Wir schaffen Wissen – heute für morgen Vienna, 08.05.2014 Paul Scherrer Institut Profile Measurements at Light Sources and FELs Volker Schlott

Wir schaffen Wissen – heute für morgen

Vienna, 08.05.2014Vienna, 08.05.2014

Paul Scherrer Institut

Profile Measurements at Light Sources and FELs

Volker Schlott

Overview

• Synchrotron Radiation Monitors for Light Sources

• Synchrotron Radiation Monitors for FELs

• Screen Monitors for FELs

• Wire Scanners for FELs

• Summary and Outlook

3rd Topical oPAC Workshop on Beam Diagnostics, May 8 th - 9th, 2014Volker Schlott,

Profile Measurements for Light Sources and FELs

Acknowledgements



…most of the material presented in this overview relies on the outstanding work of many colleaguesfrom various accelerator facilities and has been taken from their presentations and publications

…for their support in discussing the topics, which are presented, and for the provision of informationmaterial and measurement results, I would like to explicitly thank the following colleagues…:

Angela Saa-Hernandez (PSI)

Andreas Streun (PSI)

Matsamitsu Aiba (PSI)

Michael Böge (PSI)

Rasmus Ischebeck (PSI)

Gian Luca Orlandi (PSI)

Gero Kube (DESY)

Minjie Yan (DESY)

Karsten Holldack (HZB – BESSY)

Ake Andersson (MaxLab)

Toshiyuki Mitsuhashi (KEK)

Alan Fischer (SLAC)

Henrik Loos (SLAC) and many more…!!!

…this presentation is far from being complete ! It tries to give an overview of state-of-the-art systemswith a number of – hopefully instructive – examples and (latest) measurements

…important issues e.g. about the design and resolution limitations of optical systems had to be left out!

Motivation for Profile Measurements at SR Light Sources ISpectral Brilliance B is one of the key parameters for SR light sources



yyxx

photons

yx

beamNI

B

~~

horizontal beam size

x x x x 2

yxyx ,,

for „flat lattices“ (low coupling) with hy ≈ 0

y y y

use locations for sx meas. with dispersion hx = 0b-functions & dispersion are well known in SR light sources (storage rings)…

→ ex,y can be determined from beam sizes sx,y

Number of Photonssec mm2 mrad2 0.1% BW

vertical beam size

beam divergence

horizontal beam size usually determined by storage ring lattice

vertical beam size is typically minimized by reducing coupling from horizontal to vertical plane

coupling of 0.1 to < 0.01% leads to vertical beam sizes of < 10 µm to a few µm (rms)

Horizontal and Vertical Emittances of Storage Rings

existing () and planned ()

Figure taken from:

R. Bartolini, Low Emittance Ring Design, ICFA Beam Dynamics Newsletter, No. 57, Chapter 3.1, 2012 – and updated.

Motivation for Profile Measurements at SR Light Sources IICoupling Correction @ SLS...:



Iterative Minimization Procedure → BPM roll error corrections → beam-based girder alignment

→ dispersion & coupling corrections → beam size monitor tuning → random walk optimization

Illustration of SLS Beam Size (short ID straight – 2s)

courtesy of A. Streun M. Aiba, et al., Ultra Low Vertical Emittance at SLS Through

Systematic and Random Optimization, NIM-A 694 (2012) 133-139

Properties of Bending Magnet SR as a Diagnostics Tool



SR is non-invasive and “freely available” at light sources

SR covers a wide spectral range from visible to hard X-rays

SR properties & transport are exactly computable (e.g. SRW)

SR is strongly collimated in the vertical plane → but usable opening angle* depends on wavelength

SR main power (heat load) at small opening angle* → (hard) X-ray optical elements require water cooling

SR is emitted with p and s polarization

hard X-ray (@ lc ) opening angle: DY ~ 1/g

opening angle in visible: 3

1

1

c

*

Peculiarities when Imaging with Synchrotron Radiation



The object to be imaged is its own source, emitting SR in the forward directiontangential to the circular beam path of the electrons in a bending magnet

