Microbunching Workshop 2010 March 24, 2010, Frascati, Italy. Zhirong Huang

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Grant Ray
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1 Measurements of the LCLS Laser Heater and its impact on the LCLS FEL Performance Z. Huang for the LCLS commissioning team LCLS 1 1

2 Outline Introduction LCLS setup and measurements Effects on FEL performance Anomalous heating effects (next talk) Results are published as PRSTAB 13, (2010). Sorry for many repeating slides from FEL

3 m-bunching Landau Damped by Laser Heater Ti:saph 758 nm <15 MW 0.5-m undulator Injector at 135 MeV Laser heater suggested by Saldin et al., NIMA, 2004; independently by J. Galayda 14 GeV Without Laser Heater LCLS design study: Z. Huang et al., PRST 2004 (chicane suggested by T. Smith) 14 GeV With Laser Heater 3 3 Elegant simulations

4 How does a laser heater work? Dg L after undulator Modulation smeared after chicane by half chicane l R 52 L and angular spread laser peak power laser rms spot size The last half chicane time-smears the energy modulation leaving an effective thermal energy spread increase 4 4

5 transport to the tunnel Layout of the Laser Heater Optics Two cameras for monitoring laser beam MH2 & MH3 provide laser pointing control Two OTR screens for spatial alignment Fast photodiode for timing within ~10 ps 758 nm IR laser from laser room OTR screens e - MH4 shutter photo diode VHC camera MH3 wave plate power meter CH1 camera MH2 Sasha Gilevich et. al., 5 5

COTR signal after BC2 bends (OTR22 inserted) Laser Heater Temporal Overlap Laser pulse 10-20 ps, electron bunch 5-7 ps Laser timing done by minimizing the

6 COTR signal after BC2 bends (OTR22 inserted) Laser Heater Temporal Overlap Laser pulse ps, electron bunch 5-7 ps Laser timing done by minimizing the COTR signal after BC2 Laser energy 230 µj 16 ps OTR22 COTR from microbunching suppressed when laser timed L2-linac BC2 Laser delay line in mm (1ps = 0.15 mm) 6 6

7 Heating Measurements (TCAV0) YAGS2 RF deflector ON YAGS2 YAGS2 energy time Laser OFF σ E /E < 12 kev Laser: 40 µj Laser: 230 µj σ E /E 45 kev σ E /E 120 kev 7 7

8 Slice Energy Spread YAGS2 Laser energy 230 µj σ E /E 120 kev Laser OFF Double-horn when laser spot ~50% larger than transverse electron beam size More uniform heating when laser spot matches transverse size of electron beam 8 8

9 Slice-E Spread vs. Laser Energy Trickle heating no heating good heating time energy No heating 8-10 kev (limited by resolution?) Operating point (~7 uj) 20 kev compression ratio 90/14GeV = TCAV has an off-axis acceleration field which can increase slice energy spread on YAGS2 (thanks to D. Filippetto!)

10 Slice E-spread from Transverse Cavity This effect can be derived from PW theorem (e.g., S.Korepanov et al., DIPAC07, p144) Take V ymax = 1 MV (S-band), g y =0.5 mm, y =7 m, E=135 MeV, TCAV0 contributes to 7 kev slice energy spread Plan to measure TCAV off-axis acceleration soon Need to subtract TCAV0 effect in quadrature to determine slice E- spread (yet to be done) 10 10

11 E-spread vs. Undulator Gap Scan undulator gap, subtract LH-off energy spread in quadrature 11 11

12 No Emittance Growth from Heating Laser-induced E-spread in chicane does not introduce emittance growth (< 2%) Heater from 45 mj J to 250 mj, no change in slice emittance Heater off, COTR after LH chicane biases emittance results 12 12

13 Effects on COTR Modulated e-beam current from microbunching leads to many COTR problems in LCLS Laser heater suppresses COTR but not to incoherent level (small part of beam escaped heating, laser-heated energy profile non-gaussian?) OTR22 L2-linac BC2 OTR22 (heater off) OTR22 Camera image, 10 bit heater depth full Camera 5 bit depth COTR can restart from shot noise in the downstream linac + bend Alternative diagnostics (wires) used for beam profiles after compressors 13 13

14 Laser Heater Improves FEL Power 250 pc, 3 ka, 1.5 Å FEL Laser optimal heater OFF Scan unsaturated FEL w/ 12 undulators vs. heater FEL saturation power improves 2X w/ heater 14 14

15 FEL Gain Length vs. Laser Heater 250 pc, 3 ka, 1.5 Å FEL Heater OFF mbunching effectively increases E-spread to 0.03% ~ 0.04% level 0.01% at 14 GeV with compression factor Å 0.4-mm emit 0.5-mm emit Heater OFF Heater ON 15 15

16 FEL Power vs. LH after saturation nominal (6 mj) All 33 undulators inserted YAG may be saturated? Laser heater OFF 16 16

17 Summary Microbunching is a generic problem for high-brightness beams (brighter beams become a bigger problem) LCLS laser heater works well to control microbunching and to improve x-ray FEL performance. Microbunching is suppressed but not all COTR is gone alternative diagnostics necessary for bright beams 17 17

Femto-second FEL Generation with Very Low Charge at LCLS

Femto-second FEL Generation with Very Low Charge at LCLS Yuantao Ding, For the LCLS commissioning team X-ray Science at the Femtosecond to Attosecond Frontier workshop May 18-20, 2009, UCLA SLAC-PUB-13525;