Accelerator Physics Considerations for the LCLS Linac Paul Emma, SLAC April 23, 2002 Design of Compression and Acceleration Systems Technical Challenges Full System Simulations LCLS LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

use 2 compressors, 3 linacs Nominal System Design 1.5-Å SASE FEL Linac: Requirements Acceleration to 14.3 GeV (4.5 GeV min.) Bunch compression to 3.4 kA (for gex,y  1.2 mm) Emittance preservation (<20% ‘slice’, <100% ‘projected’) Final energy spread (<0.02% ‘slice’, <0.1% ‘projected’) Minimal sensitivity to system ‘jitter’ Diagnostics integrated into optics Flexible operations (15 Å, low-charge, chirp, etc.) use 2 compressors, 3 linacs LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Nominal System Design Constraints 1 km Use existing SLAC linac compatible with PEP-II operation Use existing ‘FFTB’ hall for undulator 1 km LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

2 Design Optimization Fast, computer model used to optimize design Set final e- parameters (E, sE/E, Ipk, etc.), minimize sensitivity (e.g., gun-timing  Ipk), find best RF phases, chicane strengths and locations, with reasonable constraints (RF power., optics, etc.) |R56|i < lim. |fRF|i < lim. N sz0 E0 2 R56 (BC1, BC2) fRF (L1, L2, L3) Ei (BC1, BC2) |dI/dt| < lim. |dE/dQ| < lim. explore alternate parameter sets (15 Å, low-charge, chirp, etc.) LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Nominal LCLS Linac Parameters for 1.5-Å FEL Single bunch, 1-nC charge, 1.2-mm slice emittance, 120-Hz repetition rate… SLAC linac tunnel FFTB hall Linac-0 L =6 m Linac-1 L =9 m rf = -38° Linac-2 L =330 m rf = -43° Linac-3 L =550 m rf = -10° BC-1 R56= -36 mm BC-2 L =22 m R56= -22 mm DL-2 L =66 m R56 = 0 DL-1 L =12 m R56 0 undulator L =120 m 7 MeV z  0.83 mm   0.2 % 150 MeV   0.10 % 250 MeV z  0.19 mm   1.8 % 4.54 GeV z  0.022 mm   0.76 % 14.35 GeV   0.02 % ...existing linac new rf gun 25-1a 30-8c 21-1b 21-1d X Linac-X L =0.6 m rf=180 21-3b 24-6d (RF phase: frf = 0 at accelerating crest) LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

after BC1 after X-RF after L1 after DL1 after BC2 after L3 at und. energy profile phase space time profile sz = 830 mm sz = 190 mm sz = 830 mm sz = 23 mm sz = 830 mm sz = 23 mm FINAL sz = 190 mm sz = 23 mm LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Machine Flexibility Operation at 0.2 to 1.0 nC and constant 87-m saturation length at 1.5 Å in/output beam linac setup jitter LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

LCLS Technical Challenges Coherent Synchrotron Radiation in Bends projected emittance growth micro-bunching instability Emittance Preservation in Linacs transverse wakefields misalignments & chromaticity Machine Stability jitter tolerance budget simulation of budget LCLS LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Magnetic Bunch Compression DE/E z 2sz0 DE/E z DE/E 2sz Under-compression Over-compression V = V0sin(wt) RF Accelerating Voltage Dz = R56DE/E Path Length-Energy Dependent Beamline LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Coherent Synchrotron Radiation (CSR) Powerful radiation generates energy spread in bends Induced energy spread breaks achromatic system Causes bend-plane emittance growth (short bunch is worse) coherent radiation for l > sz bend-plane emittance growth sz l L0 s DE/E = 0 Dx e– R  DE/E < 0 overtaking length: L0  (24szR2)1/3 Dx = R16(s)DE/E LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

CSR Micro-bunching* S. Heifets, S. Krinsky, G. Stupakov, SLAC-PUB-9165, March 2002 CSR can amplify small modulations on bunch current  Successive bend-systems cause micro-bunching  Growth of slice-emittance. without SC-wiggler sd  310-6 avoid! 230 fsec R. Carr Super-conducting wiggler prior to BC2 increases uncorrelated E-spread (310-6  310-5) SC-wiggler damps bunching sd  310-5 * First observed by M. Borland (ANL) in LCLS Elegant tracking LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

CSR in BC2 Chicane (animation through chicane) DE/E0 f(s) gex LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

