Transcript RF Systems, Diagnostics, and Feedback

RF Systems, Diagnostics and
Feedback
Josef Frisch
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
RF Systems: All Stations working!
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
Beam Timing Jitter (relative to TCAV) ~50 fsec.
TCAV produces Y kick that
depends on beam arrival
time realtive to RF phase
Calibration: 2.34mm Y / ps
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
RF Phase Drifts
RF phases drift at the ~ few degree S-band
level.
Beam based feedbacks used to correct for
phase drifts
Cannot fix L1X phase
Fixes average of L2 linac phase, but can still
have drift of individual stations or sectors
Machine can run stably for days.
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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 readjustment of the lattice and beam compression.
This can require significant beam time for retuning.
Injector RF reference system has several RF
dividers that can hop phase
Automated software exists to fix most, but not all phase
jumps.
Manual fixes quite time consuming.
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
Beam Diagnostics
Position 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
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
BPMS
New EPICS controlled BPMS installed in LCLS injector
and BC2 bunch compressor
Uses continuous calibration
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
BPM Performance
Noise can be measured by performing
a linear regression to predict one BPM
reading based on the other BPMS.
June 17, 2008
RF, Diagnostics and Feedback
Beam / BPM stability for 1 day at
end of Injector. Approximately
15 um RMS, Beam size ~50um RMS
at this location
Josef Frisch
[email protected]
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
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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.
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
Sample Images from YAG screens
YAG image for 1nC beam at 6MeV
Also possible to image cathode at low charge (30pC)
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
Wire Scans
Scan before addition of
10X reducer gear
Scan after addition of
10X reducer gear
June 17, 2008
RF, Diagnostics and Feedback
Wire scan at 250MeV after first bunch compressor
Asymmetric Gaussian fit (LCLS standard)
Josef Frisch
[email protected]
OTR Foils: COTR Problems
OTR after BC1, normal compression
250pC, upstream OTR foil inserted
In compressor Chicane to spoil
Longitudinal Phase space
5 Mcounts
June 17, 2008
RF, Diagnostics and Feedback
With upstream foil removed, signal
Is saturated. Neutral density filters
Give approximately 60M counts
10X increase
~60 Mcounts
Josef Frisch
[email protected]
COTR has now been observed at other labs
COTR spikes at APS Injector, at 2X
minimum bunch length
COTR at DESY/ FLASH at “max”
compression
Lumpkin et. al. Zeuthen workshop April 08.
Not yet seen at Spring-8 / SCCS
Much discussion of possible options, but so far no solid ideas except Laser Heater
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
No one sees it at the level we do!
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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 .
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
Automated Emittance Software
Emittance Application
Wire scanner application
H. Loos
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
Emittance at end of Linac 0.5nC
Ex = 1.33, Ey = 0.96. 0.5nC,
normal compression 12.6 um
bunch length.
Measurements slow since we
are using wire scanners.
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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)
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
Temporal Measurements
LCLS has 2 transverse deflection cavities
135 MeV before DL1 bend
4.3 GeV after BC2 compressor (COTR -> use fluorescent screen).
2.4 m
V(t)
e-
sz
RF off-axis screen
‘streak’
sy
S-band (2856 MHz)
single-shot, absolute bunch
transverse RF deflector
length measurement
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
Bunch length after BC1 and BC2
Approximately 5 micron minimum bunch
length observed, limited by TCAV /
fluorescent screen resolution
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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.
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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.
Bunch length monitors now
calibrated in approximate peak
Amps.
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
Diagnostics Status
BPMs
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.
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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 – runs for days with minimal problems!
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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 near future will add a steering loop at the entrance to L2
In future should either combine or cascade for higher
bandwidth
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
EPICS / MATLAB Feedbacks
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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.
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]
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.
Excellent phase noise.
Drift mostly fixed by beam feedback – but needs study.
Need a solution to occasional station drop-outs, and loss of lock in
injector.
Beam feedbacks working
Longitudinal feedbacks somewhat slow
Need to replace SLC feedbacks when VMS system is removed.
June 17, 2008
RF, Diagnostics and Feedback
Josef Frisch
[email protected]