescribedinSection5.4.7.This

approachwillbeparticularlyeffectivewhenthedurationof

theseedlaserpulseisshorterthan150fssoastofitwithin

the250fslongcoreoftheelectronbunchwhileallowingfor

±50fsjitterinthetimingofthebunch.If,instead,theseed

laseroverlapstheentireelectronbunch,thetimingofthe

radiationpulsewillbedrivenbytheaveragearrivaltime

oftheelectrons.Inthiscase,theseedlasershouldbeatleast

350fsindurationtoallowfortimingjitter.

toafewnanometers,buttheeventualcompleterangeof

photonenergiesaccessiblebytheoverallfacilitycouldbe

considerablylargerthanthis.

Asummaryofthecriticalelectronbeamparameters

usedinthebaselineFELsimulationsisgiveninTable7.

A“slice”isheredefinedasthedurationofthecoherent

FEL interactionorroughly tencooperation lengths—

about5fsfortheparameterregimeassumed.Theparam-

eterscloselyapproximate thoseobtained in the linac

simulations:seeSection5.4.3.4.

Table7 Electron beam parameters used in FEL simulations.

Parameter Value

Energy(GeV) 1.8

Peakcurrent(A) 500

Slicetransverseemittance(µm) 0.6

Sliceenergyspread(keV) 50

Lengthofcoreofbunchconditionedforlasing(fs) 250

Rangeofenergieswithinthecore(keV) ±250

Inthestraightforwardmodeofoperationpresented

here,theCWlinacwilldeliverelectronbuncheswiththe

samenominalparameters toall FELbeamlines.Most

importantly, theelectronbeamenergy,peakcurrent,

energyspread,andtransverseemittancewillallbeidenti-

caluptosmalllevelsofjitter.Becausethedifferentbeam-

linesaredesignedfordifferingFELschemes(SASE,EEHG,

etc.),overallperformancemightbeoptimizedbypresent-

ingdifferentelectronbunchestoeachbeamline.Thepoten-

tial capabilities todeliverdifferentclassesofbunches

withoutsacrificing totalbeamtimewillbe thoroughly

exploredduringdetaileddesignofthelinacandinjector.

Theradiationwillbeproducedbyin-vacuumhybrid

permanent-magnetundulatorswithamagneticgapof

4mmtoprovidetherequiredbeamclearance.An18.5-mm

periodundulatorwithK-value0.8–2.6willproduceatun-

ingrangefrom3.28nm(377eV)to1nm(1240eV)atthe

baselineoperatingenergyof1.8GeV.Aslightlydifferent

undulatorwithaperiodof20mmandwithK-value0.8–3.0

providesatuningrangefrom4.5nm(276eV)to1.2nm

(1033eV).TheseparametersaresummarizedinTable8.

Thelong-wavelengthlimitisdeterminedbythemaximum

fieldachievablewithintheconstraintsoftheclearance

requiredfortheelectronbeam,whiletheshort-wave-

lengthlimitisdominatedbytherequirementtoyieldsuf-

ficientlyhighphotonfluxwithinareasonablesaturation

length.Finally,long-period(200–400mm)modulatingundu-

136


enablefastmachineprotectionsystems.Sufficientspacein

allbeamlinesisallocatedforsuchdiagnostics.

5 .4 .5 .2 Beamline Examples

Thissectionprovidesperformanceestimatesofthethree

beamlinesenvisionedfortheNGLSFacility.Thefirsttwoare

seededFELs,thethirdaSASEFEL.SimulationsofFELper-

formance have been performed using GENESIS65 and

GINGER.66

Beamline1usestheEEHGseedingscheme66withtwo

laser-drivenmodulatorstogeneratehighlyupshiftedpho-

tons.The pulses can have a duration ranging from

5to150fs.Ifsynchronizationtotheelectronbeamrather

thantheseedlaserpulsesisacceptable,theradiationpulse

lengthwillmatchthepulselengthoftheelectronbeamcore

(250fs).Eachpulseishighlycoherent,withmodestpower

fluctuations (12%RMS)andphase fluctuations (0.2 rad

RMS),varyingonacorrelationtime-scaleof~5fs.Figure94

illustratesa50fssampleofthepredictedoutputpulse.

Beamline2,usingavariantoftheEEHGseedingscheme,

producestwosub-femtosecondpulses,eachwith~250as

duration.Thewavelengthofeachpulsecanbeindependent-

lycontrolled,andthetimedelaybetweenthetwopulsescan

becontrolledwithaprecisioncomparabletothedurationof

eachpulse.Atypicalsingle1keVphotonoutputpulseis

showninFigure95.Notethatthefigureillustratesa2fstime-

slice; the pulse itself is sub-femtosecond in duration.

Increasingthelengthoftheundulatorbeyond~1mdoesnot

increasethepeakpower;whiledifficulttoobserveinthefig-

ure,thepulsedurationbeginstoincreasewithincreasing

undulatorlengthafterthispoint.Thebandwidthoftheoutput

pulseisclosetothetransformlimit.

Beamline3isaSASEbeamlineandneedsnoexternalseed

laser.A50fssectionoftheoutputpulseisshowninFigure96.

Thepowerprofileconsistsofmultiplespikesthatareeachon

theoforder5fsinduration,butareincoherentinphasewith

respecttoeachother,andhavenearlyindependentintensity

fluctuations.WhilethepeakpoweroftheSASEFELiscompa-

rabletothatoftheseededFELofBeamline1,theSASEpulse

lacksthehighlongitudinalcoherenceorenhancedsynchroni-

zationcapabilitiesofaseededFEL.However,thisbeamline

hasamorerobustdesignandcanoperateatthefull1MHz

electronbunchrepetitionrate,andpotentiallygreater.

Each of the three beamlines incorporates a distinct

arrangementofundulators,seedlasers,chicanes,magnetic

focusingoptics,anddiagnostics,thedetailsofwhichare

describedonthenextpage.

Thelaserseedingschemesusedintheexamplebeam-

lines are based on echo-enabled harmonic generation

(EEHG)60orvariantsthereof.Thischoicehasbeenmade

basedupontheprojectedcapabilityofEEHGtogenerate

photonenergiesordersofmagnitudeabovethatoftheinput

radiation,allowingconventionalopticallaserstobeusedas

theinputseed.Thisisaccomplishedwithonlymoderate

increases(byafactorof10orless)intheenergyspreadand

peakcurrentoftheelectronbunch,andwithoutrelyingon

“fresh-bunch”techniquesthatlimittheoutputpulsedura-

tiontoafractionofthecoreoftheelectronbunch.EEHGis

alsolesssensitive61tolongitudinalvariationsintheelectron

beamthanotherschemes,whichrelaxessometolerances

ontheelectronbeam.WhileEEHGisanovelconcept,it

showsgreatpromisebasedontheory,simulations,andini-

tialexperimentalstudies.EEHGhasbeentestedwithprom-

isingresultsuptothe4thharmonic,62limitedbydiagnostic

capabilities. Further study and experimentation will be

importantinunderstandingtherequirementsforapplying

EEHGtoharmonicnumberswellabove100;theNGLSR&D

planwilladdressthedemonstrationofseedingtechniques

toshorterwavelengthspriortofinaldesignoftheFELs.

WithintheFELbeamlines,threeadversebeamdynamics

effectsmustbeconsidered:increasedenergyspreaddueto

spontaneoussynchrotronradiationineitherundulators63or

chicanes;64distortionsintheelectronbeamduetowake-

fields;andparticlelosses.Collectiveeffectsyieldslowvaria-

tionsinthepositionofthecentroidofeachslice,which,while

predictedtobesmallerthantheRMSbeamwidthsinposi-

tionandangle,willresultinamodestdegradationoftrans-

verse coherenceover theentiredurationof theoutput

radiationpulse.Eachbeamlinehasbeencarefullydesigned

totakeintoaccountphysicslimitationsandconstraints:the

largeharmonicnumberlimitstheinitialbunchingfactor,

requiringeitheranamplificationsectionor,forBeamline2,

localizedbunchcompression;incoherentsynchrotronradia-

tion (ISR) in the modulating undulators and chicanes

degradesthebeam,requiringlongelementswithlowmag-

neticfields;shot-noise-seededradiationinthemodulators

willpollutethesignal;thelaserspotsizemustbemuchlarg-

erthantheelectronbeamspotsizetominimizeradialvaria-

tionthatotherwisewouldquicklyshiftthemicro-bunching

awayfromthetargetharmonic.Electronbeamandradiation

diagnosticsarecriticalforoptimizingandmaintainingper-

formanceinthepresenceoftheseeffects.Diagnosticsalso

facilitateclosesynchronizationofexperimentalsystemsand

137


400

300

200

100

0

30

40

2020

10z (m) Time (fs)

Pow

er (M

W)

0

0

60

40

20

0

1.5

2.0

2.0

1.0

0.5

z (m)Time (fs)

Pow

er (M

W)

0.0

0.0

1.5

1.0

0.5

400

300

200

100

0

60

40

2040

20

z (m)Time (fs)

Pow

er (M

W)

0

0

Figure94 Beamline 1: Growth of X-ray power at 1.2 nm wavelength in a 50 fs section of the seeded pulse, as a function of distance z along the undulator. The final power fluctuations have RMS fluctu-ations of 12%, while the phase fluctua-tions have an RMS deviation of 0.2 radian, which are small compared to SASE fluc-tuations (see Fig. 96), and close to the transform limit.

Figure95 Beamline 2: Growth of X-ray power at 1.2 nm wavelength in a single sub-femtosecond seeded pulse, as a function of distance z along the undulator. The time window is a 2 fs section of the electron beam. Lengthening the undulator past ~1 m does not increase the peak power, but the duration of the pulse increases from 130 as to above 200 as. The pulse is almost transform limited.

Figure96 Beamline 3: Growth of X-ray power at 1.2 nm wavelength in a 50 fs section of the SASE pulse as a function of distance z along the undulator. The power profile is broken up into distinct spikes, with uncorrelated phases among spikes. The pulse has large fluctuations and is significantly further from the trans-form limit than the seeded pulses illus-trated in Figs. 94 and 95.

Beamline 1: Seeded FEL

Beamline1 isaseededFELprovidingpulseswith

highdegreesoflongitudinalandtransversecoherence.

AschematicdiagramoftheFELlayoutisshowninFigure97;

thetuningrangeisfrom1.2nmto4.5nm(1.0to0.28keV).

TheEEHGtechniqueemployed60incorporatestwomod-

ulators,bothusing~200nmlaserseeds,separatedbya

verystrongchicane(withanR56~-15mm).Thischicane

generatesa“striped”phasespacewithwell-separated

energybandsatanygivenlongitudinalpositionwithin

thebunch(seeFigure98).Afinalchicane(R56~-200μm)

transformstheseenergybandsintolongitudinalmicro-

138


Matching

10 1 1

X-ray production(8 undulators)

Undulator Chicane Laser

10 10 4.53

34.5

U U U

U

UU U

Energy bands Micobunching

Figure97 Schematic of layout for Beamline 1 utilizing EEHG seeding, showing main components, scale is in meters.

1799.8

1800.0

1800.2

Electron densityMax

0Ener

gy (M

eV)

Phase

1799.2

1799.4

1799.6

1799.8

1800.0

1800.2

1800.4

1800.6

1800.8

Phase

Phase

Electron densityMax

0

Ener

gy (M

eV)

50010001500200025003000

Curr

ent (

Amps

)

Figure98 Phase space density after the first modulating undu-lator and chicane. There is minimal density modulation at this stage. The horizontal axis is phase of the seed laser, and the full scale correlates to a distance of one seed laser wavelength of 200 nm along a section of the bunch.

Figure99 Phase space density plot of one modulation period of the electron beam after the second modulating undulator and chicane yielding maximal compression of the microbunch. The resulting modulation of the peak current at very short scales is shown in the lower half of the figure. The horizontal axis is phase of the seed laser, and the full scale correlates to a distance of one seed laser wavelength of 200 nm along a section of the bunch.

bunching (via phase space rotation, see Figure 99),

yieldinghighharmoniccontentatmultiplewavelengths.

Figure100showstheresultingbunchingspectrum.The

resulting beam is then passed through a final set of

undulatorstunestoaveryhighharmonicoftheoriginal

seedlaserpulses.TheFELbeamlinecanbeoptimizedfor

anyspecificharmonicbyadjustingthestrengthofthe

chicanes.Becauseharmonicsof the laserwavelength

willbecloselyspaced,combiningchangesinthechi-

caneswithasmalltuningrangefortheseedlaserswill

permitcontinuoustunabilityoverthewholerangeacces-

siblebytheundulators.

AsshowninFigure101,Beamline1producespulses

nominallycontainingfrom3×1011photonsat1.2nmto

5×1012photonsat4.5nm.Thesaturationlengthisconsid-

erablyreducedatlongerwavelengths.At1keV,theFEL

outputpowersaturatesat~300MW.

Thechoiceof200nmseedlasers(correspondingto

the4thharmonicofthedrivingIRlaser)wasadoptedto

reducetheharmonicjumprequiredtoreach1.2nm,while

stillutilizingconventionallasertechnology.About8MW

(peakpower)at200nmisrequiredinthefirstmodulator

and30MWpeakinthesecond.Itmaybedesirableto

havethesecond,higher-powerseedlaserextendoverthe

entireelectronbunch, thusuniformly increasing the

energyspreadthroughoutthebeam;thissuggestsaseed

laserpulselengthof700fs,yieldingarequiredenergy

perpulseof21µJat200nm,correspondingto~50W

averagepowerinthedrivingIRlaser.Alternativeschemes

involving longer seed wavelengths would typically

requirehigherpeakpower.Thehigh-powerseedlasers

arefurtherdiscussedinSection5.4.5.5.

ThefirstchicaneisrequiredtogenerateanR56ofupto

-30mmandconsistsoffourmagnets,eachabout1mlong,

139

withhalf-meterlonggapsinbetween.Thesecondchicaneis

significantlyweaker,withR56ofatmost–1mm,andcanuse

magnetswithalengthof0.5m.Themagneticlatticebefore

theradiatorsectionispredominantlycomposedofdoublet

quadrupolesinordertoallowstrongsimultaneousfocusing

inhorizontalandverticalplaneswithintheundulators.

Beamline 2: Two-color X-ray Pump / X-ray Probe

Beamline2isatwo-color,short-pulseFELemployinga

variationofecho-enabledharmonicgenerationtodelivertwo

X-raypulseswithdurationsof250fsFWHMorless(simula-

tionsshowpulsesasshortas130asmaybeobtained)while

stillcontainingoforder108photonsineachpulse.67Each

pulsemaybetunedindependentlyforphotonenergyand

timing.Thetuningrangeisfrom1.2nmto4.5nm.Beamline2

utilizesthesameenergymodulationschemeasBeamline1

butdeploystwomicrobunchingmodulatorsoperatingon

separateportionsoftheelectronbeam,eachofwhichthen

canbeforcedtoradiateindependently.Thefinalmicrobunch-

ingisproducedbyfew-cyclecarrier-envelopephasestabilized

800nmlaserpulsescombinedwithsingle-periodundulators.

