Photon Beamlines and Diagnostics 2010

Meeting Summary

Optics• To preserve the coherent wavefront of the FEL pulse• To achieve diffraction limited focii

– Limit or control the phase error– Need nm level height errors

• Metrology «» Manufacturing– Manufacturer and end-user collaboration

• Classical polishing– Spheres with <0.2 µrad slope errors

• Deterministic figuring– IBF, EEM

• Large substrates– Fast figuring - ELID– Fast metrology - stitching interferometry for 2-D maps

Optics “beyond the mirror”• Diamond optics for use in an X-Ray cavity

FEL– Need “perfect” crystals– 4 needed for tunable FEL– Demonstrated the required 99% reflectivity is

achievable– Feedback control to stabilise angle also proven– Cryo cooling to reduce heat deformation

• Proven technologies, no reason XFELO should not work

Optics “beyond the mirror”• Multilayers

– Large NA in the hard X-ray– Improved reflectivity– More convenient geometry– Delay lines– Smaller optics– Spectral filtering (isolate harmonic)– Beam splitters– BW match to FEL pulse

• Damage (Mo:Si)– Thermal in nature - diffusion– Use experience with EUV lithography

Optics / Metrology• There is a limit to what can be made by direct figuring• Change the shape with a bender

– Allow more extreme shapes– Allow change in focal length

• Be clever when setting up the bender– Measure response function of each actuator - derive

response matrix• Can work out which actuator to use to correct residual errors, shift

FL etc. Saves a lot of time!– Accuracy depends on quality of original measurements

• More accurate metrology, lower systematic errors

Optics / Metrology• Why stop at just defining the basic mirror shape with

a bender?– Add actuators along mirror surface to give control to

smaller spatial frequencies– Deterministic control

• Find response matrix with metrology • Dial in required figure• More degrees of freedom = better figure accuracy

– Adaptive control• Phase compensator• Deform mirror to correct for errors in a downstream mirror• In situ interferometry

Metrology• Profilers - the end-users favourite

– Accuracy 0.05 µrad for flats (10x improvement)• Known through comparative measurements to

ESAD (Deutsch national standard)– 0.1 to 0.2 µrad for spheres

• Systematic errors more problematic– Characterize with angle comparator– Not the best choice for the manufacturer (too

slow)

Metrology / Diagnostics• Hartmann wavefront sensors

– Developed technology for XUV and SXR• Commercially available• Simple Hartmann plate (no micro lenses)• Calibrate with spherical wavefront from pin hole

– HXR more difficult - convert to optical • Calibration more critical (imaging optics) and

harder to achieve (can’t make spherical wavefront) – Measuring beam wavefront allows

determination of source properties

Metrology / Diagnostics• Grating interferometry

– At-wavelength metrology of optics with 10 nrad sensitivity

– Source characterisation• Longitudinal position to 1 m

– Shot-by-shot potential– Facilitated by advanced nanofabrication

techniques• But an instrumentally simple and robust diagnostic

Photon Beam Transport• Time preserving monochromators

– A diffraction grating stretches a pulse– Double grating systems

• Complete reversal of pulse stretch• Complicated, reduced efficiency

– Conical diffraction• Pulse stretch down to few fs • The only practical single grating solution in VUV to XUV• Working example on HHG source (RAL)

– Classical diffraction mount (match to FEL source in SXR)• Also few fs pulse stretch• Conceptual only• Needs model of FEL source• >~250 eV only

Photon Beam Transport• Multi-beam experiments (TIMER)

– Beam splitters• Wavefront (knife-edged mirror)• Harmonic (multilayers)

– Delay lines• Multilayer

– Multiple mirrors to vary angles– Three beams coincident in time and space at

the experiment, with defined angles and energies

Photon Beam Transport• HHG Seeding

– Essential for single mode SXR FELs (?)– Successful demonstration at SCSS Test FEL and

