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Imaging Techniques of Relativistic Beams: Issues & Limitations

Imaging Techniques of Relativistic Beams: Issues & Limitations. Alex Lumpkin and Manfred Wendt , Fermilab , Presented at the LCWS11 September 27, 2011 Granada, Spain. Outline. Introduction Beam profiling with YAG:Ce * scintillation Scintillator resolution Depth-of-focus issue

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Imaging Techniques of Relativistic Beams: Issues & Limitations

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  1. Imaging Techniques of Relativistic Beams: Issues & Limitations Alex Lumpkin and Manfred Wendt, Fermilab, Presented at the LCWS11 September 27, 2011 Granada, Spain

  2. Outline • Introduction • Beam profiling with YAG:Ce* scintillation • Scintillator resolution • Depth-of-focus issue III. Optical Transition Radiation (OTR) • OTR basics • OTR point-spread-function (PSF) aspects • Microbunching instability and coherent OTR IV. Future tests V. Summary *Yttrium Aluminum Garnet, Cerium doped A. Lumpkin and M. Wendt LCWS September 26, 2011

  3. Fermilab A0 Photo Injector Beamline and diagnostics support for EEX applications 3.9 GHz TM110 Cavity A. Lumpkin and M. Wendt LCWS September 26, 2011

  4. Intro to Beam-Size Imaging • The charged-particle beam transverse size and profiles are part of the basic characterizations needed in accelerators to determine beam quality, e.g. transverse emittance. • A basic beam imaging system includes: • conversion mechanism (scintillator, optical or x-ray synchrotron radiation (OSR or XSR), Cherenkov radiation (CR), optical transition radiation (OTR), undulator radiation (UR), and optical diffraction radiation (ODR). • optical transport (lenses, mirrors, filters, polarizers). • imaging sensor such as CCD,CID, CMOS camera, with or without intensifier and/or cooling • video digitizer • image processing software beam conversion screen optical transport imaging camera video digitizer image processing & controls software A. Lumpkin and M. Wendt LCWS September 26, 2011

  5. Identify Corrections to Consider • System related • YAG:Ce powder and crystal screen spatial resolution. • Camera resolution and depth of focus. • OTR polarization effects and OTR point spread function. • Camera calibration factor. • Finite slit size (if applicable) . • Accelerator / beam related • Beta star term in spectrometers. • Macropulse blurring effects on energy spread , beam size, and beam divergence in OTR images. A. Lumpkin and M. Wendt LCWS September 26, 2011

  6. Observed vs Actual Slit Image Size • Uncorrelated terms are treated as a quadrature sum to actual image size Act (see Lyons’ textbook a). • Observed image size Obs • YAG screen effects YAG • Camera resolution Cam • Finite slit width Slit • In addition there can be macropulse effects and OTR polarization effects. and solving for the actual beam size we have, aLouisLyons, Statistics for Nuclear and Particle Physicists (1986) A. Lumpkin and M. Wendt LCWS September 26, 2011

  7. Converter Screen Properties A. Lumpkin and M. Wendt LCWS September 26, 2011

  8. YAG:Ce Powder Scintillator Screens • YAG:Ce screens, used at the A0 Photoinjector: • The screens have nominally a 5 µm grain size and are coated at 50-µm thickness on various metal substrates. • Substrates are Al or SS and 1 mm thick. • In the A0PI arrangement the scintillator was on the front surface of the substrate, and oriented at 450 to the beam direction. • Powder screens are kindly provided by Klaus Floettmann(DESY). • Observed Characteristics • The response time is about 80 ns FWHM. • There have been reports of saturation of the mechanism for incident electron beam areal charge densities ~10 fC/µm2. • This effect can cause a charge dependence of the observed image size in addition to the low-charge, screen resolution limit. A. Lumpkin and M. Wendt LCWS September 26, 2011

  9. YAG:Ce Powder vs OTR Screen • Both screen surfaces at 450 to the beam direction. • Gaussian fits to the projected beam profiles of 10 images. • Deduced YAG resolution term (page 6): 80 ± 20 μm • YAG resolution, averaged using three measurement sets: 60 ± 20 μm A. Lumpkin and M. Wendt LCWS September 26, 2011

  10. Screen Resolution vs. Thickness 2 • Scintillator screen resolution vs. thickness after applying corrections discussed on page 6. Chromox, Elettra,450 Chromox, APS/ANL,450 YAG:Ce, A0PI,450 YAG:Tb, BNL,00 YAG:Ce, single crystal SCSS and Mainz,00 YAG 5 μm grain size A. Lumpkin and M. Wendt LCWS September 26, 2011

