Low Charge LCLS Operations P Emma Z Huang

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Low Charge LCLS Operations P. Emma, Z. Huang, C. Limborg Oct. 26, 2004 Abstract:

Low Charge LCLS Operations P. Emma, Z. Huang, C. Limborg Oct. 26, 2004 Abstract: The present 1 -n. C operating point in the LCLS wastes much of the charge in large peak-current spikes at bunch head and tail. These spikes contribute to an increased resistive-wall wakefield in the FEL undulator and large CSR projected emittance growth in the BC 2 chicane. This study examines the benefits in lowering the bunch charge while maintaining the same SASE saturation length. The proposed parameters are quite reasonable with a 0. 85 -mm slice emittance in just 30 amps of peak current in the injector, and a 2 -k. A peak current in the FEL. The resulting performance is more stable, has negligible resistive-wall wakefield, greatly reduced CSR effects, and virtually no transverse wakefield emittance dilution in the linac, with no change to the baseline engineering design.

Present LCLS Design Problems Gun emittance: 1 -mm at 1 n. C, 100 A

Present LCLS Design Problems Gun emittance: 1 -mm at 1 n. C, 100 A Resistive wake in undulator LSC/CSR micro-bunching Tight RF jitter tolerances Transverse wakes in L 2 CSR De in BC 2 too much charge AC resistive-wall wakefield

wakefieldinduced cubic chirp in L 2 -linac pre-BC 2 Nominal 1 -n. C case

wakefieldinduced cubic chirp in L 2 -linac pre-BC 2 Nominal 1 -n. C case post-BC 2 cubic chirp produces current horns after BC 2 1 -n. C, 20 -mm rms bunch length is 6 k. A for a Gaussian in FEL much of charge wasted with only 3. 0 k. A in core

Low Charge Optimization Motivation: Less charge* less wake Same compression factor ~same jitter Low

Low Charge Optimization Motivation: Less charge* less wake Same compression factor ~same jitter Low gun current low emittance Chosen Scaling: Charge: 1 n. C 0. 2 n. C Gun Current: 100 30 A (10 ps 6. 5 ps) Slice emittance: 1 mm 0. 8 mm Final current: 3. 4 k. A 2. 0 k. A (same Lsat) * Also recently suggested by Max Zolotorev, LBNL

Ming Xie

Ming Xie

200 -p. C optimized Parmela output from Cecile at end of L 0 -a

200 -p. C optimized Parmela output from Cecile at end of L 0 -a section (64 Me. V) gethermal = 1 mm/mm spot radius = 0. 42 mm laser pulse = 5 ps gethermal = 1 mm/mm Thermal emit. of 1 mm per mm radius 200 k in lcls_200 k_02 nc_atendl 01. dat C. Limborg, Oct. 19, 2004

Linac Almost Unchanged Keep BC 1 & BC 2 at nominal locations Initial bunch:

Linac Almost Unchanged Keep BC 1 & BC 2 at nominal locations Initial bunch: 560 mm length at 200 p. C Optimize for 2 k. A, no final chirp, 13. 6 Ge. V and minimal DQ & Dtgun jitter sensitivity L 1, L 3, and X-band RF ~unchanged L 2 phase: -41 -39 R 56(BC 1): -39 -43 mm R 56(BC 2): -24. 7 -20. 6 mm

200 p. C less cubic chirp pre-BC 2 post-BC 2 spikes reduced 2 k.

200 p. C less cubic chirp pre-BC 2 post-BC 2 spikes reduced 2 k. A in FEL

no CSR 6 D tracking with CSR

no CSR 6 D tracking with CSR

CSR projected x-emittance growth gex 4 mm 1. 0 n. C gey BC 2

CSR projected x-emittance growth gex 4 mm 1. 0 n. C gey BC 2 0. 2 n. C BC 2 gex gey gex 1 mm

Cylindrical-Copper Resistive-Wall Wakes in FEL Undulator uses K. Bane damped resonator model for AC

Cylindrical-Copper Resistive-Wall Wakes in FEL Undulator uses K. Bane damped resonator model for AC wake

Jitter Sim. ’s: 10 sec @ 120 Hz 0. 09% rms (was 0. 10%)

Jitter Sim. ’s: 10 sec @ 120 Hz 0. 09% rms (was 0. 10%) D E / E 96 fs rms (was 120 fs) 8. 0% rms sz Ipk more stable at 0. 2 n. C E / E 7. 2% rms (was 12%) D t Dt. FWHM

830 mm 195 mm 20 mm 1. 0 n. C 1. 6% 0. 72%

830 mm 195 mm 20 mm 1. 0 n. C 1. 6% 0. 72% 560 mm shorter bunch length and smaller energy spread in linac ease alignment tol. ’s 60 mm 8 mm 0. 2 n. C 1. 2% 0. 25%

Linac Alignment Effects 300 mm rms RF-structure and 200 mm rms quad/BPM misalignments, plus

Linac Alignment Effects 300 mm rms RF-structure and 200 mm rms quad/BPM misalignments, plus steering in 10 seeds 1. 0 n. C 0. 2 n. C Transverse wakes & dispersion errors vanish at 0. 2 n. C

LSC/CSR Micro-bunching Gain nominal 1 n. C, = 1£ 10 -4, Ipk = 3.

LSC/CSR Micro-bunching Gain nominal 1 n. C, = 1£ 10 -4, Ipk = 3. 4 k. A (final current) 200 p. C, = 1£ 10 -4, Ipk = 2 k. A, 3 k. A, & 5 k. A, adjusted with L 2 chirp and laser heater power Z. Huang

Ming Xie scaling of sliced S 2 E beam 200 p. C geth =

Ming Xie scaling of sliced S 2 E beam 200 p. C geth = 1 mm/mm Lsat < 75 m Psat > 10 GW

Sector-25 RF deflector still effective, but less so (shorter bunch) V = 25 MV

Sector-25 RF deflector still effective, but less so (shorter bunch) V = 25 MV y = 133 mm V=0 y = 67 mm z = 8 m m E = 5. 88 Ge. V Q = 200 p. C

Compress still further, until back to a 12% peak-current jitter L 2 D-phase =

Compress still further, until back to a 12% peak-current jitter L 2 D-phase = -1. 6 deg 3 k. A: Saturate at 87 m with ge 1. 15 mm ( b = 25 m) 200 p. C 3 k. A

Advantages for LCLS at Low Charge Drive-laser energy 1/10 Laser-heater power 1/4 BC 2

Advantages for LCLS at Low Charge Drive-laser energy 1/10 Laser-heater power 1/4 BC 2 CSR De 1/5 Linac quad/BPM align. tol. ’s 2 L 2 transverse wake De 1/16 BC dipole field quality 1/2 Peak current jitter 1/2 (…or X-band phase tol. 3) Final timing jitter 95 fs (was 120 fs) X-ray pulse 85 fs (was 210 fs) No undulator RW-wake (Cu, AC, cyl. ) FEL power ~10 GW & ~1012 photons Undulator radiation damage reduced? Dump power 1/5 (to 330 W, was 1700 W) Less loading eases multi-bunch operation 1 -n. C operation still fully supported option

Conclusions The 200 -p. C configuration is the preferred LCLS operating point! 1 -n.

Conclusions The 200 -p. C configuration is the preferred LCLS operating point! 1 -n. C is an alternate configuration with possibly a few times more photons, but is much more challenging on all fronts Diagnostics should emphasize 200 -p. C