CSR Micro-bunching and Projected Emittance Growth 14.3 GeV at undulator entrance x versus z without SC-wiggler 230 fsec projected emittance growth is simply ‘steering’ of bunch head and tail ‘slice’ emittance is not altered 0.5 mm x versus z with SC-wiggler Workshop in Berlin, Jan. 2002 to benchmark results (www.DESY.de/csr/) – follow-up meeting in summer LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

X-band RF used to Linearize Compression (f = 11.424 GHz) S-band RF curvature and 2nd-order momentum compaction cause sharp peak current spike X-band RF at decelerating phase corrects 2nd-order and allows unchanged z-distribution 1  -40° x = p Slope linearized lx = ls/4 avoid! 0.6-m section, 22 MV available at SLAC (200-mm alignment) LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Transverse Wakefields and Component Misalignments transverse wake effect on projected emittance corrected trajectory y correctable emittance growth x y 300 mm rf-structure 150 mm quad/BPM experience at SLC… gey  2 mm @ 1 nC beam-based align correction bumps… x L2 phase adv/cell optimized sz = 195 mm wakes off wakes on L3 phase adv/cell optimized sz = 22 mm wakes on off also misaligned quads/BPMs generate dispersion  De LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Correction of Wakefield Emittance Growth Measure emittance at end of linac Launch b-oscillations of various amplitudes & phases Empirically minimize gex,y Well tested at SLC* LCLS L2 sim. with Liar SLC gey = 2-3 mm @ 1 nC with 3-km linac and 1-mm bunch length * J.T. Seeman et al., 15th International Conf. on High Energy Accelerators, Hamburg, Germany, July 1992. LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Emittance and Energy Spread Diagnostics* 4 energy spread meas. stations (optimized for small bx) 5 emittance meas. stations designed into optics (Dyx,y) slice measurements possible with transverse RF (L0 & L3) 3 prof. mon.’s (Dyx,y = 60°) ...existing linac L0 rf gun L3 L1 X L2 gex,y sE * see also D. Dowell and P. Krejcik talks LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Transverse RF deflector as diagnostic* sz V(t) S-band sx RF ‘streak’ V0 = 0 230 fsec LCLS simulation long. phase space * see P. Krejcik talk Built & used at SLAC in 1960’s meas. bunch length & slice emittance V0 = 20 MV meas. longitudinal phase space y = kt [mm] x = hDE/E [mm] LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Machine Stability Final peak current & energy jitter caused by variations of… bunch charge gun-laser timing RF phases RF voltage chicane supplies Need feedback for time scales >1 min. Design is optimized, but… system jitter tol.- budget is needed LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Jitter Budget (<1 minute time-scale) measured RF performance klystron phase rms  0.07° (20 sec) klystron ampl. rms  0.06% (60 sec) LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Start-to-End Tracking Simulations Track entire machine to evaluate beam brightness & FEL Track machine many times with jitter to test stability budget (M. Borland, ANL) Parmela Elegant Genesis space-charge compression, wakes, CSR, … SASE FEL with wakes LCLS LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Initial Beam from Parmela Tracking (C. Limborg) 1 nC 10-psec FWHM 0.7-ps rise/fall 120 MV/m gun getherm  0.3 mm 150 MeV gex,y < 1 mm gex gey 2105 to 2106 macro-particles z, DE/E x, y LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Sliced e- Beam to Evaluate FEL (Dz  0.7 mm) After full system tracking (also see talk by S. Reiche) gex gey mismatch amplitude variation zx slice 4D centroid osc. amplitude zy Lg < 4 m LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

Machine Stability Simulations Track LCLS 230 times with Parmela Elegant Genesis Include wakes, CSR, etc. + jitter budget (M. Borland, ANL) Lg Ipk gex Pout LCLS DOE Review, April 23, 2002 Paul Emma, SLAC

LCLS Summary Linac design well optimized for nominal 1.5-Å operation Design is flexible to accommodate 15-Å, low-charge, & chirp CSR growth of projected emittance – not slice Much experience on SLAC linac with wakefield control Beam diagnostics built into design Full system tracking to… Evaluate e- brightness preservation, Calculate SASE gain, Simulate pulse-to-pulse stability. LCLS Full tracking confirms saturation at 1.5 Å LCLS DOE Review, April 23, 2002 Paul Emma, SLAC