Linac Diagnostics Patrick Krejcik, SLAC April 24, 2002 Full characterization of beam phase space before bunch enters undulator Monitoring bunch-by-bunch fluctuations for feedback control Transverse and longitudinal phase space diagnostics for tuning Feedback and feedforward schemes LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Introduction and outline Transverse and “conventional” diagnostics Emphasis in this presentation will be on the “new” aspects of longitudinal measurements of RMS Bunch length Bunch length distribution Bunch arrival time Bunch phase Energy, energy spread LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Wire Scanner Transverse Emittance Diagnostics Wire scanners routinely used in the SLAC linac 4-wire scans Quadrupole scans Measurement at right is from the SLAC Damping Ring Vertical emittance shown gey=3mm Just a factor 3 within LCLS requirements Beam vertical beam profile measured during an emittance scan LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Transverse Emittance Measurement Locations and Energy and energy spread BPMs and Profile Monitors ...existing linac L0 rf gun L3 L1 X L2 gex,y sE Wire scanner beam size measurements Profile monitor high-dispersion locations LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Major longitudinal diagnostics and their locations RF transverse deflecting cavities - TC Electro optic systems - EO Terahertz radiation monitors for bunch length - Tz Energy and energy spread BPMs and Profile Monitors - DE CSR measurements from synch light from BC bends Beam phase measurements - f Zero phase crossing measurement of bunch length - Zf EO f Tz Zf TC DE CSR LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

RF deflecting cavities Brief description of principal Example of measurements taken in SLAC linac Comparison with simulation of its use in the LCLS LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Measurements at the SLAC Linac Profile Monitor Images of Damped, scavenger bunch at end of linac Transverse Cavity OFF Transverse Cavity ON LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Transverse Cavity Voltage Calibration from BPM vs.Phase y Deflection [mm] f RF Phase [ºS-Band] LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Bunch Length Reconstruction With Transverse Deflecting Cavity Relationship between Vertical beam size and RF Voltage, Bunch length, LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Linac Wakes Introduce Asymmetry Into the Transverse Deflection Scan Measurement in the SLAC linac Further degree of sophistication in the measurement Beam size scans not symmetric with voltage Provides a new capability for predicting correction of wakefield emittance growth Bunch crabbing LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Over compression case examine bunch distribution Bunch Length Measurement at a fixed RF Deflector Voltage as the Compressor is Varied Practical example of tuning of a bunch compressor in the SLAC linac Over compression case examine bunch distribution 1 nC bunch charge LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Longitudinal Bunch Distribution for an Over-Compressed Beam Streaked beam measurement shows characteristic non-Gaussian steep profile of over compression (RTL Compressor Voltage = 42 MV) LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

LITRACK Simulation of Bunch Length Tuning from under compression to over compression Beam profile for over compression (RTL Compressor Voltage = 42 MV) LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

On Sect 25 profile monitor RF deflector OFF X-Y projection of beam Simulation of RF Deflector Bunch Length Measurement of LCLS beam at Sector 25 X-Y projection of beam On Sect 25 profile monitor RF deflector OFF X-Y projection of beam On Sect 25 profile monitor RF deflector ON … In combination with quad scan gives SLICE X-EMITTANCE LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

… gives SLICE ENERGY SPREAD Simulation of RF Deflector Bunch Length Measurement of LCLS beam at High Dispersion Location Longitudinal phase space at end of linac X-Y profile monitor image of “streaked” beam at a high dispersion location in DL2 … gives SLICE ENERGY SPREAD LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Zero phase crossing measurement of bunch length Energy distribution Rotate z-coordinate of bunch into energy coordinate Measure energy spread at high dispersion location Longitudinal phase space Horizontal distribution At high h L3 RF Off L3 at zero crossing L3 at zero crossing - 180° LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Move from a destructive measurement to a non invasive diagnostic Electro optic systems Meet challenge of ultra-fast diagnostics with femtosecond resolution by Combining ultra-fast femtosecond laser technology with electro optic gating using Frequency domain analysis with chirped laser pulse Frequency resolved optical gating spectrometry of laser modulation from the electron bunch Patrick Krejcik: Move from a destructive measurement to a non invasive diagnostic LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Principal of Electro Optic Detection Electric field from bunch modulates transmission of chirped laser pulse Initial laser chirp Polarizer Analyzer EO Crystal Bunch charge Gated spectral signal Spectrometer wl t ws I Electron bunch Co-propagating Laser pulse Beam pipe LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Electro Optic Crystal Mount LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Optical Layout LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Synchrotron light monitors in the BC bends Interferometric measurements with coherent component of radiation from short bunches Auto-correlation measurements to measure bunch length Bandpass detectors D.X. Wang 1 THz incoherent Sugahara et al LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Terahertz bunch length radiation monitors Detection of amplitude of THz diffraction radiation to quantify bunch length Existing THz monitors use either electro optic modulation of a laser beam Or use micron-sized photo-emitting dipoles on GaAs substrate to rectify signal D. M. Mittleman et al Exploits a new, available technology LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Sources and expected magnitude of timing jitter Expected variation in bunch length at end of linac Expected variation in bunch arrival time at end of linac = 0.073 ps = 0.010 ps LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Electro Optic Detector also provides timing with respect to laser probe pulse Initial laser chirp Polarizer Analyzer EO Crystal Bunch charge Gated spectral signal Spectrometer wl t ws I Electron bunch Co-propagating Laser pulse Beam pipe Centroid gives arrival time of electrons w.r.t. laser pulse LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Phase and amplitude errors arise in the RF distribution LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

~ 1 ° rms longer term (~1 ps) Actual Beam Based Measurement of Relative Phase Jitter Between Bunch and the Transverse Deflecting Cavity Phase deviations calculated from transverse kick measured by fitting BPM orbit downstream of cavity ~0.1° rms short term (~100 fs) ~ 1 ° rms longer term (~1 ps) LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Timing precision is only as good as the reference signal Proposed timing and RF distribution system LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Characterization of diagnostics as either Invasive Machine setup Pulse stealing mode Machine monitoring Fast, non-destructive single bunch measurements Required for beam-based feedback LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

Feedback control of RF phase and amplitude in L0, L1,L2 RF feedback relies on beam-based measurement of: Energy plus bunch length plus (a less precise) phase measurement at each compressor Simulation using Matlab-Simulink tools Incorporates 2D tracking (Litrack) Experience with Damping Ring and PEP II RF feedback systems LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC

CONCLUSION Transverse emittance measurements and beam profiling well established at SLAC. Our recent tests with RF transverse deflecting structures show we can measure bunch length distribution with required precision. Electro optic and Terahertz diagnostics use techniques borrowed from femtosecond laser technology. Improvements in RF distribution plus beam-based feedback to meet stability requirements for LCLS. Extensive experience with beam-based feedback at SLAC LCLS DOE Review, April 24, 2002 Patrick Krejcik, SLAC