SPARC– As demanding as a user beamline

• Focus and spatial overlap with electron beam– Steering and focusing mirrors

• Temporal overlap• Diagnostics

– Spectrometer, position & angle

SXR Beamlines• Use with a FEL source

– Reduce the bandwidth of a SASE pulse with many longitudinal modes

– Remove the background of spontaneous emission– Remove SASE emission outside the spectral bandwidth

of a seeded pulse– Define the photon energy reaching the experiment

(eliminate energy jitter)– Reduce the intensity of the higher harmonics– Remove the fundamental when working on the

harmonics– Experiments requiring HSRP

SXR Beamlines• Design challenges

– 1st mirror distance / mirror lengths– Coatings (conflicting edges)– Carbon K-edge– Mirror damage (what is safe?)– Grating damage (effect of structure?)– Transport efficiency– Pulse stretch– Diffraction limited focusing (quality, aperture)

Photon Diagnostics• Experiments on FELs demand a wide range

of diagnostics of the photon beam– Pulse energy– Pulse length– Pulse spectrum– Timing jitter– Polarisation– Beam position and angle– Etc

• Non-invasive and every pulse

Photon Diagnostics• Timing and pulse length

– Streak camera• Over come limits of camera with single photon

counting and in-camera processing• Achieved 280 fs FWHM resolution• Single shot, 4.25 kHz

Photon Diagnostics• Pulse spectrum

– Grating spectrometer• On-line (minimally invasive)• Pulse by pulse analysis (detector ltd)

• Photon energy– eTOF and iTOF

• Central wavelength, modes, harmonics• On-line, non-invasive• Robust and high accuracy

Photon Diagnostics• Generation of CP in a FEL with mixed planar

and helical ID’s means measuring the DOP is essential

• Multilayer-based Polarimetry– Broad-band in SXR– Full Stokes vector– High accuracy, proven– Slow (inherently time averaged)– Needs very high quality multilayers

Photon Diagnostics• Gas-based polarimetry

– Angle resolved photo electron spectrometer using TOF

• Broad-band XUV and SXR• Full Stokes vector (need coincidence for CP)• Requires data base of gas yields• Can be shot by shot (if good cross-section)• Complicated instrument (16 TOFs)

– Also measures photon energy, photon flux and beam position

• A universal, in situ, pulse-by-pulse diagnostic

Global Perspective• Japan

– Success of SCSS Test FEL &rapid progress towards SCSS operation

– Common themes• Shot by shot diagnostics• Diffraction limited focusing• Polarization control• Pulse duration, delay and synchronisation

– X-ray autocorrelator using thin Si Bragg beam splitters and channel-cut Si for delay line

– Spectrum analysis• Crystal spectrometer• Compton back-scattering from a diamond crystal

Global Perspective• USA

– Stunningly rapid commissioning of LCLS• Excellent performance already achieved

– Huge user demand • Proof of the importance of FEL sources

– Extensive use of B4C coatings on mirrors• Have seen some carbon deposits

– Absorber uses N2 gas and Be foils• Up to 7 Torr• Measure fluorescence of nitrogen• See speckle from Be

– Gas detector compared to DESY GMD– LUSI diagnostic suite for HXR experiments

Global Perspective• Europe

– FLASH II– Huge over-subscription of FLASH has to be addressed– Complementary to FLASH– Seeding to go beyond the limits of FLASH SASE

• Timing stability• Frequency stability• Single mode• Polarisation control

– Details still being worked out

Global Perspective• EuroFEL

– Preparatory phase now been running for 2 years– Created a vibrant community of experts with

• A diverse range of expertise• But a common interest in delivering photons from a FEL source to

a users experiment– Outcomes

• Exchange ideas, experiences and knowledge• R & D collaborations• Faster solutions to common problems

– Is it useful? I invite you to draw your own conclusion:• This meeting has demonstrated Global interest in FEL sources and

common problems that face all the sources• Does anyone think they can solve every problem on their own?

Open Forum

“gardener’s question time”

Closing Remarks• Thanks to the organisers• Thanks to the speakers• Thanks to all the participants for making

the meeting a success• Reminder to submit your papers