COTR Diagnostics and Sources of Microbunching. Alex Lumpkin, Fermilab Presented at Microbunching Instability Workshop College Park, MD April 13, 2012

Transcription

1 COTR Diagnostics and Sources of Microbunching Alex Lumpkin, Fermilab Presented at Microbunching Instability Workshop College Park, MD April 13, 2012

2 OUTLINE I. Introduction II. Diagnostics with optical transition radiation (OTR) - Characterization of microbunching using Coherent OTR (COTR) III. Early microbunching interplay in a FEL IV. Mitigation options of COTR in imaging V. LCLS X-ray FEL effects VI. Summary A.H. Lumpkin ubi4 Workshop April 13,

3 Microbunching Mechanisms Microbunching of an electron beam, or a z-dependent density modulation with a period λ, can be generated by several mechanisms: The LSC-induced microbunching (LSCIM) starts from noise fluctuations in the charge distribution which causes an energy modulation that converts to density modulation following Chicane compression. This is a broadband case. (our topic). The laser-induced microbunching (LIM) occurs at the laser resonant wavelength (and harmonics) as the e-beam copropagates through the wiggler with the laser beam followed by Chicane compression. This is narrow-band. (Oct WS) In self-amplified spontaneous emission or (SASE) induced microbunching (SIM) the electron beam is also bunched at resonant wavelength and harmonics. This is narrow band. A microbunched beam will radiate coherently.(cotr) A.H. Lumpkin ubi4 Workshop April 13,

4 Coherent Optical Transition Radiation Calculations Coherent Spectral-Angular Distribution from a Macropulse, Number of Photons per Unit Frequency and Solid Angle 2 d N 2 d N = r Ω 1,// I dωd dωdω 2 ( k) I( k) E = 220 MeV 1.0 σ x, y = 0.2 mrad Single Particle OTR Spectral-Angular Distribution d d N e 1 ( 2 2 θ ) x + θ y ( γ + θ θ ) = 2 ω dω c π ω x + 4 A.H. Lumpkin ubi4 Workshop April 13, 2012 y Relative Intensity (arb. units) Angle (radians) From D. Rule and A. Lumpkin, PAC 01

5 COTRI Calculations (cont.) Coherence Function I ( k) = N + N ( N 1) H ( k) 2 B B Fourier Transform of Charge Form Factors ( k) ρ H ( k) = = Q g x ( k ) g ( k ) F ( k ) x y y z z Q = total charge of macropulse Bunching fraction = f B =N B /N Note: The coherence function reduces to just the number of particles, N, when the number of microbunched particles, N B is zero. From D. Rule and A. Lumpkin, PAC 01 5 A.H. Lumpkin ubi4 Workshop April 13, 2012

6 Schematic of APS SASE FEL Experiment Sta-5 *FEL decommissioned A.H. Lumpkin ubi4 Workshop April 13, 2012

7 300-A VLD1 Data OTR/COTR data from a) LSCIM and b) LSCIM plus SIM. Peaks about 5x brighter in b). (Dec. 2001) (a) 500 nm SPF (b) ND 1.0 Y(ch) Y (ch) 800 µm 800 µm X (ch) X (ch) Lumpkin, Dejus, Sereno: PRST-AB April 2009 A.H. Lumpkin ubi4 Workshop April 13,

8 (a) 300-A VLD2 Data OTR/COTR data from a) LSCIM and b) SIM. The right hand peak in b) is 100 times brighter. FEL at 530 nm. (a) 500-nm SPF ND 2.0 (b) Intensity 800 µm Intensity 320 µm (FWHM) X (ch) X (ch) Lumpkin,Dejus,Sereno: PRST-AB April 2009 A.H. Lumpkin ubi4 Workshop April 13,

9 Spectra in SASE and COTR Observed At and After Saturation Spectral evolution shown after undulators 5,7,9. 9 A.H. Lumpkin ubi4 Workshop April 13, 2012

10 Experimental Gain Curves Compared to GENESIS Results Experimental Gain curves for COTR (L) and UR (R) compared to GENESIS (Y. Chae). Data of OTR Image Intensity (10 5 cts) A COTR DS6 GEN VLD1 GEN-260A-Norm.@VLD1 UR Image Intensity (10 5 cts) 1e+7 1e+6 1e+5 1e+4 1e+3 1e+2 1e A UR HM-DS5 GEN-II-Norm.@VLD1 GEN-260A-N1-@VLD Z Position (VLD #) 1e Z Position (VLD #) Lumpkin,Dejus,Sereno: PRST-AB April 2009 A.H. Lumpkin ubi4 Workshop April 13,

