Low-Charge Linac Operating Point Including Recent FEL Simulations

1. Low-Charge Linac Operating Point Including Recent FEL Simulations P. Emma, W. Fawley, Z. Huang, C. Limborg, S. Reiche, J. Wu, M. Zolotorev SLAC/LBL/UCLA LCLS week, January 26, 2005

2. Present LCLS Design Problems • Gun emittance: 1-mm at 1 nC, 100 A • Resistive wake in undulator • LSC/CSR micro-bunching • Tight RF jitter tolerances • Transverse wakes in L2 • CSR De in BC2 too much charge AC resistive-wall wakefield

3. wakefield-induced cubic chirp in L2-linac Nominal 1-nC case pre-BC2 post-BC2 cubic chirp produces current horns after BC2 in FEL 1-nC, 20-mm rms bunch length is 6 kA for a Gaussian much of charge wasted with only 3.0 kA in core

4. Low Charge Optimization • Motivation: • Less charge less wake • Same compression factor ~same jitter • Low gun current low emittance • Chosen Scaling: • Charge: 1 nC0.2 nC • Gun Current: 10030 A (10 ps6.5 ps) • Slice emittance: 1 mm 0.8 mm • Final current:3 kA 2 kA (same Lsat)

5. Ming Xie

6. 200-pC optimized Parmela output from Cecile at end of L0-a section (64 MeV) gethermal = 1 mm/mm gethermal = 1 mm/mm spot radius = 0.42 mm laser pulse = 6.5 ps Thermal emittance: 1 mm per mm radius 200k in lcls_200k_02nc_atendl01.dat C. Limborg, Oct. 19, 2004

7. 200 pC pre-BC2 less cubic chirp post-BC2 spikes reduced 2 kA in FEL

8. Much less CSR projected x-emittance growth gex 4 mm gex 1.0 nC gey BC2 0.2 nC gex gex 1 mm gey BC2

9. Linac Alignment Effects Eased 1.0 nC 300 mm rms RF-structure & 200 mm rms quad/BPM misalignments, plus steering over 10 seeds 0.2 nC Transverse wakes & dispersion errors vanish at 0.2 nC

10. LSC/CSR Micro-bunching Gain nominal 1 nC, = 1£10-4, Ipk = 3.4 kA (final current) 200 pC, = 1£10-4, Ipk= 2 kA, 3 kA, & 5 kA, adjusted with L2 chirp and laser heater power gain reduced!

11. 200 pC Compress still further, until back up to a 12% peak-current jitter L2 D-phase = -1.6 deg 3 kA: Saturate at 87 m with ge 1.15 mm (b = 25 m) 3 kA (RW-wake and m-bunching OK)

12. Cylindrical-Copper Resistive-Wall Wakes in FEL Undulator 0.2 nC 1 nC uses K. Bane damped resonator model for AC wake

13. 200-pC FEL Simulations – Cu pipe • Data is smoothed from raw ~12-as resolution to ~1-fs resolution • Curve represent output power at z=130 m • For 300-kV ext. field + Cu wake case, agreement between the codes is good in overall temporal dependence with GINGER showing ~1.5X greater power than GENESIS • Compared to 1-nC case, lasing occurs over full 200-pC pulse

14. GINGER Results for 200-pC Pulse Energy vs. Z ~1012 output x-ray photons • Uncompensated 200-pC Cu & Al wake lower power ~8-10X • Gain length increased ~15% but sat. point unaffected • External field of ~150 kV/m makes up for wake • Increasing ext. field to +300 kV/m nearly doubles power over no wake case – agreeing with Huang and Stupakov prediction

15. Advantages for LCLS at Low Charge • Drive-laser energy  1/5 • Laser-heater power  1/4 • BC2 CSR De 1/5 • Linac quad/BPM align. tol.’s  2 • L2 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 undulatorRW-wake (Cu, AC, cyl.) • FEL power ~20 GW & ~1012 photons • Undulator radiation damage reduced? • Dump power 1/5 (to 330 W, was 1700 W) • Less loading eases multi-bunch operation • 1-nC operation still fully supported option

16. Conclusions • The 200-pC configuration is the preferred LCLS operating point (300 pC is OK too)! • 1-nC is an alternate configuration with possibly somewhat more photons, but is more challenging on all fronts • Diagnostics should emphasize 200-pC