  11. Identify Corrections to Consider • System related • YAG:Ce powder and crystal screen spatial resolution. • Camera resolution and depth of focus. • OTR polarization effects and OTR point spread function. • Camera calibration factor. • Finite slit size (if applicable) . • Accelerator / beam related • Beta star term in spectrometers. • Macropulse blurring effects on energy spread , beam size, and beam divergence in OTR images. A. Lumpkin and M. Wendt LCWS September 26, 2011

  12. Depth-of-Focus Issues • 1 mm (X5) / 4 mm (X24) spaced slits, 50 μm wide • Camera calibration ~30 μm / pixel. • Depth-of-focus issues in extended field of view for 450 arrangement of the YAG:Ce scintillator screen X5 X24 A. Lumpkin and M. Wendt LCWS September 26, 2011

  13. MATLAB Emittance Code • Application tool provides online emittance and C-S parameter calculations to facilitate operations. courtesy R. Thurman-Keup A. Lumpkin and M. Wendt LCWS September 26, 2011

  14. Identify Corrections to Consider • System related • YAG:Ce powder and crystal screen spatial resolution. • Camera resolution and depth of focus. • OTR polarization effects and OTR point spread function. • Camera calibration factor. • Finite slit size (if applicable) . • Accelerator / beam related • Beta star term in spectrometers. • Macropulse blurring effects on energy spread , beam size, and beam divergence in OTR images. A. Lumpkin and M. Wendt LCWS September 26, 2011

  15. Optical Transition Radiation (OTR) • OTR can be used for beam • profile / size • position • divergence • Charged particle passing a media boundary (EM dipole). OTR angular intensity distribution of a single charged particle • energy, • relative intensity • bunch length A. Lumpkin and M. Wendt LCWS September 26, 2011

  16. OTR Calculation • OTR single particle spectral-angular distribution: • Ω spatial angle • ω angular frequency • N1 # of photons • Θx,y radiation angle • E, ħ, c, πconstants • Coherent spectral-angulardistribution from a macropulse • N # of photons from per unit frequency and solid angle (typ. 1 e -> 0.001 photons) • r reflection coefficient • I interference function (double foil) • F coherence function (can be non-linear) E = 220 MeVx’, y’= 0.2 mrad A. Lumpkin and M. Wendt LCWS September 26, 2011

  17. Prototype Imaging Station • New developed imaging station in collaboration with RadiaBeam, Inc. A. Lumpkin and M. Wendt LCWS September 26, 2011

  18. OTR / YAG Configurations • Switchable assemblies to compare options. • Still tweaking! Option 1 Option 2 Impedance Screen Impedance Screen mirror YAG:Ce, plus Al on Si mirror OTR,100 µm Al plus Al on Si mirror light screen OTR, normal to beam, plus Al on Si mirror OTR foil, 1µm plus 1µm foil at 450 beam A. Lumpkin and M. Wendt LCWS September 26, 2011

  19. Test of OTR Normal to Beam • Optics focused on crystal location: • gives superposition of focused OTR and defocused OTR source from mirror. Single-Gaussian Fit σ1 = 16.2 ± 0.2 pix Double-Gaussian Fit σ1= 9.2 ± 0.1 σ2 = 28.1± 0.2 A. Lumpkin and M. Wendt LCWS September 26, 2011

  20. OTR Polarization and PSF History • 1996: Lebedev evaluated OTR resolution. • 1998: Castellano, et.al. points out the OTR PSF has a polarization feature. • Calculated a 12λ (FWHM) effect for the total widthand 0.1 rad collection angle. • 2007: FNAL & JLAB observed OTR / ODR polarization effects on the beam image size. • 2007: Xiang, et.al. calculates PSF for OTR and ODR. • 2010: OTR polarization on beam images reported by FNAL A0 photoinjector staff at the BIW10. • 2010: KEK OTR experiment uses a polarizer to analyze the details of the PSF (IPAC2010). • In 2005 the experiment was performed without polarizer. A. Lumpkin and M. Wendt LCWS September 26, 2011

  21. Point Spread Function (PSF) PSF: convolution integral of Point charge diffraction OTR response Example: M=1, E=4GeV, λ=500nm courtesy C. Liu Cross section A. Lumpkin and M. Wendt LCWS September 26, 2011 • At the diffraction limit the image of a point source radiates a ring pattern defined by the OTR point spread function (PSF): • maximum acceptance angle • magnification factor • radius of the lens

