Diagnostics RF and Feedback Josef Frisch

Beam Diagnostics Position Measurements Profile measurements BPMs Profile measurements Fluorescent screens, Wire scanners or OTR screens Transverse measurements (emittance) Use multiple profile monitors, or Quad scans Longitudinal measurements Spectrometers, Millimeter wave bunch length monitors, Transverse Deflection Cavities

BPMS New EPICS controlled BPMS installed in LCLS injector and BC2 bunch compressor Uses continuous calibration

BPM Performance Noise can be measured by performing a linear regression to predict one BPM reading based on the other BPMS. Beam / BPM stability for 1 day at end of Injector. Approximately 15 um RMS, Beam size ~50um RMS at this location

New EPICS BPMs have excellent performance. BPM Performance New EPICS BPMs have excellent performance. Good Resolution No significant reliability issues. SLC BPMs still used for most of the linac Resolution OK: Readout through EPICS very slow (< 1Hz) Beam Feedback is difficult / impractical Readout rate thorough SLC system is OK

Plan to replace the SLC control system (VMS part) early ’09. SLC BPM Software Plan to replace the SLC control system (VMS part) early ’09. Expected to fix SLC BPM readout speed problems. SLC VMS provides high level control for many systems – may take time to get this working. Eventually will replace the SLC BPMs with new EPICS modules throughout the accelerator.

Beam Profile Measurements Fluorescent Screens Good sensitivity Saturate at high intensities (>0.04pC/um2) Typical LCLS 1nC, 50 micron spot is 10X this density Used in LCLS at 135MeV and below Wire Scanners Good resolution, Nearly non-invasive, work with high intensity beams Slow, and integrated profile only Mechanical vibration problems Used in LCLS at 135MeV and above Optical Transition Radiation Monitor Good resolution, work with high intensity beams Coherent effects limit use in LCLS (discussed later in this talk) Installed in LCLS at 135MeV and up, but only useable before first bunch compressor

Sample Images from YAG screens YAG image for 1nC beam at 6MeV Also possible to image cathode at low charge (30pC)

Wire Scans Scan before addition of 10X reducer gear Wire scan at 250MeV after first bunch compressor Asymmetric Gaussian fit (LCLS standard) Scan after addition of 10X reducer gear

OTR Foils: COTR Problems OTR after BC1, normal compression 250pC, upstream OTR foil inserted In compressor Chicane to spoil Longitudinal Phase space With upstream foil removed, signal Is saturated. Neutral density filters Give approximately 60M counts 10X increase ~60 Mcounts 5 Mcounts

Profile Monitor Status Fluorescent Screens Working well Only usable to 135 MeV due to saturation Wire Scanners Vibration problem fixed – at the expense of scan speed Work – but by their nature are slow, integrated profile only OTRs Coherent effects prevent quantitative measurements after BC1. Laser heater may fix COTR problem We have no quantitative profile monitors between the end of BC1 and the end of L3. 900 Meters of accelerator, and a bunch compressor. Makes linac / BC2 tuning very difficult. AIP to add new sector 24 wire scanners .

Automated Emittance Software Emittance Application Wire scanner application H. Loos

Emittance after first bunch compressor After first bunch compressor, ex=1.15, ey=1.02 at 1nC Set for normal compression Asymmetric Gaussian fit method Note that X emittances as low as 0.68 microns at 1nC have been measured. At 250pC, typical ex = 0.8, ey = 0.7 Measurements variable, strongly dependant on steering in X-band section (L1x)

Longitudinal Measurements - Energy Gun Spectrometer 6 MeV, fluorescent screen Injector Spectrometer, or DL 1 bend. 135 MeV, OTR screen Bunch Compressor 1 250 MeV, OTR screen Bunch Compressor 2 4.3 GeV, OTR screen – nearly unusable due to coherent effects End of linac-3 13.6 GeV, Fluorescent screen (installed for SLC)

