Free Electron Laser Commissioning Meeting
Friday, 8 May 98
Recorder: C. Bohn
Next Meeting
- Date: 15 May 98
- Time: "0845"-0945
- Place: FEL Facility Break Room
Agenda for Next Meeting
Item Person Responsible Time
---- ------------------ --------
Status of Open Action Items All 5 min
Scheduling and Commissioning Program Bohn/All 50 min
New Issues All 5 min
This Week's Attendees
J. Bennett, S. Benson, C. Bohn, J. Boyce, D. Douglas, F. Dylla, J. Fugitt,
A. Hofler, C. Hovater, R. Lauze, R. Li, L. Merminga, G. Neil, M. Shinn,
Q.-S. Shu, T. Siggins, R. Walker, B. Yunn
Closed Action Items
- Walker/Bohn: Ascertain need and develop guidance for routine checks of
arc detectors and BLMs.
- Status: Operators have been instructed to test
BLMs and check cryounit arc detectors once per shift. G. Lahti is
scheduled Monday, 11 May 98, to upgrade software to automate checks of the
arc detectors.
Items of Discussion
This week's activities centered on continued commissioning of
diagnostics, including development of procedures to use them, additional
difference-orbit measurements, and developing a reproducible injector-setup
procedure that involves diagnostics downstream of the injector. None of
these activities are complete yet, but progress was made in all of them.
In addition, one shift was used to devise a BLM setup that would permit
operating the Happek interferometric bunch-length monitors as well as to
recheck the BLM settings for cw running. Regarding the latter, the voltage
of the BLM at the straight-ahead dump was adjusted, but otherwise the
existing setup looked appropriate.
One complication is a persistent drift in the rf phase of the drive
laser that began to be observable about two weeks ago. The drift causes the beam
properties to change concomitantly, and it must be eliminated prior to
trying to lase. We believe that it was introduced as a consequence of some
hardware changes in the drive-laser rf system, and we have been trying to
track down the cause using well-defined test plans. At this writing (1115,
8 May 98) we still had not done so definitively, but we have suspected
causes that we will check over the next few days.
C. Bohn summarized the status of the electron-beam diagnostics as
follows:
- MaxVideo works for beam size measurements. However, on several of the
viewers noise and dead pixels prevent its use because the signal-to-noise
ratio is too low.
- Current monitor cavity works and is in constant use.
- BPMs work; 2 cans (1 injection line, 1 linac) are bad but can reproduce
orbits. One of the bad cans is the key BPM in front of the wiggler, and it
needs to be replaced to enable setup for lasing. Replacement is scheduled
to occur tomorrow (Saturday). We don't yet know whether the BPMs have the
resolution or stability needed for FEL setup.
- Multislits work, including analysis software; we are deciding on the
best way to use them. Neither of the multislits presently gives an
emittance measurement in the standard beam setup. It may be possible to
get the needed measurements by altering quad settings and then backing out
the numbers, and we are presently working on the methodology for doing so.
- Quad/viewer works; we need the final setup for good numbers. One
complication is that we may need to turn off the raster magnets at the
straight-ahead dump to use the last viewer. The overall procedure is not
yet finalized, but it is in process of being developed.
- Multimonitor works, but the uncertainty in the measurements is presently
too large (±50%) for the data to be useful. More refinement is needed.
- M_55 cavities work as bunch-length monitors; the associated software has
been checked.
- Zero phasing works, but we need good steering through the cryomodule
cavities to get good data. There is presently no approach that is
reproducible operator to operator at the needed accuracy, so work on the
technique continues.
- Happek is scheduled to be here from 10-13 May 98. We will install his
hardware during day shift Monday and test it during swing and owl shifts
Monday and Tuesday nights next week.
As a general remark, phase-space parameters that have been measured
thus far generally fall within a factor 2-3 (far less than an order of
magnitude!) from those required for lasing. Of course, if all parameters
were double the requirement, no lasing could happen. The point is that the
data continue to suggest we're within reach but need to continue
fine-tuning the beam before installing the wiggler.
G. Neil reported that significant progress was made toward setting
up the FEL optical systems. Optical transport diagnostics were completed, aligned
and put under vacuum, and they were used to take a surrogate HeNe beam from
the FEL to the diagnostics table in the optical control room. Patch panels
to the optical control room were installed and fully wired.
Fiducialization of the first-light mirrors was completed. The
mirror-control software for save/restore was verified. The cavity mirror
alignment as viewed through position sensors drifted by less than 10 µrad
over several days, well within specification.
More discussion about the always popular topic of configuration control
arose. D. Douglas pointed out that he has difficulty iterating toward an
optimal orbit when the machine setup keeps changing. Bohn reviewed the
configuration control policies instituted to date, reiterated that the
present Golden File is Douglas's file from last Wednesday, and explained
once again how the planned shift activities are designed to converge on a
usable setup for lasing. The process should get easier, and the procedures
should get firmer, once the machine is "complete", meaning to zeroth order
that the Happek monitors are installed and checked out, and that is
scheduled for the beginning of next week. Bohn is totally open to ideas on
how to do things better and more efficiently. He also will soon convene a
meeting to discuss elements of the procedure for setting up the
longitudinal phase space.
