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Paul Scherrer Institut

Paul Scherrer Institut. Volker Schlott. Profile Measurements at Light Sources and FELs. Overview. Profile Measurements for Light Sources and FELs. Synchrotron Radiation Monitors for Light Sources Synchrotron Radiation Monitors for FELs Screen Monitors for FELs Wire Scanners for FELs

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Paul Scherrer Institut

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  1. Paul Scherrer Institut Volker Schlott Profile Measurements at Light Sources and FELs

  2. Overview Profile Measurements for Light Sources and FELs • Synchrotron Radiation Monitors for Light Sources • Synchrotron Radiation Monitors for FELs • Screen Monitors for FELs • Wire Scanners for FELs • Summary and Outlook Volker Schlott,

  3. Acknowledgements Profile Measurements for Light Sources and FELs … this presentation is far from being complete !It tries to give an overview of state-of-the-art systems with 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! …most of the material presented in this overview relies on the outstanding work of many colleagues from 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 information material 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…!!! Volker Schlott,

  4. Motivation for Profile Measurements at SR Light Sources I Profile Measurements for Light Sources and FELs Spectral Brilliance Bis one of the key parameters for SR light sources Horizontal and Vertical Emittances of Storage Rings Number of Photons sec mm2 mrad2 0.1% BW • horizontal beam size usually determined by • storage ring lattice • vertical beam size is typically minimized by • reducing coupling from horizontal to vertical plane existing () and planned () Figure taken from: R. Bartolini, Low EmittanceRing Design, ICFA Beam Dynamics Newsletter, No. 57, Chapter 3.1, 2012 – and updated. • coupling of 0.1 to < 0.01% leads to • vertical beam sizes of < 10 µm to a few µm (rms) for „flat lattices“ (low coupling) with hy ≈ 0 • horizontal beam size b-functions & dispersion are well known in SR light sources (storage rings)… → ex,ycan be determined from beam sizes sx,y use locations for sx meas. with dispersion hx= 0 • vertical beam size • beam divergence Volker Schlott,

  5. Motivation for Profile Measurements at SR Light Sources II Illustration of SLS Beam Size (short ID straight – 2s) courtesy of A. Streun Profile Measurements for Light Sources and FELs Coupling Correction @ SLS...: Iterative Minimization Procedure → BPM roll error corrections → beam-based girder alignment → dispersion & coupling corrections → beam size monitor tuning → random walk optimization M. Aiba, et al., Ultra Low Vertical Emittanceat SLS Through Systematic and Random Optimization, NIM-A 694 (2012) 133-139 Volker Schlott,

  6. Properties of Bending Magnet SR as a Diagnostics Tool Profile Measurements for Light Sources and FELs • 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: Volker Schlott,

  7. Peculiarities when Imaging with Synchrotron Radiation Profile Measurements for Light Sources and FELs • The object to be imaged is its own source, emitting SR in the forward direction • tangential to the circular beam path of the electrons in a bending magnet • SR generates a narrow forward directed cone in the vertical direction • and 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 SLS case: for l = 400 nm and DY = 2.5 mrad resolution improvementsby…: imaging at shorter wavelengths (UV or X-rays) X-ray pinhole camera interferometric techniques (UV and/or visible) Volker Schlott,

  8. Remarks about SR Imaging Systems:Optics, Cameras and Utilities Profile Measurements for Light Sources and FELs in many cases also applicable to screen monitors • 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 σof 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) Volker Schlott,

  9. Profile Measurements for Light Sources and FELs Synchrotron Radiation Monitors: X-Ray Pinhole Camera Example from ALBA, U. Iriso et al., EPAC 2006 X-ray pinhole camera resolution is limited by: with L1 = 6 m L2 = 12 m l = 12 nm (17 keV) → w = 20 mm ... blurring ... diffraction typical resolution limitation of x-ray pinhole cameras ~ 10 mm Example: PETRA III Pinhole Camera ESRF ID-25 X-Ray Pinhole Camera Ø 18 μm hole in 500 μm thick Tungsten plate courtesy of K.Scheidt, ESRF courtesy of G. Kube, DESY P.Elleaume, C.Fortgang, C.Penel and E.Tarazona, J.Synchrotron Rad. 2 (1995) , 209 Volker Schlott, 3rd Topical oPAC Workshop on Beam Diagnostics, May 8th - 9th, 2014

