1. Diagnostics RF and Feedback
Josef Frisch

2. 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

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

4. 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

5. 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

6. 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.

7. 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

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

9. 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

10. 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

11. 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 L Meters of accelerator, and a bunch compressor.
Makes linac / BC2 tuning very difficult.
AIP to add new sector 24 wire scanners .

12. Automated Emittance Software
Emittance Application
Wire scanner application
H. Loos

13. 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)

14. 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)

15. 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

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

17. 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.

18. 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.

19. 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.

20. RF Systems: All Stations working!

21. RF Station noise OK (except glitches)

22. 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

23. 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.

24. 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.

25. 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

26. 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

27. EPICS / MATLAB Feedbacks

28. 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.

29. 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.