LCLS Longitudinal Feedback and Stability Requirements P. Emma LLRF Review November 23, 2005 LCLS
Critical LCLS Accelerator Parameters Final energy 13.6 GeV (stable to 0.1%) Final peak current 3.4 kA (stable to 12%) Transverse emittance 1.2 mm (stable to 5%) Final energy spread 10-4 (stable to 10%) Bunch arrival time (stable to 150 fs) (stability specifications quoted as rms)
FEL Power Sensitivity to e- Beam 12% DIpk/Ipk 20% DP/P 0.1% DE/E 0.2% Dlr/lr
Electron Bunch Compression d DE/E d d under-compression szi ‘chirp’ z z z sz sdi V = V0sin(kz) Dz = R56d RF Accelerating Voltage Path-Length Energy- Dependent Beamline
Compression Stability d Df d z RF phase jitter becomes bunch length jitter… Compression factor:
Phase and Bunch Length Stability Example (not LCLS)
Machine Schematic with Parameters 6 MeV z 0.83 mm 0.05 % 135 MeV z 0.83 mm 0.10 % 250 MeV z 0.19 mm 1.6 % 4.30 GeV z 0.022 mm 0.71 % 13.6 GeV z 0.022 mm 0.01 % Linac-X L =0.6 m rf= -160 rf gun 23-m Linac-1 L 9 m rf -25° Linac-2 L 330 m rf -41° Linac-3 L 550 m rf 0° Linac-0 L =6 m undulator L =130 m ...existing linac 21-1b 21-1d X 21-3b 24-6d 25-1a 30-8c BC1 L 6 m R56 -39 mm BC2 L 22 m R56 -25 mm DL1 L 12 m R56 0 LTU L =275 m R56 0 1 X-klys. 3 klystrons 1 klystron 26 klystrons 45 klystrons SLAC linac tunnel research yard
Correlated or Uncorrelated Errors? Suppose the mean RF phase of all 26 Linac-2 klystrons changes by: 0.21° |DIpk/Ipk| 12% This may arise statistically with 26 random uncorrelated phase errors with rms spread of: f21/2 = 0.21°261/2 = 1.07°, or with 26 identical phase errors. Since we don’t fully understand the correlations, we choose the conservative (smallest) tolerance of 0.21° rms/klys. and then reduce this by ~N, where N (=12) is the number of major error sources.
Phase, Amplitude, and Charge Sensitivities parameter |DE/E0| = 0.1% |DI/I0| = 12% |Dtf| = 100 fs unit Dti 1.6 4.4 1.5 psec DQ/Q0 46 5.2 24 % Df0 3.5 0.65 5.9 deg-S DV0/V0 0.32 0.24 0.95 Df1 0.17 1.0 DV1/V1 0.29 0.25 0.78 DfX 5.5 1.4 7.6 deg-X DVX/VX 2.0 1.2 6.3 Df2 0.54 0.21 0.084 DV2/V2 1.1 0.13 Df3 0.35 24.8 15 DV3/V3 0.15 5.7 8.6
Longitudinal Fast-Jitter Tolerance Budget tolerances are rms values X-band X- 0.50 laser timing (w.r.t. RF) laser energy mean phase of 2 klys. 1 klys. 1 X-klys. mean phase of 26 klys. mean phase of 45 klys. mean amp. of 2 klys. 1 klys. 1 X-klys. mean amp. of 26 klys. mean amp. of 45 klys.
Jitter Simulations (Particle Tracking) 0.09% 0.004% Lg 96 fs Pout 10%
LCLS Longitudinal Beam-Based Feedback (stabilizes beam for jitter frequencies < 10 Hz @ 120-Hz rep-rate) L0 gun L3 L2 X DL1 BC1 BC2 DL2 L1 sz1 d1 1 V1 sz2 d2 2 V2 d3 V3 d0 V0 BPM CSR detector J. Wu, et al., PAC’05, May 16-20, 2005, Knoxville, TN.
CSR Relative Bunch Length Monitor Red curve: Gaussian Black curve: Uniform Blue curve: ‘Real’ J. Wu, et al., PAC’05, May 16-20, 2005, Knoxville, TN.
LCLS Feedback Performance (use CSR P/P) feedback off DIpk/Ipk0 (%) feedback on J. Wu (undulator entrance)
Feedback System Bode Plot at 120 Hz J. Wu Define fast-jitter as variations faster than 2 seconds Slow drift occurs on time-scales > 2 seconds (to 24+ hr)
Slow Drift Tolerance Limits (Top 4 rows for De/e < 5%, bottom 4 limited by feedback dynamic range) Gun-Laser Timing 2.4* deg-S Bunch Charge 3.2 % Gun RF Phase 2.3 Gun Relative Voltage 0.6 L0,1,X,2,3 RF Phase (approx.) 5 L0,1,X,2,3 RF Voltage (approx.) (Tolerances are peak values, not rms) * for synchronization, this tolerance might be set to 1 ps (without arrival-time measurement)
L1 adjustment: phase +2.1o, voltage -2.1% Compensate X-band Phase Step Error... jx (deg) x-band phase LX phase error = 5o final energy final peak current L1 adjustment: phase +2.1o, voltage -2.1% final arrival time J. Wu
Gun Timing Jitter and Energy Feedback Dtf Dt0 Dt0 E > E0 E = E0 Dtf = Dt0 without energy feedback with energy feedback