AschematicdiagramisshowninFigure102.


00 50 100

Harmonic of 200 nm150 200

Harm

onic

bun

chin

g (%

)

2

4

6

8

10

0 0

5

10

15

20

25

30

35

1 1.5 4 4.53.52Wavelength (nm)

Photons per pulseDistance to saturation

2.5 3

Phot

ons

per p

ulse

(1012

)

Dist

ance

to s

atur

atio

n (m

)

1

2

3

6

4

5

Figure100 Electron beam bunching spectrum after EEHG manipulations.

Figure101 Beamline 1: Predicted output and saturation length vs. wavelength for the EEHG-seeded FEL.

Attosecondpulse 1

Attosecondpulse 2

Matching

10 1 112

U

10 10 10

U

Undulator Chicane LaserU

U U U

Energy bands Micobunching 1 Micobunching 2

Figure102 Schematic of layout for Beamline 2 utilizing a two-color seeding scheme, showing main components, scale is in meters.

Figure103showsphotonsperpulseforBeamline2,

fortwocases:afixedlengthof1mradiatorundulatorfor

eachofthetwoX-raypulses,andforafixedoutputpulse

of250asdurationandinthiscasethelengthofradiator

undulatorrequiredisshown.

Phaserotationtoobtainhighharmonics(seeFigure99)

requiresafew-cycle800nmseedpulsedurationof3.5fs

FWHM,68andthetotalpulseenergyisroughly70µJ.The

resultingbeamentersasingleradiatingundulatorwith

significantlocalizedinitialbunchingofupto10%,enhanced

peakcurrentofupto3kA,andlargeenergyspread,which

canbetoleratedduetotheshortnumberofundulator

140

periodsusedtoextracttheattosecondpulse.Onebenefit

of thisschemeis that thebackgroundSASEradiation

fromthefinalradiatorwillbeverylow,leadingtohigh

contrast.Herethedesignoftheopticalmatchingismore

critical,becausecompressing theelectrons toavery

smallspotsize(withminimumbetafunctionscloseto

1m)withintheradiatorimprovesthenumberofphotons

produced,whereasextendingthelengthofundulatorso

astoamplifythepowerthroughFELgainisnotcompati-

blewiththegoalofattosecondpulsedurations.

FAt1.2nm,theresultingoutputpulsereachesapeak

power of 60 MW after only 0.8 m of undulator. For

theentirerangeofwavelengths,thepeakpowerand

pulsewidthremain fairlysimilar,producingoforder

108photonsperpulse.

Itisexpectedthattheamplifiedeffectofshotnoisewill

leadtosomejitterinoutputpulseparameterssuchasthe

totalnumberofphotons,thecentralphotonenergy,and

theprecisetimingofthepulse.Intheattosecondregime,

thebandwidthoftheresultingpulseisoforder1%,over-

lappingseveralharmonics.Thus,thereisnoneedforany

tuningofseedlaserwavelengths,andacombinationof

jumpingtodifferentharmonicsandtuningtheundulator

parametershouldallowforcontinuoustunability.

Beamline 3: High-Repetition-Rate SASE FEL

Beamline3isaSASEFELthatcanacceptthefullelec-

tronbeampower,ultimatelylimitedonlybythecapacityof

thebeamdump.Noexternallaserisrequired,andtuning

isaccomplishedsolelybychangingtheundulatorgap.The

totalbeamlineconsistsofsixteen3mlong,18.5mmperi-

odundulators.The1.5mbreakseachcontainsaquadru-

pole,aradiationdiagnosticinsert,andaphase-shifterto

maintainproperphaserelationshipbetweentheradiation

andthemicrobunches.Themagneticopticsconsistofa

simpleFODOlattice,yieldingabetafunctionthatranges

from9mto17mandaphaseadvanceperhalf-cellof42°.

AschematicofBeamline3isshowninFigure104.

Dependingondesiredwavelength,theSASEbeamline

willproduce1011–1012photonsperpulse(seeFigure105).

Notethat thenumberofundulatorsectionsneededto

reachsaturationdecreasesstronglyasthetargetwave-

lengthisincreased.Thepeakpowervariesfrom~300MW

to above 2 GW; the average power within a pulse is

reducedbyapproximatelya factorof twodueto the

stochastic nature of the characteristic SASE power

spikeswithintheoutputradiation.


0 0

0.2

1 1.5 4Wavelength (nm)

Photons per pulseDistance

2.5

Phot

ons

per p

ulse

(109 )

Dist

ance

to 2

50 a

s FW

HM (m

)

0.1

0.2

0.7

0.3

0.4

0.6

0.8

1

1.2

1.4

1.6

0.4

0.5

0.6

2 3.5 4.53

0 0

100Photons per pulse

Pulse FWHM

Phot

ons

per p

ulse

(109 ) a

t 1 m

Puls

e FW

HM (

as)

0.2

0.4

1.8

0.6 200

300

400

500

600

0.8

1.2

1

1.6

1.4

Figure103 Photons per pulse as a function of wavelength for two cases for Beamline 2: In the upper figure, the performance is shown for a fixed radiator of 1 m length, and the pulse duration varies as a function of wavelength. In the lower figure, the pulse length is fixed at F250 as and the length of the radiator undulator is shown. This beamline produces 2 pulses of independent wavelength.

141

5 .4 .5 .3 Undulator Design Options

Themagneticundulatorsarecriticalelementsofthe

NGLSFELs,usedinmodulatingtheelectronenergyand

providingcollimatedphotonpulsesofhighbrightness

withtailoredwavelengthandpolarizationcharacteristics.

Weplantodevelopandimplementadvanced,high-per-

formance,adjustable-gapplanarundulators,leveraging

undulatorcharacteristicstooptimizeoverallperformance

(e.g.photonflux)andtominimizeoverallfacilitycostand

risk (e.g. linac size and electron beam energy — see

Section5.4.1).

Table2describesthephotonpropertyparametersfor

thethreepreliminaryNGLSFELdesigns.Toachievethe

desired performance, various undulator technology

options were considered. As a conservative bound,

aminimum4mmvacuumapertureintheundulatorsec-

tionwaschosensoastoavoidradiationdamagetothe

undulatorandtoavoidbeaminstabilityeffectscausedby

wakefieldsinthevacuumchamberwalls.Anundulator

parameterofatleastK=0.8attheshortestwavelength

hasbeentakenasalowerboundforeffectivephotonpro-

duction.Thedesiredphotonspectral reachandrange

(approximately1nmto4nm),togetherwiththeexisting

undulator technologycapabilities, thendeterminethe

optimal undulator period λu, the required maximum

undulatorparameterK,andthebaselinemachineenergy.

Achievingthesegoals,whilesimultaneouslyreducing

technicalrisk,haveleadustoselectin-vacuumhybrid

technologyforthebaselineNGLSdesign,andleadtoa

linacenergyof1.8GeV.Parametersforthethreebaseline

undulatorbeamlinesaresummarizedinTable8.


MatchingX-ray production(16 undulators)

Undulator

10 4.5

3

70.5

U U U

U

U

0 0

10

20

30

40

50

60

70

1 1.5 3.52Wavelength (nm)

Photons per pulseDistance to saturation

2.5 3

Phot

ons

per p

ulse

(1012

)

Dist

ance

to s

atur

atio

n (m

)

0.5

1

2.5

1.5

2

Figure106 Prototype XFEL out-of-vacuum hybrid planar undulator (left) and SPring-8 in-vacuum hybrid undulator (right). Both technologies are mature for FEL application. NGLS will use in-vacuum devices.

Figure105 Beamline 3: Scaling of performance vs. wavelength for the SASE FEL, in terms of number of photons and saturation length.

Figure104 Schematic layout for Beamline 3, a SASE FEL, showing main components, scale is in meters.

142


ducting-baseddevicesandaredevelopingcomplemen-

taryand/oralternativesuperconductingtechnologies.

Bythemselves, thesehigh-fielddeviceswouldenable

expanded tuning range and modestly shorter wave-

lengths.Coupledwithacceleratorupgradestoprovide

moderatelyhigherelectronbeamenergy,suchdevices

offer the promise of much higher photon energies:

forexample,increasingbeamenergytoapproximately

2.5GeV,andusingundulatorsof~12mmperiodallows

forover3keVphotonsinthefundamental.

5 .4 .5 .4 Synchronized THz/IR Sources

Short,intensepulsesofTerahertzradiationranging

from0.5to50×1012Hzinfrequency(orequivalently,

wavelengthsfrom600μmto6μm)maybeproduced

directlyfromtheelectronbeambyathin(~1µm)metal

foilorapertureplaceddownstreamoftheFELundula-

tors.Suchsourcesareinherentlysynchronizedtothe

electronbunches,andthustotheX-raypulses.However,

therequirementthatthefoilliedownstreamoftheFEL,

toavoiddisruptionoftheFELprocessitself,limitsthe

naturaluseofsuchschemestosituationswhereatrail-

ingTHzprobepulsecouldbeused,unless theX-ray

pulseitselfcouldbedelayed.Theoppositeformat,that

ofaTHz-pumpandX-rayprobe,wouldbeexpectedto

findmanymoreapplications.Fortheseexperiments,the

NGLS timingandsynchronizationsystemwouldalso

allowtightsynchronizationoftable-topTHz,IR,oroptical

sourcestotheX-raypulses.

5 .4 .5 .5 Seed Laser Systems

TheNGLSseededFELsrequirehigh-power,tunable,

opticallasersystemswithlowpulse-to-pulsepeakpower

fluctuations(~1%),andaquasi-flat-topprofilewithlow

powerripple(~1%).Thelargeharmonicjumpsrequirethat

Table8 Undulator parameters for the three beamlines.

Beamline 1 Beamline 2 Beamline 3

Wavelengthrange(nm) 1.2–4.5(1–0.28keV)

1.2–4.5(1–0.28keV)

1–3.3(1.2-0.38keV)

MaximumundulatorparameterK(minimum0.8)

3.04 3.04 2.61

Undulatorperiod(mm) 20 20 18.5

Undulatortechnology Hybridin-vacuum

Hybridin-vacuum

Hybridin-vacuum

Permanent-magnet,planar,hybridundulatortechnol-

ogyiswelltested,utilizingNdFeBpermanentmagnet

material plus soft iron/cobalt/vanadium permendur

polepiecesinaplanararraytoproducelightwithahigh

degreeoflinearpolarization.Hybriddeviceshavebeen

used in synchrotron facilities worldwide since their

inventionatLBNLin1983.Morerecently,hybridundula-

torsarefindingapplicationastheradiatorsectionsat

FELfacilitiesatXFELinGermany(out-of-vacuum)andat

theSPring-8FEL inJapan (in-vacuum),asshownby

Figure106.Thiswell-established,yetpowerfulundulator

technology introducesnosignificant risk tobaseline

NGLSoperation.

Although the undulators will be planar, circular

polarizationcapabilitymaybeenabledbyorientingthe

finalundulatorsectionsorthogonallytothelongerbunch-

ing and radiator undulator sections upstream;69 see

Figure107.Aconventionalelectromagnetphase-shifter

positionedbetweenthecrossed-planarundulatorsec-

tionscanenablefastpolarizationswitchingandchangeof

polarizationstate.

TheperformanceoftheFELbeamlinescouldbesig-

nificantlyimprovedthroughmoreadvancedundulator

technologieswithhigherfield-strengthcapabilitiesand

shorterperiods,andwithpotentiallylowerfabrication

costs.Wehaveworkingprototypesofvarioussupercon-

Horizontal polarization Phase shifter

Vertical polarizationRadiation with

adjustable

polarization

Figure107 Planar hybrid undulator horizontal and vertical sections separated by a tunable electromagnetic phase shifter enable fast polar-ization switching and arbitrary polarization modes.

143


for the IR laseroutput reachabout500Wofaverage

power.IRlasersystemsuptopowerlevelsof30–50Wfor

10kHzoperationsalreadyexist.Thescalabilityofexisting

systemstohigherpowerlevels,andhigherrepetition

ratesiswelldefined.Thecurrentlimitationistheavail-

abilityofhigherrepetitionratepumplaserswithsuitable

powerlevels,whichwillbedevelopedinthenextdecade.

Onceagain,efficientpulsecompression,aswellasCEP

stabilizationoftheseedlasersystemalreadyexist,but

wouldrequirefurtherexpansiontothescaledupamplifier

systemandHHGstage(typicallyachievedinagascell).

HGHGisanalternateoption,requiringseedlasers

producingapproximately10µJ,100fspulsesintheUV,

oraround100MWpeakpowerataround200nm.Soft

X-rayoutputistobeachievedbysuccessiveharmonic

generationinanumberofcascadedFELstages.Atarep-

etitionrateof100kHz,werequireaboutonewattof

averagepower in theUV,andup to100Win the IR.

CommercialIRseedlaserssystemscancurrentlyreach

upto30–50W,andspecialdesignswillprovidepower

levelsinthe100Wrange.

Thecurrentseed lasersystemsavailable for these

threeoptionsarescalabletothepowerlevelsrequired,

andmostofthecriticalopticalcomponentsarecurrently

available(e.g.largeapertureTi:Sapphirecrystals,large

thermallystablecompressiongratings,largeoptics,har-

moniccrystals,etc.).Oneof themainareas for laser

developmentwillbethatofthepowerfulpumplasers,in

ordertolimitthefootprintaswellascostoftheoverall

pumppowerrequirements.ScalabilityoftheCEPstabili-

zationtechniquesaswellasefficientsub-15fscompres-

sionwouldhave tobe furtherdeveloped tomeet the

requiredseedpulsedurations.Anotherarea thatwill

requiredevelopmentwillbetheUVpulsetemporalinten-

sityprofileuniformity.Furtherdevelopmentofexisting

techniques(e.g.phasemodulation,pulsestacking,etc.)

willberequiredtoreachstabilityrequirements.

Opticssystemswill transport theseedlaserpulses

fromaremotetemperature-stabilizedlaserroom,tothe

FELinstalledintheacceleratorradiationenclosure.There

maybelocalopticalequipmentinstalledintheFELvault,

andthismustberobustandreliable(e.g.HHGgascells

forconversionoftheIRpulsetoharmonics).Thelaser

beammustoverlaptheelectronbeamforasignificant

distanceinmanycases,andactivebeampointingstabili-

zationsystemswillbeprovidedtomeasuretheinputand

outputbeampositionsandcontrolthebeampaththrough

thephasenoisewithinthepulsebebelowapproximately

0.01radiansRMS.Additionally,thefew-cycleopticalseed

lasersusedforgeneratingattosecondX-raypulsesshould

bestabilizedwithrespecttocarrierenvelopephase,pulse

duration,andamplitude.

