P. Emma FAC Meeting 7 Apr Low-Charge LCLS Operating Point Including FEL Simulations P. Emma 1, W. Fawley 2, Z. Huang 1, C. Limborg 1, S. Reiche 3, J. Wu 1, M. Zolotorev 2 FAC Meeting April 7, 2005 LCLS [1]SLAC [2]LBNL [3]UCLA

P. Emma FAC Meeting 7 Apr Present LCLS Design Challenges Gun emittance: 1-  m at 1 nC, 100 A Resistive wake in undulator LSC/CSR micro-bunching Transverse wakes in L2 CSR  in BC2 too much charge AC resistive wakefield

P. Emma FAC Meeting 7 Apr wakefield- induced cubic chirp in L2-linac cubic chirp produces current horns after BC2 much of charge wasted with only 3.0 kA in core Nominal 1-nC case 1-nC, 20-  m rms bunch length is 6 kA for a Gaussian bunch pre-BC2 post-BC2 in FEL

P. Emma FAC Meeting 7 Apr Motivation: Less charge  less wake Same compression factor  ~same jitter Lower gun current  lower emittance Chosen Scaling: Charge: 1 nC  0.2 nC Gun Current: 100  30 A (10 ps  6.5 ps) Sliced gun emittance: 1  m  0.8  m Final current: 3 kA  2 kA (same L sat ) Low Charge Optimization

P. Emma FAC Meeting 7 Apr Ming Xie 87 m final slice  = 0.85  m

P. Emma FAC Meeting 7 Apr pC optimized Parmela output at 64 MeV 200k in lcls_200k_02nc_atendl01.dat C. Limborg, Oct. 19, 2004  th /r = 1  m/mm spot radius = 0.42 mm laser pulse = 6.5 ps FWHM

P. Emma FAC Meeting 7 Apr kA pre-BC2 post-BC2 in FEL 200 pC less cubic chirp spikes reduced 80-fs FWHM X-ray pulse

P. Emma FAC Meeting 7 Apr Much less CSR projected emittance growth 0.2 nC 1.0 nC  x  4  m  x  1  m  x  y BC2 BC2

P. Emma FAC Meeting 7 Apr  x,y (  m) 0.2 nC 1.0 nC Transverse Wakes & Dispersion Errors Vanish at 0.2 nC Linac Alignment Eased 300-  m struct  m quad 200-  m BPM steer 10 seeds rms errors:

P. Emma FAC Meeting 7 Apr LSC/CSR Micro-bunching Gain Reduced 1 nC,   = 1£10  4, I pk = 3.4 kA (final current) 1 nC,   = 1£10  4, I pk = 3.4 kA (final current) 200 pC,   = 1£10  4, I pk = 2 kA, 3 kA, & 5 kA, adjusted with L2 chirp and laser heater power 200 pC,   = 1£10  4, I pk = 2 kA, 3 kA, & 5 kA, adjusted with L2 chirp and laser heater power 1-nC nominal gain reduced

P. Emma FAC Meeting 7 Apr L2  RF =  1.6º  3 kA: L sat  87 m at   1.15  m,   25 m L2  RF =  1.6º  3 kA: L sat  87 m at   1.15  m,   25 m 200 pC 3 kA (RW-wake and  -bunching OK) Compress more, until back up to 12% peak-current jitter

P. Emma FAC Meeting 7 Apr Cylindrical-Copper Resistive Wakes in Undulator uses K. Bane damped resonator model for AC wake 1 nC 0.2 nC

P. Emma FAC Meeting 7 Apr pC FEL Simulations: Cu cyl. pipe ( r = 2.5 mm ) Data smoothed from raw ~12-as resolution to ~1-fs resolution Curve represents output power at z = 130 m For 300-kV ext. field + Cu wake case, agreement between 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

P. Emma FAC Meeting 7 Apr Uncompensated 200-pC Cu & Al wake gives lower power ~8-10X Gain length increased ~15% but saturation 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/Stupakov model Uncompensated 200-pC Cu & Al wake gives lower power ~8-10X Gain length increased ~15% but saturation 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/Stupakov model ~10 12 photons GINGER Results for 200-pC Pulse Energy vs. Z

P. Emma FAC Meeting 7 Apr Drive-laser energy  1/5 Laser-heater power  1/4 BC2 CSR   1/5 Linac quad/BPM align. tol.’s   2 L2 transverse wake   1/16 BC dipole field quality  1/2 Peak current jitter  ½ (or X-band  -tol.   3) Final timing jitter 95 fs (was 120 fs) X-ray pulse 85 fs (was 210 fs) No undulatorRW-wake (even for Cu, AC, cyl.) No undulator RW-wake (even for Cu, AC, cyl.) FEL power: GW & ~10 12 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 Advantages for LCLS at Low Charge Disadvantages: Requires 20% smaller gun slice emittance 8-  m bunch (  z ) more difficult to measure

P. Emma FAC Meeting 7 Apr Summary LCLS The 200-pC configuration is the preferred LCLS operating point (300 pC is similar) Resistive-wall wake, transverse linac wakes, and CSR all become ~non-issues 1-nC is an alternate configuration with possibly more photons, but more challenging on all fronts Diagnostics must emphasize 200-pC range