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Linac Lessons Learned from LCLS-I

Linac Lessons Learned from LCLS-I. Franz-Josef Decker 14-Dec-2011 CSR is a big deal limiting performance Tolerance studies beforehand helped a lot Tor’s List Software adaptability helped speed up commissioning . CSR (Coherent Synchrotron Radiation).

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Linac Lessons Learned from LCLS-I

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  1. Linac Lessons Learned from LCLS-I Franz-Josef Decker 14-Dec-2011 CSR is a big deal limiting performance Tolerance studies beforehand helped a lot Tor’s List Software adaptability helped speed up commissioning

  2. CSR (Coherent Synchrotron Radiation) • FEL intensity [mJ] is suppressed at the shortest bunch lengths (highest peak current) we can run under- or over- compressed • Emittance growth due to CSR in BC2 (is there a cure?) Over - compressed Under - compressed

  3. Charge: 1nC  250 pC  150 pC • Instead of raising charge (like SLC or PEP-II) LCLS runs “better” with lower charges, same FEL intensity with lower charge and shorter FEL pulses • 1 nC was abandoned early on, emittance growth to Li28 • 250 pC had (has) double horizontal FEL spot and 150 pC is cleaner and more stable • User are asking for even lower charges (80,) 40, 20 pC for shorter pulses lengths

  4. Tolerance Budgets, e.g.: long. Jitter • L1S: 0.05 need 0.03 • DE/E: 0.04% • Need even better for certain experiments

  5. Energy jitter bigger than spread • Jitter: 4.0E-4 Intensity after monochromator • Spread: 3.3E-4 varies 100%

  6. Simulated FEL through Monochromator • Energy spread: 0.02% Mono width: 0.006% hard edge • Energy jitter: 0.04% Intensity jitter: 20% (blue)

  7. New jitter compensation ideas • Over-compressed: Stable area • Over-compression cancels L1S jitter: more negative phase in L2 give lower E, but also longer bunch length (less WF) so more E: 0.045-0.050% to 0.030-0.035% • Less taper (by Ops) reduces FEL intensity spikes under over compressed

  8. Before No intensity From XPP thru Mono: D. Fritz FEL spikes • No FEL intensity and very high spikes got eliminated taper Always some intensity Barely FEL spikes

  9. Tor’s List • He would love to hear about anything that impacts the design of BC1/BC2, the phase control, klystron com-plement, diagnostics, and beam operational modes. • BC1: Energy change: 250  220 MeV for less L1S jitter (un-SLEDed or lower modulator voltage) • BC2: Energy change: 4.5  4.7  5.0 GeV gives higher end energy (sometimes less chirp possible) • Phase control: new PACs and PADs are good, but not all klystrons have this control. New PACs for each SBST would help some multi-bunch issues • Klystron complement: no issues

  10. Tor’s List cont. • Diagnostics: BPMs good, Beam phase somewhat noisy (online), wire scanner o.k., but slow (except 10 sec version with bumps), (Li24 wires never installed), OTRs in BCs used in linac, BLMs (length) have filter issues (mirco-bunching) are calibrated with TCAV3, BLM after DL2 still missing • Beam operational modes: Energy change (20 min), charge change (1 hr), peak current change(2 min), rate change, … all routine by now with GUIs • CQs (dispersion Correction Quads) in BC1, BC2, DL2 also change the betatron match, absolute DL2 energy is also influenced; dispersive bumps might be better

  11. Software adaptability helped speed up commissioning, e.g. E-Loss GUI • 10-April-2009: First Lasing (YAGXRAY max_out) • 15-April: 1.3 mJ “possible” energy loss due to FEL • 17-April: DL2toDumpEnergyLoss function • 18-April: DL2 Dump Loss Matlab PV 562 • 2-May: I_pk correction: 14 MeV/kA • 24-May: E Loss GUI • 8-Aug: Gas detector calibration

  12. Other Linac Issues by Region • Early Linac (Injector): MCORs jitter near zero, RF kick jitter at load end if multi-pacting or not (L0A), quads 3% weaker if near other magnets, source of micro-bunching(?), x-band: wakefield + dispersion + rf-kick, lattice good for diagnostic, but (maybe) not for beam (space charge at focus in OTR2?), good BC bend magnets necessary, L1S stability, … • L2: wakefields in early L2 affect emittance, matching VERY critical for FEL performance, BC2 dispersion fix, … • L3: matching in Li26 (?), emittance growth with shorter bunches (Li28), Li30 match into BSY, …

  13. Summary • Linac performs well for FEL operation • Coherent effects limit higher FEL intensity (the beam wants to radiate before reaching the undulator) • Jitter, especially in the longitudinal (30 times shorter), is a real challenge • But soon energy jitter reaching levels of intrinsic SASE FEL • Tor’s List: Leave it flexible, good and enough diagnostics, new PAC (Phase & Amplitude Control) per sector • Software adaptability is good (inside AD), connections to photon side are a pain

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