WehavebaselinedourpreliminarydesignsforEEHG

using theoutputofa fiberorTi:Sapphire-based laser

chain,ateither its fundamentalorup-convertedto its

3rdto5thharmonictomodulatetheelectronbeam.Seed

laserpowerofuptoapproximately30MWat200nm

isrequiredforseedinglongX-raypulses,orper-pulse

energyrequirementsof1µJ(for10fspulses)to20µJ

(for 700 fs pulses). IR systems capable of delivering

~50–100Waveragepowerat100kHzarethusrequired,to

provide temporally and spatially filtered pulses and

accommodatefortransmissionlosses.Forsub-femtosec-

ondX-raypulsegeneration,aper-pulseenergyof~70µJ

isrequiredofthefewcyclesof800nmradiation(~3.5fs),

resultinginpeakpowerupto~20GW,oraveragepower

oforder10W,at100kHz.Chirped-pulseamplification

(CPA)ordivided-pulseamplification(DPA)basedlaser

systemsofthistypealreadyexistat10–100kHz,atpeak

powerlevels~100MW,mostlylimitedbytheavailability

ofpowerfulpumplasersatthisrepetitionrate(~500W

powerlevelsofgreenpumplaserswouldberequired).In

additiontoscalingupthepumpandIRamplifierpower

outputlevels,fortherequiredIRpulsewithinafewcycles,

carrier-envelopephase(CEP)stabilizationwillberequired

(andisalreadyimplementedinsomeCPAamplifiers).

Additionally,efficientpulsecompression(downtoafew

cycles),reliablesystemswithtunability,suitablediagnos-

tics,androbustremoteoperationwillneedtobedevel-

opedaspartoftheNGLSR&Dplan.

AnotherpossibleapproachtoseedingtheNGLSFELs

wouldbetheuseofHHGlasersources(seeSections5.1.2

and5.2.4),whichwouldsignificantlyreducetheharmonic

jumprequiredbetweentheseedlaserandFELoutput

wavelength,andreduceX-rayoutputsensitivitytoseed

laserinstability.Currently,HHGsourcesoperateinthe

EUVrange,with10kHzrepetitionratepulsesofnJenergy,

atuptoabout100eV.AnFELcanfurtheramplifytheEUV

seedinanopticalklystronconfiguration,startingfrom

pulsesofaround30–50nminwavelength,5nJinenergy,

andafewtensoffemtosecondsinduration,correspond-

ingtoaround100kWpeakpower,or500µWaverage

powerat100kHzrepetitionrate.Withcurrentlyachiev-

abletypical10-6up-conversionefficiency,requirements

144


5.4.6 Beam Dumps

TheNGLSwillutilizebothhigh-power(MWscale)and

mediumpower(100kWscale)beamdumps.Thehigh-

powerdumpswillbelocatedattheendofthespreader

sectionandonthehigh-repetition-rateFELbranchline.

Theconceptualdesignof thehighpowerdump isa

water-cooled“window”followedbyaseriesofmetal

platessuspendedinarapidlycirculatingwaterbath.The

windowisolatestheprimaryabsorberfromthevacuum

beampipeandinitiatesanelectromagneticcascadethat

broadenstheradialextentofthebeamasittransverses

thedump.Thethicknessandspacingoftheabsorption

plateswillbeselectedtomaximizetheenergyabsorp-

tionwithin themetalwhilemaintainingsafe thermal

stresslevels,followingtheexampleoftheCEBAF5GeV,

1MWdump.70

Medium-power beam dumps will be located in the

remaining100kHzrepetition-ratebranchlines.Themoder-

atebeampowerallowstheuseofsolidabsorbermaterials,

whichsignificantlysimplifiestheconstructionandopera-

tion.Thekeydesignissuesarelocalizedheating-induced

thermalstressesandoverallheatdissipation.

Themedium-powerdumpwillutilizethin,dual,edge-

cooledwindowstoconfineanargon-gas-filledvolume.As

inthecaseofthehighpowerbeamdump,thewindowwill

initiateanelectromagneticcascadethatwillbroadenthe

beamthusdiffusingtheenergy.Followingthewindow

willbeadriftspaceupstreamoftheprimaryabsorberthat

providesroomforthebeamtospreadbeforestrikingthe

useofpiezoandmotor-controlledmirrorsandCCDdetec-

tors.Existingpointingcontrolsystemscanachievebetter

than10μradprecision,whichcorrespondtotherequire-

mentofNGLS.Suchsystemshavesuccessfullybeen

implementedalready(e.g.LBNL,SLAC,andNIFatLLNL),

andwillneedtobeadaptedtothespecificbeamtrans-

portline.Superpositionwiththeelectronbeamwillbe

accomplishedusingpop-inscreensalongtheundulator,

andultimatelyfeedbackcontrolincorporatingtheelec-

tronbeamBPMs.LaserbeamfocusingintotheFELwill

bedonewithfirstsurfacereflectiveopticsforbothIRand

shorterwavelengthbeams,allowingforadjustmentin

thewaistpositionwithintheundulator.Currentcoating

technology will support the fluence levels expected.

ForHHGseedingatEUVwavelengths,aKirkpartick-Baez

mirrorpairmaybeused.

Diagnosticssystemswillberequiredtofullycharacter-

izetheseedlaserpulses.Spatialandtemporalintensity

profileswillneedtobemeasured,aswilltheenergypro-

filealongthepulse,usingwell-knowntechniquesalready

inuse.Toolstocontroltheenergychirpwillbeneeded.

Ultimately,systemswithcapabilityforfeedbackcontrol

ofpulseparameters,basedonX-rayoutputoftheFEL,

aredesired,andwillbedevelopedinfutureR&Dprojects.

Lasersystemsaredevelopingatarapidpace,driven

by other demands, and NGLS will take advantage

ofthesetechnologiesastheyevolve.Theexistinglaser

systems represent approximately 10–20% of the IR

powerlevelsrequiredforNGLS,andthescalabilitypath

iswellunderstood.

Photocathode laser

Laserheater

Timingdiagnostics

Endstationlasers

MasterClock

Seed lasers

Stab

ilize

d lin

k

Stabilized lin

k

Stabilized link

Stabilized link

Stabilized link

RF controls

Stabilized link

Figure108 Schematic view of the distribution of the master clock to a variety of remote clients over stabilized fiber links. Seed and user lasers and timing diagnostics are all synchronized at the femtosecond level.

145


lengthvariationequivalenttoseveralhundredpicosec-

ondstoananosecond.Oneofthekeydevelopmentsin

femtosecondtimingdistributionoverthepastfewyears

has been the development of stabilized optical fiber

links73,74fortransmissionofthemasterclocksignalover

afacility.

Thisapproachhasbeenrealizedinsystemsinstalledat

theLCLSforsynchronizinguserlaserswiththeelectron

beam57andattheFERMI@elettralinac54forsynchroniz-

ing the relative phase of accelerating sections.

Performanceatthe10fslevelhasbeendemonstrated

overseveralmonthsofoperation.Suchasystemwould

serveasthebackbonefortimingdistributionatNGLS.

5 .4 .7 .2 Laser-laser Synchronization

GiventhatNGLSwillproducepulsesoflessthan10fs

duration, and pump/probe experiments will require

pulsedlaserstobesynchronizedwiththeX-raypulse,

NGLSwillrequirealasertimingsystemwith10fsjitteror

less.Whileitispossibletomeasuretherelativearrival

timeoftheX-rayandopticalpulsestoaprecisionwhich

islessthanthejitterandpost-processthedataaccord-

ingly,largejittercaneffectivelyreducetherepetitionrate

bymakingmuchofthedataunusablebecausethepulses

donotoverlapatall.Thus,thegoalsofNGLSlasertiming

aretoreliablymaintainprecisionwellbelow10fsandto

aimforsub-femtosecondjitteranddrift.

Toachievefemtosecondandbetterstability,weplan

todevelopsystemsbasedonthecombspectrumofa

mode-locked oscillator. Such a spectrum has two

degreesoffreedom.75Iftwooftheopticalfrequenciesin

thecombspectrumarestabilizedwithrespecttoone

another, thenthecombcharacteristicsarestabilized,

includingcontrolofthepulserepetitionrate.Acarrier-

envelope-phasestabilizedlaserhasonecombparame-

terfixed,i.e.thecomboffsetfrequencyiszero.Theother

parametercanbefixedbylockingonecomblinetoaref-

erence optical frequency. In experiments with

Ti:Sapphireanderbiumlasers,twolaserscanbelocked

towithinsub-fsjitterusingthistechnique.76,77Wehave

previouslyshownverystableopticalfrequencytrans-

missionusinganinterferometer,78whichcanbeusedto

transmitasingleoptical frequencyfromonelaserto

another to synchronize them over a long fiber.This

wouldbethebestcandidateforsub-fssynchronization,

sincethefullopticalfrequencyof200THzisused.

primaryabsorber.Theidealabsorberhasalowatomic

numberandgoodthermalconductivity.Twomaterialsthat

fitthecriteriaarealuminumandgraphite,andbothhave

beenusedsuccessfully forbeamdumpswith roughly

comparablerequirements.

5.4.7 Timing and Synchronization Systems

TheNGLSwillrequireanexactinglevelofsynchroniza-

tionbetweenacceleratorsub-systems,userandaccelera-

tor lasersystems,anddiagnostics inordertoachieve

boththedesiredelectronandX-raybeamperformance

andtoenabletime-resolvedopticalpump/X-rayprobe

studieswithfemtosecondorbetterresolution.Withsepa-

rationsofhundredsofmeters,synchronizationofthese

systemsatthefemtosecondlevelwillbechallenging,yet

criticaltoachievingoptimumfacilityperformance.

Themajorchallengestobeaddressed71inreaching

thelevelofsynchronizationrequiredforNGLSare:trans-

missionofastabletimingsignaltomultipleremotecli-

entsoverarelativelylargefacility,synchronizingremote

clientssuchaslasersandRFsystemstothestabletiming

signalatthefemtosecondlevel,andmeasurementofthe

electronbeamarrivaltime.

Aschematicdiagramofthetimingdistributionforthe

acceleratorsystemisshowninFigure108.Thesystem

comprisesaglobaltimingdistributionsystem,andlocal

systemsthatprovidesynchronizationtotheclock.Inthis

approachwesynchronizealloftheacceleratorsystems

fromthephotocathodedrivelaser,injector,linacRFsys-

tems,andseedanduserlasers.Themasterclocksignalis

distributedthroughoutthefacilityoverstabilizedoptical

fiberlinkswithrelativestabilityontheorderoftenfemto-

seconds.Systemsthatrequireextremelylow-jittersyn-

chronizationatthefemtosecondandsub-femtosecond

level,suchasuserandseedlasersandtimingdiagnos-

tics, will use an all-optical synchronization technique

describedinmoredetailbelow.

5 .4 .7 .1 Master Clock Distribution over Stabilized

Fiber Links

Oneofthemainchallengesinreachingthelevelofsyn-

chronizationrequiredfornext-generationlightsourcesis

transmissionofamasterclockreferenceoverarelatively

largefacility.72Forexample,inafacilityofakilometerin

length,diurnal temperaturevariation results in cable

146


betweenundulatorsections.Foroperationalstabilization

andfeedbacksystemsusethediagnosticsshouldbenon-

invasive;formachinestudiesandset-up,theymaybe

intercepting.

Transverse position tolerance through most of the

acceleratorisestimatedtobeafewtensofmicrons,andin

theFELundulatorstherequirementwillbetighter,perhaps

afewmicrons.Longitudinalarrivaltimingneedstobe

withina10fstolerance,andintegratedwiththetimingand

synchronizationsystem.Longitudinalprofilemeasure-

mentsofbetterthan10fsresolutionaredesirable,and

suchsystemswillrequirefurtherR&D.Dedicatedstudies

areneededincludingtrajectorysensitivityandcorrection

analysistodeterminediagnosticssystemsspecifications.

Theelectronbeamdiagnosticsystemsinclude:

• Beampositionmonitors(BPMs),includingwarmand

coldbuttonpick-ups,striplinesforuseintheaccelera-

torandcavityBPMsforuseintheFELs

• TransverseprofilemonitorsusingYAGscreens,opti-

caltransitionradiation(OTR),andwirescanners

• Longitudinalbunchprofilemonitorssuchasstreak

camerasandelectro-opticdevices

• Beamenergymeasurementsfollowingtheinjector,

inthedispersiveregionofthebunchcompressors,

andbeforeandaftertheFELs

• Beamarrival timemonitorsusingRFcavitiesand

electro-optictechniques

• Precisiontimingdistribution(seesection5.4.7)

• Currentmonitorsusingtoroids

• Beamlossmonitors

Anequallybroadsuiteofphotonbeamdiagnosticsare

neededbothtocharacterizeandoptimizetheFELinterac-

tionbetweentheelectronandphotonbeamsandtochar-

acterizetheresultingphotonbeamasitenterstheuser

hutches.Diagnosticsforthephotonbeamsarediscussed

furtherinSection6.5.

TooptimizetheFELinteraction,thealignmentofthe

electronandphotonbeamsmustbemaintainedtowithin

afractionofthebeamsizesovermultipleFELgainlengths

andtherelativeopticalphasebetweenundulatorsec-

tionsmustbecontrolledtowithinafractionofawave-

length.Standardinstrumentationincludesbeamposition

monitors,beamsteeringdevices,andX-raydetectors.

X-raydiagnosticswillrelyonbothon-axisandoff-axis

X-raydevices.Off-axisdiagnosticsarecomplementaryto

theon-axisX-raydiagnosticsandwillallownon-invasive

monitoringoftheradiation.

5.4.8 Instrumentation and Diagnostics

Anextremelyhighbrightnesselectronbeamisneeded

toachievethedesiredFELperformance,withexacting

pulse-to-pulsestabilityrequirementsovertimeperiodsof

hourstodays.Theserequirementsdemandanextensive

suiteofhigh-resolutionelectronandphotonbeamdiag-

nostics.Systemsarerequiredforstudyingandconfigur-

ingtheacceleratorandFELsaswellasmaintainingstable

operation,andprovidingthenecessarysensorsinfeed-

backstabilizationsystems.

Tooptimize theelectronbeam, instrumentation is

neededtomeasuretransversebeamposition,emittance,

totalcharge,chargedistribution,pulselength,energy,

arrivaltime,andbeamloss.Thesesystemsareinitially

requiredtooperatewithbunchchargesofapproximately

10–500pC,andatrepetitionrateuptothebunchrateof

1MHz.Thediagnosticsshouldbe,whereverpossible,

upgradeabletohigherrepetitionrateandlargerdynamic

rangetomeasurebuncheswithchargerangingfroma

fewpCto1nC,dependingonthemodeofoperation.

Furthermore,eachofthesediagnosticsystemsmustpro-

videsignalswiththeappropriatebandwidthtobeused

in feedback controlof theaccelerator to stabilize the

electronbeam.