  22. PSF Properties PSF dependence on beam energy PSF dependence on acceptance angle courtesy C. Liu A. Lumpkin and M. Wendt LCWS September 26, 2011 • Insensitive to the beam energy • Sensitive to the acceptance angle • Polarizer mitigates PSF imaging errors • E.g. horizontal polarizer reduces verticalimage size by • With a mask blocking rays ofangle ≤Θ1, the PSF will be:

  23. OTR PSF Calculations (MATLAB) • 14.3 MeV, M=1, λ=500 nm, θmax=0.010, sigma =25 µm • This version with convolutions implemented at FNAL. Hpol PSF Total PSF At Image plane Vpol PSF Y X courtesy R. Thurman-Keup A. Lumpkin and M. Wendt LCWS September 26, 2011

  24. PSF Polarization Convolved • 14.3 MeV, M = 1, λ = 500 nm, θmax=0.010, σ = 25 µm Intensity Intensity Original Sigma = 25 μm Total PSF Sigma = 33.18 μm HorPol-HorProj PSF Sigma = 38.01 μm HorPol-VerProj PSF Sigma = 29.39 μm Hpol PSF A. Lumpkin and M. Wendt LCWS September 26, 2011

  25. PSF Polarization Convolved (cont.) • 14.3 MeV, M = 1, λ = 500nm, θmax = 0.010, σ = 10 & 50 µm Intensity Intensity Original Sigma = 50 μm Total PSF Sigma = 55.63 μm HorPol-HorProj PSF Sigma = 58.49 μm HorPol-VerProj PSF Sigma = 53.05 μ Original Sigma = 10 μm Total PSF Sigma = 22.59 μm HorPol-HorProj PSF Sigma = NA HorPol-VerProj PSF Sigma = 16.63 μm A. Lumpkin and M. Wendt LCWS September 26, 2011

  26. Polarized Beam Images at XUR • OTR Perpendicular component has 15 % smaller profile. • Beam measurements with a vertical stripe, optics generated. Total Pol.: Left Single-Gaussian Fit σ1 = 66.8 ± 0.3 μm Vert. Pol.: Right Single-Gaussian Fit σ1 = 55.1 ± 1.1 μm 10µm effect @ 55 µm (Cal.: 5.3 µm/pixel) A. Lumpkin and M. Wendt LCWS September 26, 2011

  27. KEK Experimental OTR PSF • KEK staff used vertical polarizer and small beam to observe PSF and suggested potential use of structure. • Use PSF valley for profile measurements at the PSF limit. *Legend reversed with respect to zero which included a constant background; b is the amplitude of the distribution; c is the distribution width; σ is the smoothing parameter dominantly defined by the beam size; and Δx is the horizontal offset of the distribution with respect to zero courtesy A. Aryshev A. Lumpkin and M. Wendt LCWS September 26, 2011

  28. COTR Case at 250 MeV • Estimation of OTR/COTR spectral effect for LCLS case. COTR (3 keV) OTR~1/λ2 CCD Resp. -UV- A. Lumpkin and M. Wendt LCWS September 26, 2011

  29. COTR Mitigation Test at St-5/ANL • Reduction of COTR effects with 400x40 nm BPF, but need more sensitive camera than 40dB analog CCD to see remaining OTR. COTR:ND0.5 LSO: 400x40 nm COTR:400x40 nm Y Y Y X(ch) X(ch) X(ch) I I I A. Lumpkin and M. Wendt LCWS September 26, 2011

  30. 40-MeV Injector for ASTA/FNAL • Injector being installed with First beam expected in 2012. 40-MeV Injector electron gun booster cavities flat beam transform 1st cryomodule deflecting mode cavity beam dump spectrometer magnet 3rd harmonic cavity chicane test beamlines beam dump First cryomodule (from DESY) installed at NML. Booster cavity 2 (from DESY and Saclay) installed in NML courtesy M. Church A. Lumpkin and M. Wendt LCWS September 26, 2011

  31. Summary • Scintillator resolution terms should be characterized, • Use normal incidence of beam preferred geometry to minimize depth-of-focus, effective radiation thickness in beam images. • OTR polarization effects need to be elucidated • Plan to optimize OTR PSF and optical resolution. • Plan to use linear polarizers with OTR imaging for the perpendicular profile components at ASTA. • Mitigate microbunching instability effects for profiling of bright beams. • Plan to use 400x40 nm band pass filters and LYSO:Ce* crystals after bunch compression at ASTA to suppress expected diagnostics complications due to COTR. • The future remains bright for imaging techniques! *Lutetium Yttrium oxyorthosilicate (420 nm, violet), Cerium doped A. Lumpkin and M. Wendt LCWS September 26, 2011

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