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)FEL intensity [mJ] is suppressed at the shortest

bunchlengths (highest peak current) we canrun under- or over-compressed

Emittance growth due to CSR in BC2(is there a cure?)

Under - compressed

Over -

com

pres

sed

Charge: 1nC 250 pC 150 pCInstead 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

Tolerance Budgets, e.g.: long. Jitter

L1S: 0.05 need 0.03

DE/E: 0.04%

Need even better for certain experiments

XX--bandband XX--

0.500.50

XX--bandband XX--XX--bandband XX--

0.500.50

Energy jitter bigger than spreadJitter: 4.0E-4 Intensity after

monochromatorSpread: 3.3E-4 varies 100%

Simulated FEL through MonochromatorEnergy spread: 0.02% Mono width:

0.006% hard edge Energy jitter: 0.04% Intensity jitter:

20% (blue)

New jitter compensation ideas

Over-compressed: Stable areaOver-compression cancels L1S

jitter: more negative phase in L2give 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 overcompressed

From XPP thru Mono:No FEL intensity and very high

spikes got eliminated

Before

D. Fritz

No intensity

FEL spikes

Always some intensity

Barely FEL spikes

taper

Tor’s ListHe 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

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

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 function18-April: DL2 Dump Loss Matlab PV 562 2-May: I_pk correction: 14 MeV/kA24-May: E Loss GUI 8-Aug: Gas detector calibration

Other Linac Issues by RegionEarly 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, …

SummaryLinac 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 FELTor’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