Forefficientlasing,itiscriticaltocontrolthe“slice”

values,(withinafewcooperationlengthswithinabunch),

forproperties suchasemittance, charge, energyand

energyspread.Thesemustbemeasuredbeforeandafter

bunchcompression,toallowforproperoptimization.Use

ofatransversedeflectingstructurehasbeensuccessful

inmeasuringthesliceparametersbyimposingatrans-

verseangularchirpalongthebunchandprojectingthe

resultingtransversedistributiononascreen.Thetwo-

dimensionalparticledensitydistributioninadispersive

sectionenablesmeasurementoftheaverageenergyand

theenergyspreadineachlongitudinalsliceallowingsen-

sitivetuningofthebunchcompressionprocess.Because

this diagnostic is destructive, or at least disruptive,

involvingtheinterceptionandperturbationofanelectron

bunch,itshouldbeusedatamuchlowerrepetitionrate

thanotherdiagnostics.However,thehighrepetitionrate

oftheNGLSmayallowoperationofthistechniquewith

negligibleimpactontheaveragebeamintensity.

Criticaldiagnosticsgroupswillbelocatedneartheexit

oftheinjector,theinputandexitofthebunchcompres-

sor, theendofthelinac,theexitofthespreader,and

147


NGLSwillemployinterlocksthatwillimmediatelydis-

ablebeamattheinjectorinfaultconditionsanywherein

theaccelerator,FEL,andX-raybeamsystems.Integrated,

activemonitoringofmagnet,beamcurrent,andother

parametershavethepotentialtoprovidefasterfeedback

thanradiationmonitorstoreducebeamlossesduring

bothroutineoperationandinbeamlossevents.Aswith

allsimilarfacilities,radiationmonitors,vacuumlevels,

andotherstandardmeasureswillcontinuallybemoni-

toredandinterlocked.

Environmentalradiationsafetyriskshavealsobeen

evaluated.Preliminarymodelshavebeendevelopedfor

theproductionandfateofboththeshort-livedair-activa-

tionproductsaswellaslonger-livedisotopes.Standard

containmentandmonitoringofthebeamdumpswillbe

utilized.Earlycalculationsestimateoff-sitedosestobe

manyorders-of-magnitudebelowNESHAPS limits. In

addition,shieldingaroundthelinactunnelwillbethick

enoughtoeliminateanymeasurableamountsofradio-

isotopesenteringthegroundwater.

Empiricalbeam-basedalignmentwillbecriticaltothe

operationoftheNGLSandcanprovidetherequiredaccu-

racybyrecordingbeampositionexcursionsfromBPMs

underdeliberatebeamenergyvariations.Resultsfrom

BPMinformationanalysiscanfeedquadrupolerealign-

ments,transversetrajectoryfeedback,andbeamsteering

atundulatorentrances.

5.4.9 Radiation Protection

Theanalysisofradiationhazardsandcontrolshasbeen

integratedintotheprojectattheearlieststages.Experience

atothermegawattclassacceleratorshasbeenusedto

developpreliminaryradiationtransportmodelsofradia-

tionfieldsexpectedatthebeamdump,collimatorsand

otherlocationswherebeamlossisexpected.Fromthis,

appropriateshieldingstrategieswillbedevelopedwith

particularattentiontobepaidtothebeamdumpsandpho-

tonbeamlines.Thesestrategieswillbuildupontheexperi-

encegainedatsimilarfacilities.Standardsearch/secure

proceduresandinterlocktechnologiestoexcludeperson-

nelfromactivebeamareaswillbeemployed.

References

1. Barletta, W.A., et al., Free electron lasers: Present status and future

challenges, Nuclear Instruments and Methods A 618 (2010) 69-96.

2. Benson, S., et al., X-ray sources by energy recovery linacs and their

needed R&D, to be published in Nuclear Instruments and Methods A.

DOI: 10.1016/j.nima.2010.07.090.

3. Bei, M., et al., The potential of an ultimate storage ring for future light

sources, Nuclear Instruments and Methods A 622 (2010) 5188-535.

4. See, for example, Popmintchev, T., et al., Phase matching of high har-

monic generation in the soft and hard X-ray regions of the spectrum,

Proceedings of the National Academy of Sciences 106 (2009) 10516, and

Popmintchev, T., Chen, M.-C., Arpin, P., Murnane, M.M., and Kapteyn,

H.C., The attosecond nonlinear optics of bright coherent X-ray genera-

tion, Nature Photonics 4 (2010) 822-832.

5. See, for example, Schmuser, P., Dohus, M., and Rossbach, J., Physical

and technological challenges of an X-ray FEL, Ch. 9 in Ultraviolet and Soft

X-Ray Free-Electron Lasers, Springer Tracts in Modern Physics 229

(2008). DOI: 10.1007/978-3-540-79572-8.

6. Ding, Y., et al., Measurements and Simulations of Ultralow Emittance and

Ultrashort Electron Beams in the Linac Coherent Light Source, Physical

Review Letters 102 (2009) 254801.

7. Rimjaem, S., et al., Recent emittance measurement results for the upgrad-

ed PITZ facility, in Proceedings of FEL 2009, Liverpool, UK (2009), p. 251-254.

8. Carlsten, B.E., Nonlinear subpicosecond electron-bunch compressor,

Nuclear Instruments and Methods A 380 (1996) 505-516.

9. Serafini, L., and Rosenzweig, J.B., Envelope analysis of intense relativis-

tic quasilaminar beams in rf photoinjectors: A theory of emittance com-

pensation, Physical Review E 55 (1997) 7565-7590.

10. See for example, Wangler, T.P., RF Linear Accelerators, 2nd edition (2008),

Wiley-VCH Verlag GmbH & Co KGaA. ISBN: 978-3-527-40680-7.

11. Veshcherevich, V., and Belomestnykh, S., Buncher cavity for ERL, in

Proceedings of PAC 2003, Portland, OR, USA (2003), p. 1198-1200.

12. Ferrario, M., et al., Experimental Demonstration of Emittance

Compensation with Velocity Bunching, Physical Review Letters 104 (2010)

054801.

13. Bazarov, I.V., and Sinclair, C.K., Multivariate optimization of a high bright-

ness dc gun photoinjector, Physical Review Special Topics - Accelerators

and Beams 8 (2005) 034202.

14. Floettmann, K., Lidia, S.M., and Piot, P., Recent improvements to the

ASTRA particle tracking code, in Proceedings of PAC 2003, Portland, OR,

USA (2003), p. 3500-3502. ASTRA is available from: http://www.desy.

de/~mpyflo/Astra_dokumentation.

15. Miltchev, V., et al., Measurements of Thermal Emittance for Cesium

Telluride Photocathodes at PITZ, in Proceedings of FEL 2005, Stanford,

CA, USA (2005), p. 560-563.

148


31. Saldin, E.L., Schneidmiller, E.A., and Yurkov, M.V., Klystron instability of a

relativistic electron beam in a bunch compressor, Nuclear Instruments

and Methods A 490 (2002) 1-8.

32. Murphy, J.B., Krinsky, S., and Gluckstern, R.L., Longitudinal wakefield for

an electron moving on a circular orbit, Particle Accelerators 57 (1997) 9-64.

33. Saldin, E.L., Schneidmiller, E.A., and Yurkov, M.V., On the coherent radia-

tion of an electron bunch moving in an arc of a circle, Nuclear Instruments

and Methods A 398 (1997) 373-394.

34. Heifets, S., Stupakov, G., and Krinsky, S., Coherent synchrotron radiation

instability in a bunch compressor, Phys. Rev. ST Accel. Beams 5 (2002) 064401.

35. Huang, Z., and Kim, K.-J., Formulas for coherent synchrotron radiation

microbunching in a bunch compressor chicane, Phys. Rev. ST Accel.

Beams 5 (2002) 074401.

36. Venturini, M., Microbunching instability in single-pass systems using a

direct two-dimensional Vlasov solver, Phys. Rev. ST Accel. Beams 10

(2007) 104401.

37. Di Mitri, S., Cornacchia, M., Spampinati, S., and Milton, S., Suppression of

microbunching instability with magnetic bunch length compression in a

linac-based free electron laser, Phys. Rev. ST Acc. Beams 13 (2010) 010702.

38. Saldin, E.L., Schneidmiller, E.A., and Yurkov, M.V., FAST: a three-dimension-

al time-dependent FEL simulation code, Nuclear Instruments and Methods

A 429 (1999) 233-237.

39. Emma, P., et al., First results of the LCLS laser-heater system, to be pub-

lished in Proceedings of PAC 2009, Vancouver, BC, Canada (2009), paper

WE5RFP041.

40. Dowell, D.H., Hayward, T.D., and Vetter, A.M., Magnetic Pulse

Compression Using a Third Harmonic RF Linearizer, in Proceedings of PAC

1995, Dallas, TX, USA (1995), p. 992-994. DOI: 10.1109/PAC.1995.505106.

41. Emma, P., RF harmonic compensation for linear bunch compression in the

LCLS, LCLS Report No. LCLS-TN-01-1, SLAC (2001). Available from: http://

www.slac.stanford.edu/cgi-wrap/getdoc/slac-tn-05-004.pdf.

42. Vogel, E., et al., Test and commissioning of the third harmonic RF system

for FLASH, in Proceedings of IPAC’10, Kyoto, Japan (2010), p. 4281-4283.

43. Borland, M., elegant: A flexible SDDS-compliant code for accelerator sim-

ulations, APS Note No. LS-287, ANL (2000). Available from: http://www.osti.

gov/bridge/servlets/purl/761286-LMl4vP/native/761286.pdf.

44. Qiang, J., Ryne, R.D., Habib, S., and Decyk, V., An object-oriented parallel

particle-in-cell code for beam dynamics simulation in linear accelerators,

Journal of Computational Physics 163 (2000) 434-451.

45. Qiang, J., Ryne, R.D., Venturini, M., Zholents, A., and Pogorelov, I., High res-

olution simulation of beam dynamics in electron linacs for x-ray free elec-

tron lasers, Phys Rev. ST Accel. Beams 12 (2009) 100702.

46. Novokhatski, A., Timm, M., and Weiland, T., Single Bunch Energy Spread in

the TESLA Cryomodule, TESLA Report No. TESLA 99-16, DESY (1999).

Available from: http://flash.desy.de/reports_publications/tesla_reports/

tesla_reports_1999/.

16. Dowell, D.H., Bethel, S.Z., and Friddell, K.D., Results from the average

power laser experiment photocathode injector test, Nuclear Instruments

and Methods A 356 (1995) 167-176.

17. See, for example, fiber lasers from Polaronyx, Clark, IMRA, Calmar and

Amplitude Systems.

18. Sharma, A.K., Tsang, T., and Rao, T., Theoretical and experimental study of

passive spatiotemporal shaping of picosecond laser pulses, Phys. Rev. ST

Accel. Beams 12 (2009) 033501.

19. Zhou, S., et al., Efficient temporal shaping of ultrashort pulses with birefrin-

gent crystals, Applied Optics 46 (2007) 8488-8492.

20. Cialdi, S., Vicario, C., Petrarca, M., and Musumeci, P., Simple scheme for

ultraviolet time-pulse shaping, Applied Optics 46 (2007) 4959-4962; see also

Ghigo, A., Vicario, C., Petrarca, M., and Cialdi, S., Laser temporal pulse

shaping: Comparative study between LCM-SLM and DAZZLER techniques,

CARE Report No. CARE_Report-2007-032-PHIN (2007).

21. Staples, J.W., et al., Design of a VHF-band RF photoinjector with mega-

hertz beam repetition rate, in Proceedings of PAC 2007, Albuquerque, NM,

USA (2007), p. 2990-2992.

22. Baptiste, K., et al., A CW normal-conductive RF gun for free electron laser

and energy recovery linac applications, Nuclear Instruments and Methods

A 599 (2009) 9-14.

23. Sannibale, F., et al., Status of the LBNL normal-conducting CW VHF elec-

tron photo-gun, to be published in Proceedings of FEL 2010, Malmo,

Sweden (2010), paper WEPB36.

24. Sertore, D., et al., First operation of cesium telluride photocathodes in the

TTF injector RF gun, Nuclear Instruments and Methods A 445 (2000) 422-

426.

25. Arthur, J., et al., Linac Coherent Light Source (LCLS) conceptual design

report, SLAC Report No. SLAC-R-593, SLAC (2002). Available from:

http://www-ssrl.slac.stanford.edu/lcls/cdr/.

26. Brinkmann, R., et al., TESLA technical design report, part 2: The accelera-

tor, DESY Report No. DESY-01-011, DESY (2001). Available from:

http://tesla.desy.de/new_pages/TDR_CD/PartII/accel.html.

27. Bocchetta, C.J., et al., FERMI@Elettra FEL: Conceptual design report,

Sincrotrone Trieste Report No. ST/F-TN-07/12, ELETTRA (2007).

Available from: http://www.elettra.trieste.it/FERMI/index.php?n=Main.

CDRdocument.

28. Marangos, J., et al., New Light Source (NLS) project: conceptual design

report (2010). Available from: http://www.newlightsource.org.

29. Kur, E., et al., Accelerator design study for a soft X-ray free electron laser

at the Lawrence Berkeley National Laboratory, LBNL Report No. LBNL-

2670E, LBNL (2009). Available from:http://www.escholarship.org/uc/

item/1f86p730.

30. Borland, M., et al., Start-to-end simulation of self-amplified spontaneous

emission free electron lasers from the gun through the undulator, Nuclear

Instruments and Methods A 483 (2002) 268-272.

149


63. Saldin, E.L., Schneidmiller, E.A., and Yurkov, M.V., Calculation of energy dif-

fusion in an electron beam due to quantum fluctuations of undulator radia-

tion, Nuclear Instruments and Methods A 381 (1996) 545-547.

64. Giani, S., Bagulya, A.V., and Grichine, V.M., Synchrotron radiation energy

loss distribution, Nuclear Instruments and Methods A 452 (2000) 179-184.

65. Reiche, S., GENESIS 1.3: a fully 3D time-dependent FEL simulation code,

Nuclear Instruments and Methods A 429(1999) 243-248.

66. Fawley, W., A user manual for GINGER and its post-processor XPLOTGIN,

LBNL Report No. LBNL-49625 (2002).

67. Zholents, A., and Penn, G., Obtaining two attosecond pulses for X-ray stim-

ulated Raman spectroscopy, Nuclear Instruments and Methods A 612

(2010) 254-259.

68. Cavalieri, A.L., et al., Intense 1.5-cycle near infrared laser waveforms and

their use for the generation of ultra-broadband soft-x-ray harmonic conti-

nua, New Journal of Physics 9 (2007) 242.

69. Kim, K.-J., Circular polarization with crossed-planar undulators in high-

gain FELs, Nuclear Instruments and Methods A 445 (2000) 329.