11 Schematic of the APS Injector Area Both rf PC gun and rf TC gun beams available for acceleration, compression, and diagnosis. 150 MeV FS3 FS4 FS5 L5 Sta MeV (Decommissioned) A.H. Lumpkin ubi4 Workshop April 13,

12 Enhanced OTR Images Seen Examples of images and profiles for L2 phase at 12 deg. (L-uncompr.) and L2 phase at 14.9 deg.(r-compr.) Y Y X X Intensity Intensity X A.H. Lumpkin ubi4 Workshop April 13, X

13 Mitigation Techniques Diagnostics COTR mitigation options include: Spectral differences between OTR and COTR can be used to sort photons. (APS/ANL) Scintillators can be used to enhance S/B ratio in combination with a bandpass filter. (APS/ANL) Temporal gating can sort prompt COTR and delayed scintillator light, both MCP gate and CCD shutter options. (DESY, ANL, and FNAL examples.) Angular distributions of COTR and IOTR are displaced in some cases. Proposed at µbi-08 by R. Fiorito. Specific COTR angles can be spatially filtered versus 4π scintillator light. (DESY and SACLA) Instability suppression by laser beam heating, reversible beam heating, noise suppression,dispersion, drive laser shorter pulse. A.H. Lumpkin ubi4 Workshop April 13,

14 LCLS COTR Case: BC1 Estimate OTR spectral effect in LCLS OTR12 case OTR Rel. Intensity Model CCD response (x10) COTR or OTR gained up (3 kev) OTR vs. COTR Relative Intensity OTR -UV- COTR* (3 kev) CCD Resp. (x10) Wavelength (nm) *3-keV curve based on Ratner et al., FEL08 A.H. Lumpkin ubi4 Workshop April 13,

15 COTR Mitigation Test at 325 MeV Reduction of COTR effects with 400x40 nm BPF, but need more sensitive camera than 40dB analog CCD to see remaining OTR. LSCIM COTR:ND0.5 COTR:400x40 nm LSO: 400x40 nm Y Y Y X(ch) X(ch) X(ch) I I I A.H. Lumpkin ubi4 Workshop April 13,

16 Spectral Tests on COTR ICCD with GaAs PC data; Conditions such that unfiltered COTR needed ND2 to avoid saturation of ICCD. LSO:Ce,400x40nm, ND1.0 OTR,450x40 nm, ND0.0 A.H. Lumpkin ubi4 Workshop April 13,

17 Spectrometer results ( ) Grating offset adjusted to probe the red/nir end of the COTR spectra using ICCD readout with GaAs PC. 880 nm X(ch) 650 nm ITT web site 880 nm 880 nm Wavelength(ch) 900 nm Intensity 1250 nm offset, 10 µm Slit, ND1.0, ICCD A.H. Lumpkin ubi4 Workshop April 13,

18 Converter Options Reconsider possible scintillators based on Ce doping of yttrium aluminum garnet (YAG), lutetium oxyorthosilicate (LSO), and yttrium aluminum perovskite (YAP). Converter Spectrum (FWHM)*, Peak YAG:Ce , 526 nm LS0:Ce , 415 nm YAP:Ce , 369 nm Efficiency Response Time (FWHM) Comment 1.0* 89 ns* 460 µm T 0.46* 40 ns* 530 µm T ~ ns 460 µm T OTR Broadband * ~10 fs Surface *Berg,Lumpkin,Yang: Linac 2000 A.H. Lumpkin ubi4 Workshop April 13,

19 Spectral Modulation Effect Seen Increased L2 gradient and achieved more COTR with compression. (Dec. 14, 2008.) X Position a) X position 26 nm 550 nm 26 nm Wavelength (nm) Wavelength (nm) b) A.H. Lumpkin ubi4 Workshop April 13,

20 Spectral Intensity Modulation Table I: Summary of observations on the COTR spectral modulation with the two gratings (12-08). dν = -c dλ/λ 2 Grating Lines/mm Wavelength (nm) Peak Sep. (nm) Mod. Freq. (THz) Mod. Period (µm) A.H. Lumpkin ubi4 Workshop April 13,