SR generates a narrow forward directed cone in the vertical directionand a stripe of light along the mid-plane in the horizontal direction

Imaging situation is similar to a telescope → F-number should be large: F = f / d ≈ 5 m / 30 mm ≈ 165 (SLS case @ 400 nm)

→ Airy-disk radius: rA = 1.22 F ∙ l ≈ 80 µm

spatial resolution limit due to diffraction, when imaging at visible wavelengths

my 802

SLS case: for l = 400 nm and DY = 2.5 mrad

resolution improvements by…: imaging at shorter wavelengths (UV or X-rays)

X-ray pinhole camera

interferometric techniques (UV and/or visible)

Remarks about SR Imaging Systems: Optics, Cameras and Utilities



CCD or CMOS cameras are typically used as 2-D sensors / detectors → sufficient sensitivity (high QE), low read-out noise (cooling) and good linearity (visible and UV)

→ data acquisition rates of several kHz, data transfer rates of several hundred Hz

→ small pixel sizes < 5 µm and large number of pixels with RoI selection

SR transfer from X-rays to visible by phosphors (e.g. P43) or YAG:Ce crystals → grain size phosphors and thickness of YAG:Ce crystal might limit resolution

→ saturation effects avoided by attenuation with filters (e.g. Al or Mo for X-rays, NDF for visible)

Imaging quality, measurement resolution and accuracy depends on…: → sufficient magnification of imaging system (e.g. large number of line pairs per mm)

→ sensor / detector size should be ≥ 3 σ of the imaged object

→ sufficiently small pixel sizes and large number of points (pixel) for beam size fit

→ low background noise level and good pointing stability (mech. and thermal stability of set-up)

→ knowledge of optics set-up (e.g. experimental determination of point spread function)

in many cases also applicable to screen monitors

Synchrotron Radiation Monitors: X-Ray Pinhole Camera

courtesy of K.Scheidt, ESRF

P.Elleaume, C.Fortgang, C.Penel and E.Tarazona, J.Synchrotron Rad. 2 (1995) , 209

ESRF ID-25 X-Ray Pinhole Camera

X-ray pinhole camera resolution is limited by:

blurr w L1 L2

12L1

diff 12

4

L2

w

... blurring

... diffraction

Example from ALBA, U. Iriso et al., EPAC 2006

with L1 = 6 mL2 = 12 ml = 12 nm (17 keV)

→ w = 20 mm

typical resolution limitation of x-ray pinhole cameras ~ 10 mm

Example: PETRA III Pinhole CameraØ 18 μm hole in 500 μm thick Tungsten plate

courtesy of G. Kube, DESY



Synchrotron Radiation Monitors: X-Ray Pinhole Array

W.B. Peatman, K. Holldack, J.Synchrotron Rad. (1998) 5, 639-641

BESSY II X-Ray Pinhole Array

diffraction limited resolution: ~ 11 mm

simultaneous measurement of...: → beam size through single pinhole image

→ beam divergence through envelope



IBIC-2012, Tsukuba, October 2nd, 2012Volker Schlott,

Design and Expected Performance of the New SLS Emittance Monitor

T. Mitsuhashi, Spatial Coherency of the SR at the Visible Light Region and its Application for Electron Beam Profile Measurement, Proc. PAC 1997, Vancouver, p. 766

• double slit Michelson interferometer adapted for beam size measurements by Toshi Mitsuhashi• van Citert-Zernike’s theorem relates transverse distribution f(y) via FFT with spatial coherence g(y)

I y0,D I1 I2 sinc a D

Ry0

1 cos

2D

y0

R

Intensity of Interference Pattern

→ spatial coherence

2 I1I2I1 I2

Imax Imin

Imax Imin

provides rms beam size

R D

1

2ln

1

Synchrotron Radiation Monitors: Principle of Interference Monitors

Synchrotron Radiation Monitors: ATF Interference Monitor (with Mirror Optics)

T. Naito and T. Mitsuhashi, Phys. Rev. ST Accel. Beams 9 (2006) 122802

Schematic Set-Up of ATF Interference Beam Size Monitor

Example of an Interferogram Beam Size vs. Shutter Time and Fit of Interferogram

sy = 4.73 ± 0.55 mm

minimal measured beam size:



Synchrotron Radiation Monitors – Principle of the p-Polarization Method Å. Andersson, et al., Determination of Small Vertical Electron Beam Profile and Emittance at the Swiss Light Source, NIM-A 592 (2008) 437-446

• imaging of vertically polarized SR in the visible / UV

• phase shift of p between two radiation lobes → destructive interference in the mid plane → Iy=0 = 0 in FBSF (filament beam spread function)

• finite vertical beam size → Iy=0 > 0 in FBSF• fringe visibility depends on vertical beam size σy

• modeling by SRW* (Synchrotron Radiation Workshop)

E x,y E 0 sinc2xc p, x

12 1 2K1 3

1

2

c

1 3 2

0

sin2pp, y

d

2-D Electric Field Distribution (in image plane)

2-D Intensity Distribution (in image plane)

I x,y ~ sinc 2 x cos 1

2

with 2 y

O. Chubar & P. Elleaume, Accurate and Efficient Computation ofSynchrotron Radiation in the Near Field Region, EPAC 1998



Synchrotron Radiation Monitors: SLS p-Polarization Monitor



• operating wavelength: variable (266 nm)• opening angle: 7 mradH x 9 mrad V

• finger absorber to block main SR intensity

• imaging by toroidal mirror• magnification: 1.453• surface quality of optics: < 20 nm (l/30 @ 633 nm)

• p-polarization or interferometric method selectable

calibration & alignment

SLS: vertical beam size sy = 3.6 µm ± 0.6 µm for by = 13.5 m → vertical ey = 0.9 pm (natural limit from 1/g: ey,min = 0.2 pm)

Not Treated Here: Imaging SR Monitors with X-Ray (Focusing) OpticsReflective Optics:→ Kirkpatrick-Baez mirror scheme of grazing incidence (q < 0.5°) with pair of ellipsoidal / cylindrical curved mirrors

Example: Advanced Light Source Diagnostics Beam Line

T.R. Renner, H.A. Padmore, R. Keller, Rev. Sci. Instrum. 67 (1996) 3368

Diffractive Optics:→ Fresnel Zone Plates: spacing of rings (e.g. Si, Au) result in constructive interference of light waves in focal point

Examples: X-Ray Beam Imager at Spring-8

S. Takano, M. Masaki, H. Ohkuma, Proc. DIPAC05, Lyon, France (2005) 241 and NIM A556 (2006) 357

Fresnel Zone Plate Monitor at ATF (KEK)

K. Ida et al., NIM A506 (2003) 49 and H. Sakai et al., Phys. Rev. ST Accel. Beams 10 (2007) 042801

Refractive Optics:→ many (30 – 100) Compound Refractive Lenses made from Al or Be for focusing hard X-ray radiation (20 keV)

Example: PETRA III Diagnostics Beam Line for Emittance Measurements

G. Kube et al., Proc. IPAC‘10, Kyoto, Japan (2010), MOPD089, 909



Coded Aperture X-ray Monitor:→ pseudo-random array of pinholes projects a mosaic of pinhole camera images onto a detector (from x-ray astronomy)

Example: X-ray Monitor at ATF-2 Extraction Line (KEK)

J.W. Flanagan, M. Arinaga, H. Fukuma, H. Ikeda, T. Mitsuhashi, Proc. IBIC’12, Tsukuba, Japan (2012) 237

…and possibly many more!