70. Wiseman, M., Sinclair, C.K., Whitney, R., Zacrecky, M., and Vetterlein, R.,

High power electron beam dumps at CEBAF, in Proceedings of PAC 1997,

Vancouver, BC, Canada (1997), p. 3761-3763. DOI: 10.1109/PAC.1997.753410.

71. Byrd, J.M., et al., Femtosecond synchronization of laser systems for next

generation FELs, to be published in Proceedings of FEL 2010, Malmo,

Sweden (2010), paper THAOI1.

72. Byrd, J.M., et al., Enabling instrumentation and technology for 21st century

light sources, Nuclear Instruments and Methods A 623 (2010) 910-920.

73. Wilcox, R., Byrd, J.M., Doolittle, L., Huang, G., and Staples, J.W., Stable

transmission of radio frequency signals on fiber links using interferometric

delay sensing, Optics Letters 34 (2009) 3050.

74. Kim, J., Cox, J.A., Chen, J., and Kartner, F.X., Drift-free femtosecond timing

synchronization of remote optical and microwave sources, Nature

Photonics 2 (2008) 733-736.

75. Cundiff, S.T., Ye, J., and Hall, J.L., Optical frequency synthesis based

on mode-locked lasers, Review of Scientific Instruments 72 (2001) 3749-

3771.

76. Bartels, A., Diddams, S.A., Ramond, T.M., and Hollberg, L., Mode-locked

laser pulse trains with subfemtosecond timing jitter synchronized to an

optical reference oscillator, Optics Letters 28 (2003) 663-665.

77. Swann, W.C., et al., Fiber-laser frequency combs with subhertz relative

linewidths, Optics Letters 31 (2006) 3046-3048.

78. Wilcox, R., and Staples, J.W., Synchronizing lasers over fiber by transmit-

ting continuous waves, in Proceedings of the 2007 Conference on Lasers

and Electro-Optics, Baltimore, MD, USA (2007), paper CThHH4. DOI:

10.1109/CLEO.2007.4452764.

47. Bane, K.L.F., Wakefields of sub-picosecond electron bunches, from the

2005 Workshop on the Physics and Applications of High Brightness

Electron Beams, Erice, Italy (2005). SLAC Report No. SLAC-PUB-11829,

SLAC (2006). Available from: http://slac.stanford.edu/pubs/slacpubs/11750/

slac-pub-11829.pdf.

48. Borland, M., Simple method for particle tracking with coherent synchro-

tron radiation, Phys. Rev. ST Accel. Beams 4 (2001) 070701.

49. Aune, B., et al., Superconducting TESLA cavities. Phys. Rev. ST Accel.

Beams 3 (2000) 092001.

50. Kugeler, O., Neumann, A., Anders, W., and Knobloch, J., Adapting TESLA

technology for future cw light sources using HoBiCaT, Review of Scientific

Instruments 81 (2010) 074701.

51. Knobloch, J., Anders, W., and Xiang, Y., Cryogenic Consideration for CW

Operation of TESLA-type Superconducting Cavity Modules for the BESSY

FEL, in Proceedings of EPAC 2004, Lucerne, Switzerland (2004), p. 976-978.

52. Harms, E., et al., Status of 3.9 GHz superconducting RF cavity technology at

Fermilab, in Proceedings of PAC 2007, Albuquerque, NM, USA (2007), p.

2254-2256.

53. Doolittle, L., Ratti, A., Monroy, M., Champion, M., and Ma, H., Operational

Performance of the SNS LLRF Interim System, in Proceedings of PAC 2003,

Portland, OR, USA (2003), p. 1464-1466.

54. Byrd, J., Doolittle, L., Ratti, A., Staples, J.W., Wilcox, R., Stettler, M.,

D’Auria, G., Ferianis, M., Milloch, M., and Rohlev, A., Plans for precision RF

controls for FERMI@Elettra, in Proceedings of PAC 2007, Albuquerque,

NM, USA (2007), p. 2310-2312. LBNL Report No. LBNL-63221, LBNL (2007).

55. Wilcox, R., et al., Stable transmission of radio frequency signals on fiber

links using interferometric delay sensing, Optics Letters 34 (2009) 3050-

3052.

56. Fairley, D., et al., Beam Based Feedback for the Linac Coherent Light

Source, in Proceedings of ICALEPCS 2009, Kobe, Japan (2009), p. 644-646.

57. Byrd, J.M., et al., Femtosecond Synchronization of Laser Systems for the

LCLS, in Proceedings of IPAC’10, Kyoto, Japan (2010), p. 58-60.

58. Venturini, M., and Zholents, A., Modeling microbunching from shot noise

using Vlasov solvers, Nuclear Instruments and Methods A 593 (2008) 53-56.

59. Ding, Y., and Huang, Z., Statistical analysis of crossed undulator for polar-

ization control in a self-amplified spontaneous emission free electron

laser, Phys Rev. ST Accel. Beams 11 (2008) 030702.

60. Stupakov, G., Using the Beam-Echo Effect for Generation of Short-

Wavelength Radiation, Physical Review Letters 102 (2009) 074801.

61. Huang, Z., Ratner, D., Stupakov, G., and Xiang, D., Effects of energy chirp on

echo-enabled harmonic generation free-electron lasers, in Proceedings

of FEL 2009, Liverpool, UK (2009), p. 127-129.

62. Xiang, D., et al., Demonstration of the echo-enabled harmonic generation

technique for short-wavelength seeded free electron lasers, Physical

Review Letters 105 (2010) 11480.

6 Experimental systems

6.1 Introduction

IntheinitialcomplementofbeamlinesfortheNGLS,we

areplanningforthreeseparateFELswithacoreoperating

rangefrom0.28to1.2keV.Thegeneralcharacteristicsof

eachofthesethreelinesareshowninTable2.Theintent

hereistooutlinethegeneraldesignissuesandpresent

directionsfortheirsolutionratherthantoprovideadefini-

tivedesignineachcase.Inthecaseofeachofthesebeam-

linesystems,ultra-shortpulseswillbeused,intothesub-fs

regime.Thisthereforepresentsoneoftheprimarychal-

lenges, the preservation of the extremely small pulse

length.Theenergyrangetobeaccessedrequiresgrating

optics,butgratingsextend the temporal lengthof the

beam.Thegrazingincidence(focusing)opticsthathaveto

beusedinthesoftX-rayenergyregionalsotypicallyinduce

wavefronttilt,againresultinginpulselengthening.Below

welookatthepracticalconsequencesoftheseeffects.

Asecondchallengeisthatinmostexperiments,there

isa requirement forsingleor twowavelengthpump-

probemeasurements,i.e.thattheprimarybeammustbe

splitwithonebeampumpingthesampleandasecond

beamwithvariabledelayprobingthesample.Inthehard

X-raydomainthisactioniseasilyperformedbycrystal

optics,wherethinpelliclepartiallytransmittingcrystals

arepractical.InthesoftX-rayenergydomain,wedonot

havetheequivalentopticalelements,andsoalternative

strategies as described below must be employed.

Thefinalchallengeisfromtransientandaveragepower

loadontheopticscreatingdamage,orwavefrontdistor-

tion.Thepeakpoweronthe1stopticalelementswillbe

similartoFLASH,butduetothemuchhigherrepetition

rate,theaveragepowerwillbemuchhigher.Theaverage

power density will exceed that experienced at

3rdgenerationsynchrotronundulatorbeamlinesandwill

presentasignificantchallenge.Mostoftheexperienceto

dateonimpulsivedamagetomirrorcoatinghasbeenon

diamond-likecarbonduetoitsexcellentreflectivityatthe

smallgrazinganglestypicallyusedforenergieslessthan

200eV.IntheNGLScasehowever,weneedtohavehigh

reflectivitythroughoutthewholesoftX-rayenergyregion

including thecarbonK-edgeregionand this imposes

somechallengesforcoatingmaterials.Inthefollowing

sectionswehaveoutlinedtheoveralldesignissuesand

suggestedsolutionsusingexistingFELtechnologyas

wellasindicatingwheretheemphasisofourR&Dactivi-

tiesneedstobeplaced.

6.2 OverallBeamlineDesign

Thedesignandoperationof theFLASHbeamlines

offerssomeguidanceon theoverall issuesandsolu-

tions,1,2buthere,oneofthemainissueswillbethepres-

ervationandmanipulationofultra-shortpulsescloseto

thetime-bandwidthlimitthatneedfurtherconsideration.

Agratingwilllengthenapulsebyatimeδt=Nmλ/c,

whereNisthenumberofilluminatedgrooves,misthe

diffractiveorder,λisthewavelengthandcisthespeedof

light.Forexample,foraresolvingpowerof10,000we

wouldneedthesamenumberofgroovestobeilluminat-

edandhenceforthe1stdiffractiveorderatthecarbon

K-edge,wewouldhaveapulsebroadeningof150fs.

Thepulselengthofatransformlimitedlightsourceis

howeverδT~λ2/δλcandsorealizingthatthefundamental

resolvingpowerofagratingisequaltothenumberofillu-

minatedgrooves,N,wecanseethatthegrating-induced

temporalsmearingoptimallycorrespondstothatimplied

by the time-bandwidth product.The design feature

requiredthereforeistoilluminatethecorrectnumberof

groovesforthewavelengthresolutionrequired.

152

6 . EXPERIMENTAL SYSTEMS

would be done with a single elliptical mirror in each

plane,butalthoughthesearefreeofpoint-to-pointpulse

stretching,afinitewidthobjectisimagedinaplanetilted

withrespecttotheopticalaxis.Tocounterthis,focusing

would require two mirrors in each plane in order to

achieveanimagefreeoftilt.Sucherectfieldimagingfor

widefieldsistypicallydonewithhyperboloid-ellipsoid

pairsasinWolterX-raytelescopedesign,buthereasthe

angularapertureissmaller,bentparaboloidcombina-

tionswillprobablybesufficientandwouldthestarting

pointforourdesignstudies.

ALShighpoweropticalcomponentshaveeitherside

orinternalwatercoolinganddealwithhighpowerden-

sities.InthecaseofNGLShoweverathighrepetition

rate,wewillbegoingtosignificantlyhigheraverage

powers.Thiscanbedealtwithbymovingopticsfurther

fromthesource,byavoidinguseofan intermediate

focusinthebeamlinesandpossiblybyusingafirstmir-

roratextremegrazingincidencethatisdivergent,thus

increasingthebeamsizeondownstreamoptics.Water

coolingcanbepushedbeyondthehighestheat load

ALSopticsbyinternaljetcooling,butbeyondthis,we

willhave tocryo-coolsomeof thehighestheat load

components.ThishasbeendoneforexampleintheALS

BL5.0wiggleropticalsystem,wherethecrystaloptics

arecooledto120Ktotakeadvantageofthezeroexpan-

sioncoefficientofSiatthistemperatureandthehigher

thermalconductivity. Inthiscase,weextractroughly

250Wofpower,similartotheNGLScase.

Due to the very high power density, intermediate

focusingtoasetofentranceslitsshouldprobablybe

avoidedandsowewillneedinterchangeofgratingswith

differinglinedensities.Thisgratinginterchangeistypi-

callydoneinsoftX-raybeamlinestodaysothatoptimum

diffractionefficiencycanbeachievedoverawidewave-

lengthrange,buthereaswellitisrequiredforselection

oftheappropriatesource-limitedresolutionandhence

time-bandwidth-limitedpulselength.Afurtherissueis

thatthewavefrontenteringtheopticalsystemistiltedat

theexit,duetotheuseofgrazingincidenceoptics.One

waytocorrectthisistousetwogratingsintheclassical

mountinginsubtractivedispersionmodeassuggested

byVilloresi3andsuccessfullydemonstratedbyseveral

groups.4Afirstfocusinggratingproducesadispersive

focusatwhichtherequiredwavebandisselectedbyaslit;

asecondgratingthenreimagesthistoafocus.Thissec-

ondaryfocuscanbefreetofirstorderofprimarypulse

stretchingandwavefronttilt.Thepenaltyisthatthenor-

mallylowdiffractionefficiencyofagratingismadesub-

stantiallyworseinthetwogratingarrangement.

Thesimplestarrangementhoweverwouldbetouse

anerectfieldvariablelinespacing(VLS)monochromator

(Figure109).5Inthissystemtheexitwavefrontisapproxi-

matelyperpendiculartotheopticalaxis.Thecombination

ofanentranceslitlessformofthisdesignwithanarrayof

gratingsdesignedtoproduceselectionofarangeofreso-

lutionswouldbeastartingpointforourdesignstudies.

Lightfromthemonochromatorwouldberefocusedtothe

sampleusinggrazing incidenceoptics.Normally this

Figure109 VLS grating monochromator as used at the ALS (left) and a K-B mirror assembly used for microfocusing.

153


siveworkatFLASH,impulsivedamageshouldnotbean

issue.R&Dinthisareawouldconcentrateonassessing

cumulative damage due to high average power that

mightbepresentbelowthesingleshotdamagelimit.

6.4 SplitandDelay

Severaloftheexperimentsplannedfortheinitialcom-

plementofbeamlineswilluseX-raypump—X-rayprobe

techniques.Wewillthereforehavetosplitoneormore

softX-raybeamsandprovideavariabledelaypath,with

twobeamsthatareideallyco-linear.Thetwomainways

todothisaretoa)beamsplitusingagrating; inthis

methodzeroandfirstorderlightareusedandb)splitthe

wave-frontusingaknifeedgemirrorpartiallyinserted

intothebeam.Thefirstmethodhassomeadvantages,

butforultrafastpulsesisnotpractical,asasecondgrat-

ingwouldhavetobeusedtocompensatepulsestretch-

ing.ThesecondschemehasbeenpioneeredatFLASH9

andisshowninFigure110below.

ThisarrangementisbasedonagrazingincidenceMach-

Zehnderinterferometer.Therighthanddiagramshowsthe

principleinwhichamirror(SM1)ispartiallyinsertedinto

thebeam,thusgeneratingtwobeams.Thesebeamsare

directed towards mirror pairs DL1-DL3 and DL2-DL4

respectivelyandrecombinedatmirrorsSM3andSM4.

Eachpathhasfivereflectionsandadelayisintroduced

betweenthepathsbylateralmotionofthe“interferome-

ter”table.SuchasystemhasbeenbuiltattheALSand

usedforVUVFourierTransformSpectroscopybasedon

thesameoverallgeometryandcouldbeadaptedforthis

purpose.ThesystemasusedatFLASH,andplannedfor

LCLS,hasdemonstratedadelayrangeof10psandadelay

precisionof0.2fs.Themainmodificationwewouldmake

tothissystemwouldbetousemirrorsatmoreextreme

grazinganglestosupporthighthroughputtoabove1keV.