R. Lauze asked that we prepare a glossary of FEL terminology as an
aid to CEBAF operators. New hires should be showing up soon, and they will
benefit from this head start. R. Walker will begin compiling the glossary.
M. Shinn presented a synopsis of measurements of drive-laser noise.
Her principal observations are:
- The drive laser performs best when running on its internal rf system.
Performance is comparable to the required value for (FEL) lasing: <3% p-p
at 10 Hz > f > 50 kHz, but worse than measured when the laser was new (1.3%
p-p on 3/30/95).
- When running on external rf, amplitude noise is higher. Closing the
phase-locked loop results in amplitude noise of order 18-19%. This needs
to be fixed.
- The electro-optic cells do not contribute to the amplitude noise.
- Beam scraping on the optical-transport aperture increases amplitude
noise by about 25%.
She offered three proposed solutions:
- Determine causes of degraded laser performance on internal rf and fix.
- Determine why the rf control system causes increased amplitude
instability and fix.
- Add a "noise eater" after the EO cells (preferably after the aperture).
One is available already, but it would decrease the maximum laser output to
5 W.
As a result of her talk and her recommendations, Michelle was asked to
prepare a plan to do the first two fixes so we can provide resources and
schedule the activities.
Over the next two weeks the basic program is to do orbit and
phase-space
optimization during day shifts, cw runs during swing shifts, and beam
diagnostics/beam quality measurements during owl shifts. Progress from
each shift will feed into the other shifts as together we iterate toward a
usable lasing setup. Shift assignments for the next two weeks are
(remember to use the CEBAF operator!):
11-15 May 1998:
- Owl -- Merminga, CEBAF operator, Piot on standby
- Day -- Douglas, CEBAF operator, Benson on standby
- Swing -- Shinn, CEBAF operator, Siggins on standby (Krafft also on standby for Happek checks on 11-12 May)
18-22 May 1998:
- Owl -- Walker, CEBAF operator, Piot on standby
- Day -- Bohn, CEBAF operator, Neil on standby
- Swing -- Siggins, CEBAF operator, Hill on standby
Status of the remaining top-level MUST-priority prerequisites to lasing is:
1.Stable, easily recoverable 1.1 mA, 38 MeV beam with sufficiently good
beam quality.
Status: Must evolve from the iterative commissioning process described above.
2. Working diagnostics:
- Happek bunch-length monitor (or backup).
Status: Happek and crew are to be here from 10-13 May 98. The M_55
cavities are working well and can serve as bunch-length monitors. The one
in the injector may prove to be a sensitive aid in phasing the buncher
cavity and cryomodule.
- Multislit transverse-emittance monitors (#1 is working routinely, #2 is
not working).
Status: In work -- see status above.
- Quad/Viewer transverse emittance diagnostic at wiggler.
Status: In work -- see status above.
3. Adequate injector setup.
Status: Working toward a procedure for reproducible setup.
4. Detailed procedures and test plans well communicated to MCC staff.
Status: Ongoing. See, for example, list at end of these minutes.
5. Working optical transport, including Laser Safety System for the
Optical Control Room.
Status: See summary above. LSS still needs installation. Total
remaining work could be done within one week, i.e., it will be done by the time the
accelerator is ready for lasing.
6. Radiation monitor at wiggler location (to protect wiggler).
Status: Complete. Gathering data in conjunction with commissioning.
New Issues
None, but the FEL ARR is scheduled for 27 May 98, and presently we do have
some "issues" to be addressed in the context of the ARR subsequent to
providing photons to users.
New Action Items
- Bohn: Arrange meeting to outline elements of procedure to set up
longitudinal phase space. (due 11 May 98)
- Walker: Prepare glossary of FEL terms as an aid to CEBAF operators.
(due 15 May 98)
- Shinn: Prepare plan for fixing amplitude noise in drive laser. (due 12
May 98)
Old Action Items
- Walker: Work with Operations Department to develop requirements for an
FEL Alarms Handler software page for use in monitoring the FEL from the MCC
Control Room.
- Status: Done in draft, but will take time to finalize. It
could possibly take a few weeks for the Software Group to implement the
requirements, depending specifically on what they turn out to be. (due 29
May 98)
- Piot: Establish off-normal setting of quadrupole MQJ0F03 to enable
routine measurements with Multislit #1. (due 5 May 98)
- Bohn/Fugitt: Decide on standard inputs for the patch panel at the FEL
console in the MCC Control Room. (due 8 May 98)
- Jordan/Hill: Provide fixed configuration for the patch panel at the FEL
console in the MCC Control Room. (due 15 May 98)
- Douglas/Bohn/Fugitt: Establish Draconian back-out procedure for cases
where Restore files fail to recover the electron beam. (due 8 May 98)
Test Plans in Work
- Measure Beam Stability Benson, 15 May 98
(dE/E, dx, dy, dx', dy')
Procedures in Work
Set Up Longitudinal Phase Space Douglas, 15 May 98
Set Up Transverse Phase Space Douglas, 15 May 98
Set Up Beam Steering Douglas, 15 May 98
Emittance Growth from CSR
- Tomographic Phase-Space Map Piot, 30 June 98
- Back Leg Thread Douglas, 30 June 98
Procedures Needed for Recirculation
Thread Beam around Machine, Top-Level Douglas, finalize 30 June 98
RF Stability during Energy Recovery Merminga, 30 June 98