Excursion: Bunch Compressor Synchrotron Radiation Monitors in FELs



Energy Spread Measurements with SITF BC SR Monitor Optics Set-Up of SITF BC SR-Monitor

SwissFEL Test Injector BC Layoutmovable bunch compressor chicane

One Motivation for Transverse Profile Measurements in FELs

High Electron Beam Density is required for best FEL performance



gain length with FEL parameter

and efficient energy transfer from electron beam to photon beam

depends (among others) on transverse beam sizes and normalized emittances

LCLS Example: Gain Length vs. Emittance E-XFEL SASE-1: Saturation Length vs. Emittance and Wavelength

D.H. Dowell, et al., LCLS Drive Laser Shaping Experiments, Proc. FEL’09 463

R. Brinkmann, et al., Possible operation of the European XFEL with low emittance beams, NIM-A 616 (2010) 81-87

SwissFEL ARAMIS: Gain Length vs. Emittance

courtesy of Sven Reiche

Transverse Profile Measurements in Free Electron Lasers



Non-invasive SR monitors can only be used in chicanes (e.g. BCs, switchyards, collimators)

→ use screen monitors (2D, destructive) and/or wire scanners (1D, partially destructive)

OTR or scintillator screens are used in diagnostics sections and for matching control

→ sliced and projected emittance and energy spread measurements

wire scanners might be used for online beam size / emittance monitoring in LINACs

Typical transverse beam profiles to be measured in FELs (e.g. SITF, PSI)

…and a comparison

to the «real world»

Scintillator Screens as 2D Transverse Profile Monitors



electronbeam

scintillator

camera

Schematic Set-Up and Main Properties of Scintillators as Screen Monitors

electrons passing the scintillator crystal excite atoms and molecules

multiple scattering in scintillator crystal increases beam divergence

visible light from scintillator crystal is radiated in 4p

photons are created along the beam pass through scintillator crystal

thickness of scintillator crystal and observation angle affect resolution

scintillator crystals are very sensitive and radiation resistant

Study of Scintillator Crystals as Transverse Profile Monitors (Gero Kube et al. @ MAMI, Mainz)

Horizontal and Vertical Beam Sizes for Different Scintillator Crystals & Thicknesses Horizontal and Vertical Beam Sizes in BGO for Different Observation Angles

G. Kube, et al., Resolution Studies of Inorganic Scintillation Screens for High Energy and High Brilliance Electron Beams , Proc. IPAC 2010, Kyoto, Japan, 906

Scintillator Properties see e.g.: http://scintillator.lbl.gov/ or http://www.crytur.cz/pages/33/scintillation-materials-data

http://scintillator.lbl.gov/

http://scintillator.lbl.gov/

http://www.crytur.cz/pages/33/scintillation-materials-data

http://www.crytur.cz/pages/33/scintillation-materials-data

Optical Transition Radiators as 2D Transverse Profile Monitors



Schematic Set-Up and Main Properties of OTR as Screen Monitors

OTR is generated when relativistic charged particles pass the boundary of two media with different dielectric (optical) properties

incoherent Optical TR provides good linearity for profile measurements

OTR is radiated in forward and backward direction with an angle of 1/g

surface quality of OTR screen affects profile imaging quality (beam size)

OTR is widely used as beam profile monitors in LINACs

OTR screen could be thin metal foil or silicon wafer (with Al layer)

electronbeam

OTR foil

camera

Q = 1/g

backwardOTR

forwardOTR

H. Loos, et al., Observation of Coherent Optical Transition Radiation in the LCLS LINAC, SLAC-PUB-13395, September 2008

BUT…: Coherent OTR has been observed for highly highly brilliant electron beams

Coherent OTR images from LCLS showing full saturation of camera Coherent OTR was first observed at LCLS

SACLA and FLASH validated COTR observations

LCLS: Profile Measurements with Wire Scannersbut only 1D…

COTR Suppression: Temporal Separation with Scintillator & Gated CCD



OTR is an instantaneous process

Scintillation has a “long” decay time

Coherent OTR occurs in case of micro-bunching

Scintillation is insensitive to micro-bunchingelectron buncharrives at screen

t0

time

OTR: t ~ fs - ps

scintillation: t ~ 100 ns

ICCD gating time

COTR Mitigation Tests @ FLASH using Scintillator and ICCD (Minjie Yan et al. @ FLASH, Hamburg)

no delay at ICCDCOTR and CSR on OTR screenCOTR and CSR on scintillator (LuAG screen)

100 ns delay at ICCDno signal from OTR screenScintillation light only from LuAG screen