6.3 MirrorDamage

Theissueofmirrordamagehasbeenextensivelystud-

iedatFLASH,mainlyintheVUVandatLCLSforthehard

X-rayregime.NGLSpulseenergiesaresimilartothoseat

thesefacilitiesandsoinprinciplewecanadoptsimilar

solutions.Onespecialproblemhowever is that these

coatings are based on either diamond-like carbon or

boroncarbide.Inourcase,wewishtotunethroughthe

carbonK-edgeregionandnormallyuseofacarboncoat-

ingintheregionfrom280-1000eVwouldbeprecluded.

However thereare fewalternatives. Inorder toavoid

excessiveinstantaneouselectronicheating,wehaveto

usealowZcoating.AtFLASH,diamond-likecarbonis

extensivelyusedbecausethemainoperatingenergiesare

lessthan280eV.AtLCLS,B4Cisextensivelyusedbecause

thecarbonK-edgeissubstantiallylowerinenergythan

theminimumoperationalenergy.OnNGLS,thecarbon

K-edgeiswithinourenergyrange,andbyitselfaprime

targetforsomeexperiments.OtherlowZmaterialsare

limited,withperhapsBethemainalternative.Analterna-

tiveissimplytouseB4C,butatamuchlowergratingangle

thanconventionallyused.Thiswouldsignificantlyreduce

thecarbonK-edgereflectivitymodulation.Separation

betweenthevariousFELbeamlinesisprimarilyprovided

bytheFELswitchyard,notbytheoptics,andconsidering

theverylowdivergenceofthelight,mirrorlengthshould

notbeanissue.Howevershadowingofthediffraction

gratinggroovesatverysmallgrazinganglesisaconcern

andtheoptimizationofthegrooveshapeandcoatingto

optimizeefficiencywillhavetobecarefullyconsidered.

Theissueof impulsivedamageofamirrorcoating

underFELilluminationhasbeenextensivelystudiedfor

diamondlikeandamorphouscarbon6,7andforB4C.8In

bothcasestheconclusionsaresimilarinthatthesingle

shotdamagethresholdisintheregionof0.1–0.3J/cm2

wellabovethevaluesforNGLSoptics.Basedonexten-

Figure110 Mirror based split and delay line. (From Sorgenfrei et al.9)

DL1 DL3

DL2 DL4SM4

SM3

SM2

SM1

y

zSM2

154


Nq=Eγ /ε,whereEγistheincidentphotonenergyandεvar-

iesbymaterial(ε=3.6eVforsilicon).Fluctuationsinener-

gylosslimitthespectroscopicresolutionofthedetector

toσq2=FNq,whereFistheFanofactor(F~0.12forSi).

ForNGLS, silicon is an idealdirect-detection sensor:

Figure111showstheefficiency(probabilityofphotocon-

versionwithinthesensitivevolume)forX-raydetection

ina200μmthicksiliconsensorwith3nmofnativeoxide.

Above~8keV,thesensorstartstobecometransparent—

limitingdetectionefficiency.Atlowerenergies,X-raysare

absorbedpriortoenteringthesensitivevolume.

Inordertofullydepletethesensor(andthusensure

fullchargecollectionwiththeminimalpointspreadfunc-

tion)athinconductingentrancewindowisrequired—

and the thickness of this window determines the

low-energyX-rayefficiency.Weareactivelyengagedin

R&Donthinwindows:forNGLS,thetotalthicknessofthe

deadlayerbeforethesensitivevolumemustbe≤10nmin

ordertomaintain>50%efficiencyatthecarbonK-edge.

Figure111 Efficiency for 200 μm thick silicon detectors.

Readoutspeedanddataprocessingwillbecrucialfor

NGLS.Wehavedeveloped102framepersecond(fps)

2Ddetectors,12andhavesuccessfullydeployedthese

detectorsatALS,APSandLCLS.Thisdetectorisbasedon

aunique,thick,fully-depletedMOSCCDstructuredevel-

opedatLBNL.13Wearetransformingthisdetectorfrom

102fpsto103-4fpsbyadoptingafullycolumn-parallel

architecturetogetherwithadvancedcustomintegrated

circuitreadoutin65nmCMOS.

To reach the105 fps rateneeded forNGLS,anew

detectorarchitecturewillbeneeded.Today,at200fps,the

6.5 Diagnostics

AseachofthebeamlineswillbecapableofX-raypump

—X-rayprobecapability,andmeasurementofdelayand

pulselengthwillbeanessentialcomponentofeachexper-

iment,wewillneedin-beamlinediagnosticsofthetempo-

ralcharacteristicsofthebeam.Thereareseveralwaysthat

this can be achieved, and based on on-going work at

FLASHandnowattheSXRbeamlineatLCLS,thepre-

ferredmethod,especiallyforultra-shortpulsesislikelyto

evolve.Forultrashortpulses,itislikelythatasecstreaking

methodswillbeused,butforpulsesof10fswecanuse

simplerandmorerobustmethods.Oneoftheseisbased

onthemodulationoftheopticalreflectivityofasemicon-

ductorbyaVUVorsoftX-raypulse.10Theimpulsivecre-

ation of electron hole pairs in the semiconductor

modulatesthereflectivity,andbytiltingonewave-front

withrespecttotheother,aCCDcanbeusedto“image”the

arrivaloftheX-raypulse.InanX-ray—X-rayexperiment,

thetwobeamswouldbephysicallyseparated,butwould

beimagedinthesameway;afslasersynchronizedtothe

X-raysourcewouldprovidetheimagingwindowandthe

separationof the twoX-ray inducedreflectivityedges

couldgiveaprecisemeasurementofX-raypump-probe

delay.Theprecisionofthissystemispresently40fsovera

time window of many ps, with substantial area for

improvementtothe10fsscale.Beyondthis,streakingof

photoelectronenergiesproducedbyX-rayillumination

ofagasjetimmersedinanultrafastlaserfieldcanyield

highprecisionpulseshapeanddelaymeasurementsinthe

sub-fsregime,butoververylimitedtemporalwindows.11

6.6 Detectors

Worldwide,asnewFELsourcescomeonline,corre-

spondingdetectordevelopmentsareneededinorderto

beabletorecordtheshot-by-shotdatagenerated.The

greatestchallengesarefor(2-dimensional,area)~mega-

pixeldetectorsabletoreadoutattheFELrepetitionrate,

withnomemoryofthepreviousshot.Whereasearlier

2DX-raydetectorswerebasedonscintillatingphosphors

fiber-opticallycoupledtoacharge-coupleddevice(CCD),

moderndetectorsarebasedondirectdetectionofX-rays

inasemiconductorsensor.Inafullydepletedsemicon-

ductor detector, the number of electron-hole pairs,

Nq,generatedbyanincidentphoto-convertingX-rayis

10 100 1000 10000

E (eV)

10

0

80

60

40

20

Effic

ienc

y (%

)

155


NGLS,anadvancedversionofthehigh-speeddetector14

developedforTEAMprojecttogetherwithon-chippro-

cessingisthemostpromisingcandidate:inaggressive

CMOStechnologies,theimagingareaofareticle-scale

devicecanbesmall,leavingtherestoftheICareafreefor

imageprocessingandcompression.Aspartofouron-

goingR&Dprogram,weareprototypinganimagesensor

in65nmCMOSinordertodeterminesuitabilityforan

ultra-high-speed2DX-raydetector.Wearealsoworking

on algorithm development based on X-ray FEL data.

Thesealgorithmswillbetestedfirstinfirmware,andthen

portedtosiliconfortheNGLSdetector.

LBNLFastCCDgenerates1.5Tb/hr.Atthisdatarate,itis

stillpossible,ifnotalwaysefficient,tosimplyreadall

dataoutandstoreit.Athigherframerates,datatransmis-

siontoarchivalstoragebecomesimpossible.Forthefully

column-parallelCCDdescribedabove,hundredsofhigh-

speedseriallinkswillcarrydatatoafirmwaredatapro-

cessor, which will perform compression and feature

extractionpriortotransmittingthedatatostorage.At

105fps,amegapixeldetectorwouldgenerateaTbevery

5seconds—whichisimpossibletoreadoutconvention-

ally:thedatacansimplynotbemovedoffthedetector

silicon.On-chipprocessingisrequired.ForsoftX-raysat

References

1. Martins, M., et al., Monochromator beamline for FLASH. Review of

Scientific Instruments, 2006. 77(11): p. 115108.

2. Tiedtke, K. and et al., The soft X-ray free-electron laser FLASH at DESY:

beamlines, diagnostics and end-stations. New Journal of Physics, 2009.

11(2): p. 023029.

3. Villoresi, P., Compensation of optical path lengths in extreme-ultraviolet

and soft X-ray monochromators for ultrafast pulses. Applied Optics, 1999.

38(28): p. 6040-6049.

4. Nugent-Glandorf, L., et al., A laser-based instrument for the study of ultra-

fast chemical dynamics by soft X-ray-probe photoelectron spectroscopy.

Review of Scientific Instruments, 2002. 73(4): p. 1875-1886.

5. Hettrick, M.C., In-focus monochromator: theory and experiment of a new

grazing incidence mounting. Applied Optics, 1990. 29(31): p. 4531-5.

6. Juha, L., et al., Radiation damage to amorphous carbon thin films irradiat-

ed by multiple 46.9 nm laser shots below the single-shot damage thresh-

old. Journal of Applied Physics, 2009. 105(9): p. 093117.

7. Chalupsky, J., et al., Damage of amorphous carbon induced by soft X-ray

femtosecond pulses above and below the critical angle. Applied Physics

Letters, 2009. 95(3): p. 031111.

8. Hau-Riege, S.P., et al., Multiple pulse thermal damage thresholds of mate-

rials for X-ray free electron laser optics investigated with an ultraviolet

laser. Applied Physics Letters, 2008. 93(20): p. 201105-3.

9. Sorgenfrei, F., et al., The extreme ultraviolet split and femtosecond delay

unit at the plane grating monochromator beamline PG2 at FLASH. Review

of Scientific Instruments. 81(4): p. 043107.

10. Theophilos, M., et al., Single-shot timing measurement of extreme-ultravio-

let free-electron laser pulses. New Journal of Physics, 2008(3): p. 033026.

11. Kienberger, R., et al., Sub-femtosecond X-ray pulse generation and mea-

surement. Applied Physics B-Lasers & Optics, 2002. 74(Suppl S): p. S3-S9.

12. Denes, P., et al., A fast, direct X-ray detection charge-coupled device.

Rev Sci Instrum, 2009. 80(8): p. 083302.

13. Holland, S.E., et al., Fully depleted, back-illuminated charge-coupled

devices fabricated on high-resistivity silicon. Electron Devices, IEEE

Transactions on, 2003. 50(1): p. 225-238.

14. Battaglia, M., et al., Characterisation of a CMOS active pixel sensor for

use in the TEAM microscope. Nuclear Instruments and Methods in

Physics Research Section A: Accelerators, Spectrometers, Detectors

and Associated Equipment, 2010. 622(3): p. 669-677.

7 Future upgrades

NGLS,asdescribedinSection5,includesastate-of-the-

artelectronsource,superconductinglinac,andthreeinitial

FELbeamlines,togetherwiththeconventionalfacilities

required to house the facility.The linac and FELs are

designedtobeabletoobtainwavelengthsasshortas1nm

atrepetitionratesat1MHzorhigher.

Inordertoaccommodateinevitabletechnicaladvances,

theNGLSdesignconceptembracesastrategyofphased

implementationofFELbeamlines.TheNGLSfacilitywillbe

configuredtotakeadvantageofas-yetunrealizedadvanc-

esinthephysicsandtechnologyofhighbrightnessbeams,

newconcepts inFELoperation, and improvements in

undulatorsandX-rayoptics.Thepreliminarydesignpre-

sentedhereusesahighlyflexibleelectrongun,injector,

andlinacthatcanprovideawidevarietyofelectronbunch

structuresandtimepatterns,sothatNGLSwillbecapable

ofincorporatingawiderangeofFELtechnologies.

The electron beam spreader shown in Figure 76 is

intrinsicallymodular.Capacitycanbeincreasedbyadding

additional spreader elements, FELs, X-ray beamlines

togetherwith thecorrespondinghousingasshown in

Figure112.Capabilitycanbeincreasednotonlybyadding

advancedFELs,butalsobyincreasingtheelectronbeam

energywithacombinationofextra linacsectionsand

NGLS

Capacity Expansion

Capability Expansion

Future Buildout

Figure112 Upgrade paths.

158

5 . FUTURE UPGRADES

onlysmallincrementstobeamenergy.Atsufficientlyhigh

repetitionrates,FELoscillatorscombinedwithharmonic

radiatorsmightprovidethebenefitsofseededoperation

withouttheneedforexternallasers.Significantimprove-

mentsinHHGforEUVorsoftX-rayproductioncanbelev-

eragedtoprovideseedingsignalsrequiringlessstringent

harmonicjumps.Ifthelinaccouldoperatereliablyatmul-

tipleparameterpoints,soastodeliverbunchesofdiffer-

ent charge, duration, and/or energy to different FEL

beamlines,theflexibilityandversatilityofthemachine

wouldberedoubled.

Manyoftheseareasofimpactare,orwill,bethesub-

jectofsignificant researcheffortsworldwide,and the

NGLSprojectwillbepoisedto takeadvantageofany

incrementalorrevolutionaryimprovements.

gradientincreases.Forexample,increasingbeamenergy

to approximately 2.5 GeV, and using undulators of

~12mmperiodwillproduceover3keVinthefundamen-

tal,and10keVinthe3rdharmonicatpowerlevelsesti-

matedtobe1%ofthoseachievableinthefundamental.

Similarly,anincreaseinbeamenergytoapproximately

4GeV,togetherwithdevelopmentsinundulatortechnol-

ogythatprovidefor10mmperioddevices,wouldpermit

lasinginthefundamentalat10keV.

Inaddition,modest improvements incavityquality

factorscouldtranslateintosignificantlyhigherbeamener-

giesatfixedoperatingcosts.Novelundulatordesigns—

including very-short-gap superconducting undulators

orpossiblyevenelectromagneticundulators—could

dramaticallyexpandtheNGLSwavelengthwithperhaps

8 Management

8.1 CostEstimate

ThepreliminaryTotalEstimatedCostwithoutcontin-

gency(TEC)oftheprojectis$635M,andthepreliminary

TotalProjectCost (TPC) is$997Minthen-yeardollars

(withthescheduleshowninTable11,andthefundingpro-

fileshowninTable10).Theestimateincludesallcosts

associatedwithConceptualDesignandR&D,together

withalltechnicalandconventionalconstruction,acceler-

atorcommissioning,projectmanagementandcontin-

gency.These estimates were developed during pre-

conceptual design, and are based on recent US

experiencewithsuperconductingaccelerators,freeelec-

tronlasers,andrecentUniversityofCalifornia/LBNL

conventionalconstructionexperience.Abreakdownof

thecostisshowninTable9,withapreliminaryfunding

profileshowninTable10.