  10. Profile Measurements for Light Sources and FELs Synchrotron Radiation Monitors: X-Ray Pinhole Array BESSY II X-Ray Pinhole Array W.B. Peatman, K. Holldack, J.Synchrotron Rad. (1998) 5, 639-641 diffraction limited resolution: ~ 11 mm simultaneous measurement of...: → beam size through single pinhole image → beam divergence through envelope Volker Schlott, 3rd Topical oPAC Workshop on Beam Diagnostics, May 8th - 9th, 2014

  11. Design and Expected Performance of the New SLS Emittance Monitor Synchrotron Radiation Monitors: Principle of Interference Monitors 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 ToshiMitsuhashi • van Citert-Zernike’s theorem relates transverse distribution f(y) via FFT with spatial coherence g(y) Intensity of Interference Pattern → spatial coherence provides rms beam size Volker Schlott,

  12. Profile Measurements for Light Sources and FELs 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 minimal measured beam size: sy = 4.73 ± 0.55 mm Example of an Interferogram Beam Size vs. Shutter Time and Fit of Interferogram Volker Schlott, 3rd Topical oPAC Workshop on Beam Diagnostics, May 8th - 9th, 2014

  13. Profile Measurements for Light Sources and FELs Synchrotron Radiation Monitors – Principle of the p-Polarization Method Å. Andersson, et al., Determination of Small Vertical Electron Beam Profile and Emittanceat 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) 2-D Electric Field Distribution (in image plane) 2-D Intensity Distribution (in image plane) O. Chubar & P. Elleaume, Accurate and Efficient Computation of Synchrotron Radiation in the Near Field Region, EPAC 1998 Volker Schlott, 3rd Topical oPAC Workshop on Beam Diagnostics, May 8th - 9th, 2014

  14. calibration & alignment Profile Measurements for Light Sources and FELs Synchrotron Radiation Monitors: SLS p-Polarization Monitor • operating wavelength: variable (266 nm) • opening angle: 7 mradHx 9 mrad V • finger absorber to block main SR intensity • imaging by toroidalmirror • magnification: 1.453 • surface quality of optics: < 20 nm (l/30 @ 633 nm) • p-polarization or interferometricmethod selectable 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) Volker Schlott, 3rd Topical oPAC Workshop on Beam Diagnostics, May 8th - 9th, 2014

  15. Profile Measurements for Light Sources and FELs Not Treated Here:Imaging SR Monitors with X-Ray (Focusing) Optics …and possibly many more! Reflective 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 Volker Schlott, 3rd Topical oPAC Workshop on Beam Diagnostics, May 8th - 9th, 2014

  16. Profile Measurements for Light Sources and FELs Excursion:Bunch Compressor Synchrotron Radiation Monitors in FELs SwissFEL Test Injector BC Layout movable bunch compressor chicane Energy Spread Measurements with SITF BC SR Monitor Optics Set-Up of SITF BC SR-Monitor Volker Schlott, 3rd Topical oPAC Workshop on Beam Diagnostics, May 8th - 9th, 2014

  17. OneMotivation for Transverse Profile Measurements in FELs Profile Measurements for Light Sources and FELs High Electron Beam Densityis 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 SwissFEL ARAMIS: Gain Length vs. Emittance 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 courtesy of Sven Reiche Volker Schlott,

  18. Transverse Profile Measurements in Free Electron Lasers Profile Measurements for Light Sources and FELs • 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 profilesto be measured in FELs (e.g. SITF, PSI) …and a comparison to the «real world» Volker Schlott,

