Patrick Krejcik May 3-6, 2004 Patrick Krejcik R. Akre, P. Emma, M. Hogan, (SLAC), H. Schlarb, R. Ischebeck (DESY), P. Muggli (USC Los Angeles), A. Cavalieri (University of Michigan) Sub-Picosecond Electron Bunch Length Measurements at SLAC

Patrick Krejcik May 3-6, 2004 Motivation behind ultra-short electron bunches Compressing electron bunches from a linac to reach very high peak currents (kAmps) Enables them to lase in a long undulator 4 th generation light sources: LCLS, TESLA … Ultra-short pulse for probing experiments down to femtosecond level Short pulse x-rays SubPicosecond Pulsed Souce, SPPS Advanced accelerator studies Plasma wakefield acceleration

Patrick Krejcik May 3-6, 2004 Diagnostics Challenge for Measuring Sub-Picosecond Bunch Length The SPPS e- bunch is 80 fwhm (12 m rms) Conventional streak camera technology ~1/2 ps Ideally look for resolution <100 fs Single pulse measurement important in linacs Reconstruction of bunch length charge profile Fast, relative measurements for feedback control timing measurement relative to fs laser Diagnose new instabilities – microbunching instability

Patrick Krejcik May 3-6, 2004 Bunch length diagnostic comparison Device TypeInvasive measurement Single shot measurement Abs. or rel. measurement Timing measurement Detect bunching RF Transverse Deflecting Cavity Yes: Steal 3 pulses No: 3 pulsesAbsoluteNo Coherent Radiation Spectral power No for CSR Yes for CTR YesRelativeNoYes Coherent Radiation Autocorrelation No for CSR Yes for CTR NoAbsolute (2 nd moment only) No Electro Optic Sampling NoYesAbsoluteYesNo Energy Wake-loss YesNoRelativeNo

Patrick Krejcik May 3-6, 2004 Relative bunch length measurement at SPPS based on wakefield energy loss scan Energy change measured at the end of the linac as a function of the linac phase (chirp) upstream of the compressor chicane Shortest bunch has greatest energy loss Predicted wakeloss___ (P. Emma) For bunch length z __ Predicted shape due to wakeloss plus RF curvature

Patrick Krejcik May 3-6, 2004 Bunch Length Measurements with the RF Transverse Deflecting Cavity y y Asymmetric parabola indicates incoming tilt to beam Cavity on Cavity off Cavity on - 180° Bunch length reconstruction Measure streak at 3 different phases z = 90 m (Streak size) m 30 MW

Patrick Krejcik May 3-6, 2004 Calibration scan for RF transverse deflecting cavity Beam centroid [pixels] Cavity phase [deg. S-Band] Bunch lenght calibrated in units of the wavelength of the S-band RF Further requirements for LCLS: High resolution OTR screen Wide angle, linear view optics

Patrick Krejcik May 3-6, 2004 OTR Profile Monitor in combination with RF Transverse Deflecting Cavity Simulated digitized video image Injector DL1 beam line is shown Best resolution for slice energy spread measurement would be in adjacent spectrometer beam line.

Patrick Krejcik May 3-6, THz LCLS BC2 Bunch length monitor spectrum BC2 bunch length feedback requires THz CSR detector Demonstrated with CTR at SPPS Bunch profile 200 fs Bunch spectrum >> z THz spectromet er THz power detector B4 Bend Bunch Compress or Chicane CSR Vacuum port with reflecting foil

Patrick Krejcik May 3-6, 2004 Far-Infrared Detection of Wakefields from Ultra-Short Bunches Wakefield diffraction radiation wavelength comparable to bunch length Pyroelectric detector foil LINAC FFTB Comparison of bunch length minimized according to wakefield loss and THz power GADC Linac phase Wake energy loss THz power

Patrick Krejcik May 3-6, GHz 1.2 mm BC1 Bunch Length Monitor CSR Power spectral density signal for bunch length feedback Spectral lines accompanying micro-bunching instability – Z. Huang. Spectral lines accompanying micro-bunching instability – Z. Huang.

Patrick Krejcik May 3-6, 2004 Dither feedback control of bunch length minimization - L. Hendrickson Dither time steps of 10 seconds Bunch length monitor response Feedback correction signal Linac phase ping optimum Jitter in bunch length signal over 10 seconds ~10% rms

Patrick Krejcik May 3-6, 2004 Transition radiation is coherent at wavelengths longer than the bunch length, >(2 ) 1/2 z P. Muggli, M. Hogan Limited by long wavelength cutoff Interferometer for autocorrelation of CTR 12 m rms e-e- Variable Position Mirror Interferometer Pyro Detector 12.5 µm Mylar Beam Splitters R T µm Mylar Vacuum Window (3/4 dia) Ref. Pyro Detector Alignment Laser 1 µm Titanium Foil

Patrick Krejcik May 3-6, 2004 z =14 µm z =9 µm Mylar resonances Gaussian, z =20 µm, d=12.7 µm, n=3 Mylar window+splitter Effect of Mylar Window and Beam Splitter Smaller measured width: Autocorrelation < bunch ! Modulation/dips in the interferogram Simple model: Fabry-Perot resonance: =2d/m, m=1,2,… Signal attenuated by Mylar: (RT) 2 per sheet Fabry-Perot resonance: =2d/m, m=1,2,… Signal attenuated by Mylar: (RT) 2 per sheet P. Muggli, M. Hogan

Patrick Krejcik May 3-6, 2004 Electro Optic Bunch Length Measurement Probe laser Defining aperture Beam axis M1 M2 EO xtal Geometry chosen to measure direct electric field from bunch, not wakefield Modelled by H. Schlarb electrons

Patrick Krejcik May 3-6, 2004 Resolution limit in temporal-to-spectral translation BW limited pulse Short chirp Long chirp Temporal profile Spectral profiles However, recent work shows this limit can be overcome with noncollinear cross correlation of the light before and after the EO crystal S.P. Jamison, Optics Letters, 28, 1710, 2003

Patrick Krejcik May 3-6, 2004 ErEr ErEr P Elevation view End view Plan view electrons EO Xtal Temporal to spatial geometry under test at SPPS ErEr Principal of temporal-spatial correlation Line image camera polarizer analyzer xtal

Patrick Krejcik May 3-6, 2004 Jitter determination from Electro Optic sampling ErEr Principal of temporal-spatial correlation Line image camera polarizer analyzer EO xtal seconds, 300 pulses: z = 530 fs ± 56 fs rms t = 300 fs rms single pulse A. Cavalieri centroid width

Patrick Krejcik May 3-6, 2004 EO resolution limit due to wakefields – H. Schlarb r e-e- Apparent change in z when measured at increasing radii relative to the aperture from the edge of the laser mirror negligible perturbation if EO crystal is closer to beam than mirror edge. Apparent change in z when measured at increasing radii relative to the aperture from the edge of the laser mirror negligible perturbation if EO crystal is closer to beam than mirror edge.

Patrick Krejcik May 3-6, 2004 thanks!

Patrick Krejcik May 3-6, 2004 Timing system requirements Synchronization of fiducials in low-level RF with distribution of triggers in the control system 1/360 s Linac 476 MHz Main Drive Line Sector feed Fiducial detector Master Pattern Generator SLC Control System Event Generator 360 Hz Triggers 8.4 ns±10 ps 128-bit word beam codes 119 MHz 360 Hz fiducials phase locked to low level RF