Apreliminaryannualoperatingcostestimatehasbeen

performedbasedonexperiencewiththecurrentoperat-

ingcostsfortheAdvancedLightSource.Thepreliminary

estimateof$97MinFY11dollarsislargerthantheALS

operatingcostduetothelargeracceleratorandtechnical

staffrequiredforthemorecomplexNGLS,togetherwith

higherpowercosts.

Table9 Cost breakdown in escalated M$.

Construction Cost (M$)

Injector $31

Linac $233

X-rayProduction $44

ExperimentalSystems $52

ConventionalFacilities $201

ProjectManagement $74

TotalEstimatedCost(TEC) $635

ConceptualDesign $10

R&D $54

Commissioning $27

OtherProjectCosts(OPC) $91

Contingency 37% $271

TotalProjectCost(TPC) $997

Profile FY11 FY12 FY13 FY14 FY15 FY16 FY17 FY18 FY19 FY20 FY21 FY22

OPC $2 $10 $10 $15 $16 $7 $4 $0 $0 $0 $6 $21

TEC $0 $0 $15 $20 $34 $93 $156 $250 $160 $110 $64 $4

TPC $2 $10 $25 $35 $50 $100 $160 $250 $160 $110 $70 $25

Table10 Funding profile in escalated M$.

160

8 . MANAGEMENT

• Theelectronbeamspreader,whichdistributesthe

electronbeamtotheindividualFELs

- High switching rates with minimum beam

perturbationandbeamlossarerequired—see

Section5.4.4

• Electronbeamdumps,which requireengineering

design,notR&D—seeSection5.4.6

• AdvancedFELoperation(seeSection5.4.5),including:

- Seedingschemes:inparticular,EEHG-seededFELs

radiating at a high harmonic of the seed laser

requiredemonstrationofphysicsandtechnology

atbothshorterwavelengthsandfarhigherhar-

monicjumpsthancurrentlyachieved.

- Polarizationcontroland tunabilityusingcross-

polarizedundulators.

- Synchronization,timing,andfeedback-and-control

systemsforsub-femtosecondoperation.

Inaddition,inordertotakefulladvantageofthetrans-

formationalcapabilitiesofNGLS,advancesinkeytechni-

calareaswillenhancethescientificproductivityofNGLS.

Forthisreason,R&Dwillalsobeperformedon

• Shortperiodundulators

AsdescribedinSection5.4.5.3,thebaselineundula-

torsforNGLSwillbebasedonwell-provenhybridperma-

nentmagnettechnologies.Superconductingtechnologies

havethepromisetoeliminatemovingpartsandreduce

costsandprovideellipticalpolarization,alongwiththe

capabilitytoprovideshortperiods,andthusextendthe

wavelengthreachofNGLS

• X-raybeamtransport

AsdescribedinSection6.3,thehighaveragepowerof

NGLSwillrequireadvancesinsomeopticalcomponents.

• High-speeddetectors

AsdescribedinSection6.6,advancesinhighframe

ratedetectorsarerequiredinordertobenefitfromthe

NGLSrepetitionrate.

• High-powerseedlaserandpumplasersystems

AsdescribedinSection5.4.5.5high-power,andcarri-

er-envelopephase-stabilized lasersare required,with

robustandreliableperformance.

• High-dynamic-rangediagnostics

AsdescribedinSection5.4.1,thecapabilitiesofNGLS

tooperateatverylowcharge(afewpC)andveryhigh

charge(upto1nC),willallowsignificantadvancesinper-

formance,andrequiredevelopmentsindiagnosticssys-

tems(seeSection5.4.8andSection6.5).

8.2 Schedule

The preliminary schedule of major milestones is

showninTable11.TheNGLSconceptualdesignreport

(CDR)wouldbedevelopedforCD-1,andtheperformance

baselinewouldbeapprovedatCD-2.Transportofelectron

beamthroughthelinacwouldmarkthestartofcommis-

sioning (CD-4a).The project would be complete with

deliveryofphotonstotheexperimentalhall(CD4-b),fol-

lowedbythestartofoperationasauserfacility.

Table11Preliminary Major Schedule Milestones.

Milestone Date

CD-0 ApproveMissionNeed FY11

CD-1 PreliminaryBaselineRange FY13

CD-2/3a PerformanceBaseline/Long-leadProcurement FY15

CD-3b StartofConstruction FY16

CD-4a StartofCommissioning FY21

CD-4b StartofOperations FY22

8.3 RiskManagementandR&DNGLScost,scheduleandperformanceriskswillbe

minimizedbyusingproventechnologyinthebaseline

designwhereverpossible.Nonetheless,inordertodeliv-

ertheperformancedescribedabove,NGLSwillrequire

certain,specificR&Dandengineeringadvancesassum-

marizedinthissection.Onacostbasis,themajorityof

NGLSislowrisk:theNGLSlinacisaconservativeimple-

mentationofcurrentsuperconductingacceleratortech-

nology,anddoesnotrepresentasignificanttechnical

risk;similarly,theconventionalconstructioniscompara-

bletootherequivalentfacilities.

Fourprincipletechnicalsystemsrequiretechnology

maturation,whichwillbeaddressedbyearlyR&Dinthe

project,andnoneofwhichareexpectedtoimpactthe

proposedconstructionschedule:

• The high-repetition-rate, low-emittance electron

injector.Asdescribed inSection5.4.2 the injector

requires

- High quantum efficiency, long lifetime photo-

cathodes—seeSection5.4.2.2

- Ahigh-powerdrivelaserthatmatchesthephoto-

cathodematerialspropertiesandthathastrans-

verseandlongitudinalpulseshapingcapabilities

—seeSection5.4.2.3

- Ahigh-powerelectrongun—seeSection5.4.2.4

161

8 . MANAGEMENT

8.5 Environment,SafetyandHealth

8.5.1 Integrated Safety Management SystemEnvironment,SafetyandHealth(ES&H)requirements

willbesystematicallyintegratedintomanagementand

workpracticesatalllevelssothattheNGLSprojectisexe-

cutedwhileprotectingthepublic,theworker,andtheenvi-

ronment.NGLSIntegratedSafetyManagementSystem

documentsandpolicieswillmakeitclearthattherespon-

sibilityforsafetyandenvironmentalprotectionstartswith

theNGLSDirectorandflowsthroughthemanagement

chainfromseniormanagementtolinesupervisors,and

finallytotheworkers.ItistheresponsibilityofNGLSman-

agementtoensurethatstaffaretrainedandareresponsi-

bleforES&Hintheirassignedareas.TheNGLSproject

workwillbeexecutedinaccordancewithdefinedinstitu-

tionalES&Hpoliciestoensurehazardsareidentifiedand

mitigated;workisauthorizedafterES&Hanalysisiscom-

pleted;andoversightofworkisconductedbyNGLSman-

agementandstaff.Continuousassessmentandoversight

oftheprojectwillbeconductedbytheproject,institutional

EH&Sandassuranceorganizations,andtheDOE.

8.5.2 National Environmental Policy ActTheDOEwill complywith the requirementsof the

NationalEnvironmentalPolicyAct(NEPA)anditsimple-

mentingregulationspriortotakinganyactiononthepro-

posedproject that couldhaveadverseenvironmental

effects.ANEPAevaluationwillbepreparedtoevaluatethe

potentialenvironmentalconsequencesofconstructing

andoperatingtheNGLS.Itisplannedthatthiswilltake

theformafullEnvironmentalImpactStatement(EIS).

8.5.3 Fire Hazards AnalysesAfirehazardanalysis(FHA)willbedevelopedtodeter-

minethefiresafetyrisksassociatedwiththeNGLSproject.

8.5.4 Safety Assessment DocumentIn compliance with DOE Order 420.2B “Safety of

AcceleratorFacilities,”aSafetyAssessmentDocumentwill

bepreparedthatidentifiesthespecificES&Hhazardsand

themeansfortheirmitigation.Inparticular,theradiation

hazardsassociatedwiththisfacilitywillbefullyanalyzed

andallappropriateshielding,interlock,andadministrative

controlswillbedevelopedandimplemented.

8.4 Organization

NGLSwillbeexecutedasaprojectwithinthePhoton

SciencesDirectorateatLBNL.Forthedesign,construc-

tion and operation of NGLS, technical staff will be

matrixed from theAccelerator and Fusion Research,

EngineeringandFacilitiesDivisions—analogoustothe

constructionandoperationoftheAdvancedLightSource.

Scientific coupling to LBNL’s Physical Biosciences,

Genomics, Life Sciences, Chemical Sciences,

EnvironmentalEnergyTechnologies,MaterialsSciences,

EarthSciences,ComputingandGeneralSciencesand

AdvancedLightSourcedivisionswillguidethedevelop-

mentofthefacilityandtheinitialexperimentalprogram.

Figure113showstheorganizationofNGLS.NGLSwillbe

executed as a scientific and technical collaboration

betweennumerouslaboratories.Inparticular,itisantici-

patedthatthesuperconductinglinacwillbedeveloped

byaDOEpartnerlaboratory.

Department of Energy

Deputy Secretary, Acquisition ExecutiveUnder Secretary of EnergyDirector, Office of Science

Director, Office of Basic Energy SciencesDirector, Scientific User Facilities Division

NGLS Program Manager

Berkeley Site Office

Site ManagerFederal Project Director

Lawrence Berkeley National Laboratory

Laboratory Director

Project Advisory Committee

NGLS Project Office

Project DirectorProject Manager

Deputy Project Manager

Accelerator Systems

Experimental Systems

LINACInjector FELs

ConventionalFacilities

Advisory Committees

ScienceMachine

ManagementConventional Facilities

Figure113 NGLS Organization.

Appendices

Appendix1

X-ray Interactions and Non-Disruptive Probing

SoftX-raysareanincisiveprobeofelectronicstruc-

ture.However,animportantrequirementisthattheprobe

pulsenotdisruptthesystemthatweseektounderstand.

Thisisequallytrueforsystemsinacorrelatedground

state,andforsystemspreparedinaperturbativenear-

equilibriumexcitedstatebyatailoredexcitationpulse.

TheX-rayprobeinteractionwiththematerialunderstudy

mustremaininthelinearregime,andthisplacesrestric-

tionsonthetolerablepulsefluenceforultrafastprobes.

NonlinearX-rayprobeinteractionmaybemanifestin

severalformsdependingontheexperimentaltechnique,

andonthematerialpropertiesunderinvestigation.For

example,photoelectronspectroscopyisoneofthemost

informativeprobesofelectronicstructure—reportingon

boththeenergyandmomentumoftheelectronicstates

of an ordered solid. However, at fluences above

~5x106ph/pulse(50µmspot)space-chargeeffectsdis-

tortthephotoelectronspectrumanddegradetheenergy

resolution.Thushighrepetitionrateatmoderatefluxper

pulseisessentialtoachievetherequiredcountratesfor

photoemissionspectroscopy.

Photon-inphoton-outtechniquescantoleratesomewhat

higherpulsefluenceandstillremaininthelinearinteraction

regime.Forexample,XESmeasurementsonSisamples

atFLASHindicateanacceptableupperpulsefluencelimit

ontheorderof10mJ/cm2.Recentresonantdiffraction

studiesofcharge-orderinginnickelatesamplesatLCLS

indicateasafeupperboundof~1mJ/cm2.Thesefluence

levelsareconsistentwithultrafastvisiblespectroscopy

researchoverthepastseveraldecades,wheretheimpor-

tanceofmaintainingalinearprobeinteractioniswell

established.For1keVphotonsandcharacteristicspotsizes

of50µm(e.g.forprobingheterogeneousmaterialseven

smallerfocalspotsmayberequired),thiscorrespondstoan

upperlimitontheusablefluenceof~108photonsperpulse.

However,photon-inphoton-outspectroscopy tech-

niquesarephotonhungry,owingtothesmallinelastic

cross-sections.The most demanding experiments at

3rdgenerationsoftX-raysynchrotronsourcesrequirean

averagefluxinexcessof~1012ph/s/(10meVbandwidth).

Inordertoachievetheseaveragefluxlevelswithsoft

X-raylasers(whilerestrictedtolessthan108ph/pulseas

describedabove),theymustoperateinthe10-100 kHz

regime.The most demanding and most informative

experimentsofthefuture,experimentsthatarepresently

wellbeyondourreach,willpushthisrequirementintothe

MHzregime,andwillrequireevenbetterenergyresolution.

Similarestimatesontheappropriatefluencelimitcan

bederivedbasedonsimpleconsiderationsoftheX-ray

absorptioncross-section,andthedepositedenergyper

atom.InthesoftX-rayrange(0.1-1keV),typicalatomic

absorptioncross-sectionsareontheorderof1018 cm2.

Ifoneconsiders109photons/pulseat1keV,ina30µm

focalspot(10-5cm2area),thiscorrespondsto1017eV/cm2

(16mJ/cm2),or0.1eVperatom.Thetypicalenergyofa

covalentbondis~1eVperatom,sothisfluencelevelis

sufficienttobreak~10%ofthecovalentbondsinamole-

culeorsolid.Thisinteractionlevelisfarfromlinearor

non-disruptive.

Fromanotherperspective, this fluence levelcorre-

spondsto~10%valencetoconductionbandexcitationin

a1eVgapsemiconductor.Thisisthenominalthreshold

atwhich“non-thermal”melting isknown tooccur in

semiconductors.Theseelectronicexcitationlevelsare

sufficientlyhightodirectlydestabilizethelattice.Based

ontheseconsiderations,theincidentfluenceperpulse

shouldbe~1 mJ/cm2or less inordertobesafely ina

linear interaction regime. High repetition rate will be

essentialtoprovidethehighaverageX-rayfluxrequired

bytheexperiments,whilekeepingthefluxperpulsesafely

inthelinearinteractionregime.

164

APPENDICES

Nanoscale Coherent Imaging

and Microscopy with a Soft

X-Ray Laser

Date:October16–17,2009

OrganizingCommittee:

JohnCorlett(LBNL),Robert

Schoenlein(LBNL)

Attendees: 102 from Lawrence Berkeley National

Laboratory, University of California-Berkeley, Davis,

LawrenceLivermoreNationalLaboratory,PacificNorthwest

NationalLaboratory,SLACNationalAcceleratorLaboratory,

Sandia National Laboratory, Heimholz Center Berlin

(Germany),DOEOfficeofScience,CanadianLightSource

Inc.(Canada),PrincetonUniversity,StonyBrookUniversity,

PrincetonUniversity,ArizonaStateUniversity,Stanford

University, University of Wisconsin-Milwaukee,

Brookhaven National Laboratory, Royal Holloway

UniversityofLondon(UK),McMasterUniversity(Canada),

SincrotroneTrieste(Italy)

Imaging and Defining

Function: Chemical Sciences

Drivers for Next Generation

Soft X-ray Light Sources

Date:November30–

December3,2009


OliverGessner(LBNL)


Laboratory, SLAC National Accelerator Laboratory,

Lawrence Livermore National Laboratory, Argonne

NationalLaboratory,UniversityofNebraska,Louisiana

StateUniversity,UniversitätKassel(Germany),Western

Michigan University, University of Arizona, Vienna

UniversityofTechnology(Austria),UniversityofHeidelberg

(Germany),KansasStateUniversity,BrownUniversity,

NorthwesternUniversity,WashingtonStateUniversity,

UniversityofColorado,PrincetonUniversity,ETHZurich

(Switzerland),USDepartmentofEnergy,Universityof

California-Berkeley, and Davis, Stanford University,

ImperialCollegeLondon(UK),NationalResearchCouncil

Canada,FrankfurtUniversity(Germany),TohokuUniversity

Appendix2–Workshops

The international user community has been fully

engagedindefiningthescience-basedrequirementsfor

anextgenerationlightsource.Aseriesofworkshopshave

been,andcontinuetobe,organizedatLBNLwiththe

goalsofrefiningtheserequirements,andunderstanding

newareasofsciencethatNGLSwillenable.Asummary

ofrecentworkshopsislistedbelow.