  19. Scintillator Screens as 2D Transverse Profile Monitors Profile Measurements for Light Sources and FELs Schematic Set-Up and Main Properties of Scintillators as Screen Monitors • electrons passing the scintillator crystal excite atoms and molecules scintillator electron beam • scintillator crystals are very sensitive and radiation resistant • visible light from scintillator crystal is radiated in 4p • multiple scattering in scintillator crystal increases beam divergence • photons are created along the beam pass through scintillator crystal camera • thickness of scintillator crystal and observation angle affect resolution 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 Volker Schlott,

  20. Optical Transition Radiators as 2D Transverse Profile Monitors Profile Measurements for Light Sources and FELs 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 OTR foil electron beam forward OTR • OTR screen could be thin metal foil or silicon wafer (with Al layer) • OTR is radiated in forward and backward direction with an angle of 1/g backward OTR Q = 1/g • incoherent Optical TR provides good linearity for profile measurements • surface quality of OTR screen affects profile imaging quality (beam size) camera • OTR is widely used as beam profile monitors in LINACs 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 Scanners • but only 1D…  H. Loos, et al., Observation of Coherent Optical Transition Radiation in the LCLS LINAC, SLAC-PUB-13395, September 2008 Volker Schlott,

  21. COTR Suppression:Temporal Separation with Scintillator & Gated CCD Profile Measurements for Light Sources and FELs OTR: t ~ fs - ps • OTR is an instantaneous process scintillation: t ~ 100 ns t0 • Coherent OTR occurs in case of micro-bunching • Scintillation has a “long” decay time time electron bunch arrives at screen • Scintillation is insensitive to micro-bunching ICCD gating time COTR Mitigation Tests @ FLASH using Scintillator and ICCD(Minjie Yan et al. @ FLASH, Hamburg) no delay at ICCD • COTR and CSR on OTR screen • COTR and CSR on scintillator (LuAG screen) 100 ns delay at ICCD • no signal from OTR screen • Scintillation 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 Volker Schlott,

  22. COTR Suppression:Spatial Separation – Central Mask in Imaging System Profile Measurements for Light Sources and FELs • SACLA COTR image behind BC-3 • OTR is emitted at an angle Q ~ 1/g • at beam energies > 1 GeV(e.g. SACLA BC-3):Q < 0.5 mrad • (C)OTR emission for small (10 µm) beam sizes at Q ~ l/2s≈ 100 mrad • scintillation light is emitted in 4p • central mask in imaging system successfully suppresses COTR intensity 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) S. Matsubara et al., Improvement of Screen Monitor with Suppression of Coherent OTR Effect for SACLA, Proc. IBIC 2012, Tsukuba, Japan, 34 Volker Schlott,

  23. COTR Suppression:Spatial Separation – SwissFEL Profile Monitors Profile Measurements for Light Sources and FELs • entire screen (large RoI) can be observed without • depth-of-field issues by following Scheimpflug imaging principle • detector (CMOS sensor) is tilted by 15° for 1:1 imaging • to avoid astigmatism • use YAG or LuAG scintillator crystals instead of OTR • observation of beam profile according to Snell’s law of refraction • beams can be imaged, which are smaller than scintillator thickness COTR Mitigation for SwissFEL Screen Monitors(R. Ischebeck et al., PSI, Switzerland) • COTR suppression tests at LCLS (full compression, 20 pC) • SwissFEL SCM resolution test with 10 pC rms beam size: 8 µm SwissFEL Screen Monitor Design patent pending Coherent OTR Measurements with SwissFEL SCM at LCLS to be published by R. Ischebeck et al. at IBIC 2014, Monterey, USA Volker Schlott,

  24. Summary Profile Measurements for Light Sources and FELs • 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” emittancesand energy spread • → optimize matching into LINACs, transfer and undulator lines • µm resolutionshavebeenachievedevenforlow beam charges (fewpC) • Coherent OTRfromhighlybrilliantbeamsor beam withmicro-bunchingstructures • Solutions for COTR suppressionhavebeenworked out andpresented: • → 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 Volker Schlott,

  25. Profile Measurements for Light Sources and FELs 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 Volker Schlott,

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