21 ANL MCP Gate Results Recent ANL tests with MCP-CCD (See J. Dooling Talk) A.H. Lumpkin ubi4 Workshop April 13,

22 FLASH : Gated ICCD on COTR MCP gate used to reject prompt COTR emissions M.Yan,GSI WS 2011 A.H. Lumpkin ubi4 Workshop April 13,

23 FNAL Digital CCD Shutter Option The present digital CCDs provide a time shutter option at the 10-µs level at the video readout rate. For single pulse or pulses at or less than video rate (1-30 Hz) this could be used to reject the prompt COTR. A proof-of-principle test on 10-µs shutter was done with OTR and LYSO:Ce at FNAL photoinjector in Full rejection ratio not known yet. >50 LYSO:Ce OTR A.S. Johnson et al. IPAC12 A.H. Lumpkin ubi4 Workshop April 13,

24 Temporal Profile Measurement at SACLA under COTR 5712 MHz HEM11 < 10 μm resolution 20 fs resolution 24 Courtesy of the SACLA Team A.H. Lumpkin ubi4 Workshop April 13, th Microbunching Instability WS, Univ. of Maryland April 11-13,

25 Profile Measurement using Spatial Mask and Fluorescence Screen Deflector ON Deflector Off Courtesy of the SACLA Team 4 th Microbunching Instability WS, Univ. of Maryland April 11-13, 2012

26 X-ray FEL Impact of µb Instability Laser heater simulations performed for FIR modulations. A.H. Lumpkin ubi4 Workshop April 13,

27 Laser heater-lcls (135 MeV) Emma,Huang: ubi-ii A.H. Lumpkin ubi4 Workshop April 13,

28 Microbunching Instability at LCLS Laser Heater Off e - energy SXR Spectrometer Dump YAG Screen J. Welch et al., FEL10 A.H. Lumpkin ubi4 Workshop April 13,

29 Microbunching Instability at LCLS Laser-induced energy modulation used to suppress ubi at LCLS in x-ray FEL, but COTR interferences still exist. Laser Heater ON No Instability e - energy J. Welch et al., FEL10 A.H. Lumpkin ubi4 Workshop April 13,

30 SUMMARY Interplay of the microbunching mechanisms is possible. LSCIM can lead to prebunching an FEL at resonance condition. Studies of COTR strength after chicane: possible 2-D spatial imaging, 2-D angular distributions, x,y-spectral imaging, longitudinal imaging. At some point LSCIM or laser heater might interfere with LIM. Energy modulation due to LSCIM can be measured directly. Benchmark codes on microbunching fractions. Mitigation techniques for diagnostics have been demonstrated for moderate to extensive COTR. Intrinsic suppression of µbi by laser beam heating, reversible beam heating, and dispersive elements. Future tests planned at ASTA/Fermilab; need modeling. A.H. Lumpkin ubi4 Workshop April 13,

31 Acknowledgements Many discussions with D. Dowell, Z. Huang, H. Loos of SLAC and W. Fawley (LBNL), D. Rule (NSWC), and R. Fiorito (UMD). Support from K.-J. Kim, R. Gerig, and H. Weerts of the Argonne Accelerator Institute. The collaborations on ANL linac tests of N. Sereno, J. Dooling, S. Pasky, Y. Li of ANL. The support of M. Wendt of Fermilab. Two PRST-AB articles available: April and August Earlier reports in BIW08, FEL08, Linac08, µbunching II. A.H. Lumpkin ubi4 Workshop April 13,

32 APS Spectral Filtering on LSO:Ce 400x10 nm BPF used to suppress COTR spikes on 500- µm thick LSO:Ce crystal scintillations into CCD camera. COTR Spikes Intensity COTR Spikes Intensity X Position (ch) X Position (ch) A.H. Lumpkin ubi4 Workshop April 13,

33 40-MeV Injector for ASTA/FNAL Injector being installed with First beam expected in MeV Injector electron gun booster cavities 3 rd harmonic cavity flat beam transform chicane deflecting mode cavity spectromet er magnet beam dump 1 st cryomodule test beamlines beam dump Booster cavity 2 (from DESY and Saclay) installed in NML Courtesy of M. Church First cryomodule (from DESY) installed at NML. A.H. Lumpkin BIW12 April 17,

34 Prototype Imaging Station New developed imaging station in collaboration with RadiaBeam, Inc. A.H. Lumpkin BIW12 April 17,