M. Yan et al., Suppression of Coherent Optical Transition Radiation in Transverse Beam Diagnostics by Utilizing a Scintillation Screen with a Fast Gated CCD Camera Proc. DIPAC 2011, Hamburg, 440

COTR Suppression: Spatial Separation – Central Mask in Imaging System



OTR is emitted at an angle Q ~ 1/g

scintillation light is emitted in 4p

at beam energies > 1 GeV (e.g. SACLA BC-3): Q < 0.5 mrad

central mask in imaging system successfully suppresses COTR intensity

(C)OTR emission for small (10 µm) beam sizes at Q ~ l/2s ≈ 100 mrad

SACLA COTR image behind BC-3

S. Matsubara et al., Improvement of Screen Monitor with Suppression of Coherent OTR Effect for SACLA, Proc. IBIC 2012, Tsukuba, Japan, 34

COTR Mitigation @ SACLA using Scintillator and Spatial Mask (S. Matsubara et al., Spring8, Japan)

Image of vertically focused beam behind SACLA BC-3 (full compression)

COTR Suppression: Spatial Separation – SwissFEL Profile Monitors



entire screen (large RoI) can be observed without depth-of-field issues by following Scheimpflug imaging principle

observation of beam profile according to Snell’s law of refraction

detector (CMOS sensor) is tilted by 15° for 1:1 imaging to avoid astigmatism

beams can be imaged, which are smaller than scintillator thickness

use YAG or LuAG scintillator crystals instead of OTR

SwissFEL SCM resolution test with 10 pC

rms beam size: 8 µm

Coherent OTR Measurements with SwissFEL SCM at LCLSto be published by R. Ischebeck et al. at IBIC 2014, Monterey, USA

COTR suppression tests at LCLS (full compression, 20 pC)

COTR Mitigation for SwissFEL Screen Monitors (R. Ischebeck et al., PSI, Switzerland)

SwissFEL Screen Monitor Designpatent pending

Summary



SR Monitors are used for non-invasive transverse profile measurements at light sources

Different SR Monitor types cover wide spectral ranges from the visible to hard X-rays

Spatial resolutions in the order of a few µm have been achieved for vertical beam sizes

High resolution SR monitors are required for “next generation light sources” where low emittance lattices (few 100 pmrad) and low coupling (< 0.01 %) in higher brilliances

Screen Monitors provide 2D transverse profile information in FELs (LINACs) → determine projected & “sliced” emittances and energy spread → optimize matching into LINACs, transfer and undulator lines

µm resolutions have been achieved even for low beam charges (few pC)

Coherent OTR from highly brilliant beams or beam with micro-bunching structures

Solutions for COTR suppression have been worked out and presented: → temporal separation (gated ICCD) → spatial separation (spatial mask or special observation geometries) → wire scanners as 1D profile monitors have not been presented due to limited time



Thank you for your attention…!!!

…and I hope you feel motivated to continue working on these beam dynamics and diagnostics issues for a an even «brighter» future of SR Light Sources and FELs

Further Reading / Information…:

• US Particle Accelerator Schools on Beam Diagnostics using Synchrotron Radiation (2008 and 2010) http://uspas.fnal.gov/materials/08UCSC/UCSC_BeamDiagn.shtml http://uspas.fnal.gov/materials/10UCSC/UCSC_BeamDiagnostics.shtml

• CERN School on Beam Diagnostics (2008) https://cas.web.cern.ch/cas/France-2008/Lectures/Dourdan-lectures.htm

• Workshop on Scintillating Screen Applications in Beam Diagnostics http://www-bd.gsi.de/ssabd/proceedings.htm

http://uspas.fnal.gov/materials/08UCSC/UCSC_BeamDiagn.shtml

http://uspas.fnal.gov/materials/10UCSC/UCSC_BeamDiagnostics.shtml

https://cas.web.cern.ch/cas/France-2008/Lectures/Dourdan-lectures.htm

http://www-bd.gsi.de/ssabd/proceedings.htm

Documents

Wir schaffen Wissen – heute für morgen Vienna, 08.05.2014 Paul Scherrer Institut Profile Measurements at Light Sources and FELs Volker Schlott