Temporal Measurements LCLS has 2 transverse deflection cavities 135 MeV before DL1 bend 4.3 GeV after BC2 compressor (COTR -> use fluorescent screen). off-axis screen 2.4 m sy RF ‘streak’ V(t) S-band (2856 MHz) transverse RF deflector e- sz single-shot, absolute bunch length measurement

Bunch length after BC1 and BC2 Approximately 5 micron minimum bunch length observed, limited by TCAV / fluorescent screen resolution

Relative Bunch Length Measurement Need a non-invasive bunch length measurement. An electron bunch will radiate when is crosses an impedance change. Radiation will be coherent at wavelengths long compared to the electron bunch length. Monitoring the total radiated energy at wavelengths comparable to the bunch length will provide a RELATIVE measure of the bunch length for use in feedback.

BC2 Bunch Length Monitor Bunch length monitor signal for BC2 while compression is varied in L2. BC2 bunch length monitor similar to BC1 monitor except no focusing optics is used, detector is positioned directly above Silicon vacuum window.

Diagnostics Status BPMs Profile monitors Wire scanners Injector BPMS working fine! SLC BPM readout too slow New software should fix this Profile monitors Fluorescent screens OK Wire scanners OK OTR has COTR problem Expect Laser Heater to help Wire scanners Work fine after vibration fix with gear reducer. Emittance measurements working Longitudinal (energy, bunch length) measurements working Relative bunch length monitors working.

RF Systems: All Stations working!

RF Station noise OK (except glitches)

RF Phase distribution noise Saw correlation between RF phase in L2 linac and beam energy New RF distribution installed in April, PEPII phase shifter removed. Believe this has fixed the noise, but not as easy to measure correlation

RF Phase Drifts RF phases drift at the ~ few degree S-band level. Beam based feedbacks used to correct for phase drifts Works in injector. Works for L2 to hold energy and bunch length in BC2 Average phase in L2 is controlled, but distribution of energy gain may change if the phase reference drifts: may have some effect on lattice.

RF Reliability While the RF systems are in general reliable, the large number of stations results in fairly frequent drop-outs, maybe 10/day. A RF station drop-out frequently requires re-adjustment of the lattice and beam compression. This can require significant beam time for retuning. Need to improve the reliability of RF stations OR Need software to automatically re-adjust the accelerator after a RF station failure.

Beam Based Feedbacks – Longitudinal Control energy in DL1 with L0B amplitude Control energy in BC1 and bunch length after BC1 (measured with pyro detector) using L1S phase and amplitude Orthogonal combination of phase and amplitude to control energy and bunch length Control energy and bunch length in BC2 using phases of 2 stations (24-1, 24-2). Single 5X5 feedback loop. Loop works – response a bit slow, can get lost

Beam Based Feedbacks - Transverse Cathode steering: Center laser beam on RF gun cathode. Gun Launch: Center beam on BPM between L0A and L0B Injector Launch: Position and angle through L0B, and around DL1 X cavity launch: Position in X-band cavity (important for emittance) 4 independent feedbacks running. No significant problems In future should either combine or cascade for higher bandwidth

EPICS / MATLAB Feedbacks

OLD SLC Feedbacks L2: Beam orbit in sector 23, using correctors L3: Beam orbit in sector 26, using correctors BSY: Beam energy at end of Linac using phases of 2 sectors. Feedbacks run at full rate in ancient SLC micros. Will need to write new feedbacks when VMS system or SLC BPMs are replaced.

Overall Status Diagnostics in good shape – except coherent problems with OTRs. Hope to fix with laser heater. Many discussions with other R+D groups 3 lab collaboration at Spring 8, Beam instrumentation workshop, Zeuthen high brightness beams workshop. RF systems working well. Think that noise problems with RF distribution now solved. Drift mostly fixed by beam feedback – but needs study. Need a solution to occasional station drop-outs Beam feedbacks working Longitudinal feedbacks somewhat slow Need to replace SLC feedbacks when VMS system is removed.