Toward Control of Matter:

Energy Science Needs for

a New Class of X- Ray

Light Sources



AliBelkacem(LBNL),John

Corlett(LBNL),RogerFalcone

(LBNL/UC Berkeley),Graham

Fleming(LBNL/UC Berkeley),

BillMcCurdy(LBNL/UC Davis),DanNeumark(LBNL/UC

Berkeley),RobertSchoenlein (LBNL)

Attendees:89fromArgonneNationalLaboratory,Arizona

StateUniversity,BESSY(Germany),BrookhavenNational

Laboratory, EPF Lausanne (Switzerland), ETH Zurich

(Switzerland),FrontierCollaborativeResearchCenter,High

Energy Accelerator Research Organization, Lawrence

BerkeleyNationalLaboratory,KansasStateUniversity,

Louisiana State University, University of Groningen

(Netherlands), University of Oregon, University of

Washington,UniversityofWisconsin-Madison,Universite

PierreetMarieCurie(France),WesternMichiganUniversity,

MaxPlanckInstitute(Germany),MassachusettsInstituteof

Technology,NationalResearchCouncil,OakRidgeNational

Laboratory, Ohio State University, Oxford University,

PaulScherrerInstitut(Switzerland),PrincetonUniversity,

Radboud University (Netherlands), Sandia National

Laboratory,SLACNationalAcceleratorLaboratory,Stanford

University,UniversityofCalifornia-Berkeley,Davis,Irvine,

Santa Barbara, University of Oregon, University of

Washington,UniversityofWisconsin-Madison.

165

APPENDICES

FEL Design Workshops

Workshop on X-Ray FEL R&D



JonathanWurtele (LBNL/UC

Berkeley),AlexanderZholents

(LBNL)



UniversityofOregon,UniversityNijmegen(Netherlands),

UniversityofWisconsin,ArgonneNationalLaboratory,

UniversityofIllinois,MaxPlanckResearchDepartment

(Germany),UniversityofHamburg(Germany),University

ofColorado,WashingtonStateUniversity,HongDing

InstituteofPhysics(China),ChineseAcademyofSciences

(China), Helmholtz-Zentrum Berlin (Germany),Tokyo

Institution ofTechnology (Japan),Tokyo Institute of

Technology(Japan),UniversityofCalifornia-Berkeley,

Davis,andSanDiego,LosAlamosNationalLaboratory,

StanfordUniversity

Compact X-Ray FELs Using

High-Brightness Beams

Date:August5–6,2010


JonathanWurtele(LBNL/UC

Berkeley),JohnCorlett(LBNL),

MarcoVenturini(LBNL)



ArgonneNationalLaboratory,UniversityofCalifornia-

Los Angeles, Naval Postgraduate School, RAND

Corporation,DaresburyLaboratory(UK),Universityof

Wisconsin, Cockroft Institute (UK), University of

Strathclyde,Glasgow(UK)

(Japan),MaxPlanckResearchDepartment (Germany),

UniversityofHamburg(Germany),AMOLF(Netherlands),

Helmholtz-Zentrum Berlin (Germany), University of

Hamburg(Germany)

Condensed Matter Science

for the Next Generation

Light Source

Date:May5–7,2010


RobertSchoenlein(LBNL),

ZahidHussain(LBNL),Robert

Kaindl(LBNL)



UniversityofOregon,UniversityNijmegen(Netherlands),

UniversityofWisconsin,ArgonneNationalLaboratory,

UniversityofIllinois,MaxPlanckResearchDepartment

(Germany),UniversityofHamburg(Germany),University

ofColorado,WashingtonStateUniversity,HongDing

InstituteofPhysics(China),ChineseAcademyofSciences

(China), Helmholtz-Zentrum Berlin (Germany),Tokyo

Institution ofTechnology (Japan),Tokyo Institute of

Technology(Japan),UniversityofCalifornia-Berkeley,

Davis, San Diego, Los Alamos National Laboratory,

StanfordUniversity

166

APPENDICES

Appendix3–ListofAcronyms

1D ............... one-dimensional2D ............... two-dimensional3D ............... three-dimensional4D ............... four-dimensional6D ............... six-dimensionalACP ............ amorphous calcium

phosphateADK............ adenylate kinaseAF ............... antiferromagneticAFM ........... atomic force microscopyAH .............. aromatic hydrocarbonALS ............ Advanced Light SourceAMOLF ...... Foundation for

Fundamental Research on Matter’s Institute for Atomic and Molecular Physics

APEX.......... Advanced Photoinjector EXperiment

APPES ....... Ambient Pressure Photoelectron Spectroscopy

APPLE........ Advanced Planar Polarized Light Emission

APS ............ Advanced Photon Source

ARPES ....... Angle-Resolved Photoemission Spectroscopy

ASTRA ....... A Space-charge TRacking Algorithm

ATP ............ adenosine triphosphateBCS ............ Bardeen-Cooper-

SchriefferBES ............ Basic Energy SciencesBESSY ....... Berliner

ElektronenSpeicherring-gesellschaft für SYnchrotronstrahlung

BNL ............ Brookhaven National Laboratory

BPM........... beam position monitorBW ............. bandwidthBZ............... Brillouin ZoneC-band....... “compromise” band CA .............. carbonic anhydraseCAD............ computer aided designCARS ......... Coherent Anti-Stokes

Raman SpectroscopyCCD ............ charge coupled deviceCD .............. circular dichroismCD .............. Critical DecisionCDR ............ Conceptual Design

Report

CEBAF ....... Continuous Electron Beam Accelerator Facility

CEP ............ carrier-envelope phaseCFEL ........... Center for Free Electron

Laser Science, DESYCMOS ........ complementary metal–

oxide semiconductorCMR ........... colossal

magnetoresistanceCNT ............ Classical Nucleation

TheoryCO-LIF........ carbon monoxide laser

induced fluorescenceCOLTRIMS COLd Target Recoil Ion

Momentum Spectroscopy

CPA ............ chirped pulse amplification

CRS .............. coherent (i.e., stimulated) Raman scattering

cryo-EM .... cryogenic Electron Microscopy

CSR ............ coherent synchrotron radiation

CT ............... charge transferCW ............. continuous waveCXDI........... Coherent X-ray

Diffractive ImagingDC .............. direct current (i.e., non-

oscillatory)DESY .......... Deutsches Elektronen

SYnchrotronDNA ........... deoxyribonucleic acidDNS ........... direct numerical

simulationDOE ............ Department of EnergyDPA ............ divided pulse

amplificationEEHG.......... Echo-Enabled Harmonic

GenerationEF-G ........... Elongation Factor-GEGR ............ exhaust gas

recirculationEIS.............. Environmental Impact

StatementELEGANT .. ELEctron Generation

ANd Tracking codeEM.............. electron microscopyEPAC .......... European Particle

Accelerator ConferenceEPFL ........... École polytechnique

fédérale de LausanneERL ............. energy recovery linacES&H ......... Environmental, Safety,

and HealthESRF .......... European Synchrotron

Radiation FacilityET ............... electron transferEUV ............ extreme ultraviolet

EXAFS........ Extended X-ray Absorption Fine Structure

FEL ............. free electron laserFERMI ........ Free Electron Laser for

Multidisciplinary Investigations

FET ............. field effect transistorFHA ............ Fire Hazards AnalysisFLASH........ Free Electron Laser in

HamburgFM .............. ferromagneticFODO ......... FOcusing-DefOcusingFROG.......... Frequency Resolved

Optical GatingFTIR............ Fourier Transform

InfraRed spectroscopyFWHM ....... full width at half

maximumfXS ............. fluctuation X-ray

scatteringFY ............... fluorescence yieldFY ............... Fiscal YearGTP ............ guanosine triphosphateHAP............ hydroxyapatiteHGHG......... High Gain Harmonic

GenerationHHG ........... High Harmonic

GenerationHOM .......... higher-order modeHOMO........ Highest Occupied

Molecular OrbitalHTC ............ high temperature

superconductorIC ................ integrated circuitID................ insertion deviceILC .............. International Linear

ColliderIMPACT ..... Integrated-Map and

Particle ACcelerator Tracking code

INFN .......... Instituto Nazionale di Fisica Nucleare

IOT ............. Inductive Output TubeIR ................ infraredISR ............. incoherent synchrotron

radiationIXS ............. Inelastic X-ray

ScatteringJLAB .......... Thomas Jefferson

National Accelerator Facility (Jefferson Lab)

KEK ............ Kō Enerugī Kasokuki kenkyū kikō (High Energy Research Organization)

L-band ....... “long” waveLASA.......... Laboratorio Acceleratori

e Superconduttività Applicata (MIlano)

laser........... Light Amplification by Stimulated Emission of Radiation

LBNL .......... Lawrence Berkeley National Laboratory

LCLS........... Linac Coherent Light Source

LES ............. large eddy simulationLIF .............. Laser Induced

FluorescenceLINAC ........ LINear ACceleratorLLNL........... Lawrence Livermore

National LaboratoryLLRF ........... low-level radio-

frequencyLSC............. Longitudinal Space

ChargeLUMO ........ lowest unoccupied

molecular orbitalMAX........... National Electron

Accelerator Laboratory for Synchrotron Radiation Research (Sweden)

MBE ........... molecular beam epitaxyMD ............. molecular dynamicsMOSFET .... metal-oxide-

semiconductor field-effect transistor

mRNA ........ messenger RNAMtrC .......... outer membrane

decaheme cytochrome c lipoprotein

NA .............. numerical apertureNEG............ non-evaporable getterNEPA ......... National Environmental

Policy ActNESHAPs .. National Emission

Standard for Hazardous Air Pollutants

NEXAFS..... Near Edge X-ray Absorption Fine Structure

NGLS ......... Next Generation Light Source

NIF ............. National Ignition FacilityNIR ............. near-infraredNLC ............ Next Linear ColliderNLS ............ Next Light SourceNM ............. non-magnetic metalNMR .......... nuclear magnetic

resonanceNSF ............ National Science

FoundationNSLS.......... National Synchrotron

Light SourceOCP ............ octacalcium phosphateOEC ............ Oxygen Evolving

Complex

167

APPENDICES

OmcA......... outer membrane decaheme cytochrome

OO .............. orbital orderOPC ............ Other Project CostsOTR ............ optical transition

radiationPAC ............ Particle Accelerator

SchoolPAH ............ polycyclic aromatic

hydrocarbonPEEM ......... PhotoEmission Electron

MicroscopyPES ............ PhotoElectron

SpectroscopyPGA............ phosphoglyceratePITZ ........... Photo Injector Test

Facility – ZeuthenPS............... photosystemQE............... quantum efficiencyqp ............... quasiparticleR&D ........... Research &

Developmentredox ......... oxidation-reductionRF ............... radio-frequencyRIXS ........... Resonant Inelastic X-ray

ScatteringRMS ........... root-mean-squareRNA ........... ribonucleic acidRST ............ Reference Structure

TomographyRT ............... room temperatureRuBisCo .... ribulose

1,5-bisphosphate carboxylase oxygenase

RuBP.......... ribulose 1,5-bisphosphate

S-band....... “short” waveSASE.......... Self-Amplified

Spontaneous EmissionSAXS ......... Small-Angle X-ray

ScatteringSBA............ “Santa Barbara”

mesoporous silicateSC............... superconductingSCRF .......... superconducting

radio-frequencySCSS.......... Spring-8 Compact SASE

SourceSFL ............. Stabilized optical Fiber

LinkSHG............ second harmonic

generationSLAC .......... SLAC National

Accelerator LaboratorySMD........... single molecule

diffractionSNS............ Spallation Neutron

SourceSONICC ..... Second-Order Nonlinear

optical Imaging of Chiral Crystals

SPARC ....... Sorgente Pulsato Auto-amplificata di Radiazione Coerente

SPIM.......... Selective Plane Imaging Microscopy

SPring-8 .... Super Photon Ring – 8 GeV

SR............... storage ring

SRS ............ Spontaneous Raman scattering

STM ........... scanning tunneling microscopy

SXPCS ....... Soft X-ray Photon Correlation Spectroscopy

SXR ............ soft X-rayTEAM......... Transmission Electron

Aberration-corrected Microscope

TEC............. Total Estimated CostTEM ........... transmission electron

microscopyTESLA ........ Tera-Electron-volt

Superconducting Linear Accelerator

TEY ............. total electron yieldTJNAF ....... Thomas Jefferson

National Accelerator Facility

TMV ........... Tobacco Mosaic VirusTOF............. time-of-flightTPC ............ Total Project CostTR ............... time-resolvedTR- XAS ..... Time-Resolved X-ray

Absorption Spectroscopy

tRNA .......... transfer RNATRPES........ Time-Resolved

PhotoElectron Spectroscopy

TRS ............ time-reversal symmetryUHV............ ultra-high vacuumUK .............. United Kingdom

URA............ uniformly redundant array

USA............ United States of AmericaUV .............. ultravioletVHF ............ very high frequencyVLS............. variable line spacingVUV ............ vacuum ultravioletWAXS ........ Wide-Angle X-ray

ScatteringX-band....... “cross” bandXANES....... X-ray Absorption Near

Edge StructureXAS ............ X-ray Absorption

SpectroscopyXCARS ....... X-ray Coherent Anti-

Stokes Raman Spectroscopy

XES ............ X-ray Emission Spectroscopy

XFEL ........... (European) X-ray Free Electron Laser

XFROG ....... Cross-Correlated Frequency Resolved Optical Gating

XMCD ........ X-Ray Magnetic Circular Dichroism

XPCS.......... X-ray Photon Correlation Spectroscopy

XPS ............ X-ray Photoelectron Spectroscopy

XUV ............ eXtreme UltraViolet, or X-ray UltraViolet

YAG ............ Yttrium Aluminum GarnetZ ................. atomic number

(TOC continued)

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