Free Electron Laser Commissioning Meeting
Friday, 21 Nov 97
Recorder: C. Bohn
Next Meeting
- Date: 5 Dec 97
- Time: 0830-0930
- Place: FEL Facility Break Room
Agenda for Next Meeting
Item Person Responsible Time
---- ------------------ --------
Status of Action Items All 5 min
Progress Report on Commissioning Legg/Bohn 15 min
Scheduling Update Bohn 15 min
CW Setup Legg 15 min
New Issues All 5 min
Agenda for next week All 5 min
This Week's Attendees
J. Bennett, G. Biallas, C. Bohn, D. Douglas, D. Engwall, J. Fugitt, K.
Jordan, G. Krafft, R. Legg, L. Merminga, G. Neil, P. Piot, J. Preble, Q.-S.
Shu, B. Yunn
Closed Action Items
The following actions from last week's report were closed out:
- Bohn/Walker: Develop maintenance call list for FEL. Status: Was
thought to be complete, but some involved parties have requested more
detail. R. Legg will follow up to find out what is wanted, and we will try
to provide it.
- Preble: Coordinate with D. Eardley to complete commissioning of the
vacuum-interlock system. Status: Vacuum-interlock system is commissioned
and working.
- Piot: Do preliminary tests of tomography software obtained from www.
Status: Software has been verified to work. There remains the question of
task priority vs. available time. Piot will follow up with J. Song and G.
Krafft.
- Yunn: Model cryounit with cavity performance set at 12 MV/m and 8 MV/m
for the entrance and exit cavities, respectively. Feed information to J.
Preble, and iterate as required. Status: Yunn did this, and also
established the settings for 12, 8.5 MV/m gradients. The latter are in the
FEL Control Room. He will undoubtedly need to do more modeling as
measurements of the injector energy evolve during commissioning. This will
be a perpetually ongoing activity and so will not be formally tracked
herein as an action.
- Wiseman: Determine vacuum-pump requirements for the first-light dump
line. Status: L. Dillon-Townes completed code calculations with various
assumptions about the level of beam- induced outgassing. Based on
measurements of pressure in the injector dump during commissioning with
beam, we will decide whether a more powerful vacuum pump will be needed on
the first-light dump line. If so, we may be able to switch the planned 40
l/s pump with one of the 240 l/s pumps planned for use in the
energy-recovery dump line.
- Biallas: Check on availability of spare CEBAF air-core correctors.
Status: Enough spare AT correctors have been found. However, they are
designed for 1.5-inch pipes and will need to be rotated sideways to
accommodate the 2-inch pipe aperture. Accordingly, a question arises
whether they can provide sufficient field. One of them is now in the
Magnet Test Stand for measurement.
Items of Discussion
As mentioned last week, commissioning over Thanksgiving break and
weekend
will be on a voluntary basis, with no formally established shift
assignments. Work on Thanksgiving Day remains verboten! Plans for this
weekend call for diagnostics checkout, and if all requisite installation is
completed, we plan to run beam through the cryomodule into the
energy-recovery dump next week. If not, we will do more injector
commissioning. As is oft repeated, we must run tune-up beam to the
first-light dump before Christmas in preparation for timely GEN target
production.
R. Legg and J. Preble presented this week's commissioning highlights:
- Last Saturday M. Shinn replaced the broken flow tube in the drive laser
and turned it on. That evening we resumed injector commissioning. By
week's end we had run tune-up beam into the injector dump. The average
current ranged up to roughly 1 µA with bunch charges of order 1 pC. The
measured beam momentum out of the cryounit was 7.5 MeV/c, to within ±10%,
with the two cryounit cavities operating on crest. This value is lower
than expected based on the nominal accelerating gradients of the cavities
set during the srf commissioning process, and we need to understand why.
Electron-beam profiles on the viewers are consistent with simulation
(PARMELA) results. With characteristic swashbuckling style, Preble seized
the opportunity to investigate further the cryounit calibration during
swing shift this evening since there were no plans to run with beam.
- SRF commissioning of the cryomodule was completed, resulting in a total
available accelerating voltage of >47 MV. RF commissioning of the
cryomodule should be completed this evening.
L. Merminga presented data on rf phase and amplitude noise in both the
cryounit and the cryomodule. The unregulated phase noise (microphonics) is
dominated by 60 Hz. The open-loop (unregulated) phase noise is about 1.5
degrees rms in a typical cavity. This is about three times larger than in
CEBAF, but it is almost entirely compensated by lower external Q and is
accordingly not a problem. Presumably the source of the 60 Hz is a
multiplicity of pumps. Regulated phase noise is about 0.004 degrees rms in
a typical cavity. Unregulated amplitude noise (dV/V) is typically about
1%, but regulation reduces it typically below 0.01%, i.e., below 10^-4.
However, there are strong peaks in the regulated amplitude noise at 60 Hz,
so noise correlation could conceivably occur. If desired in the future, we
can probably reduce the noise by adjusting the pumps. The Lorentz
coefficient (ponderomotive force) in the cryomodule cavities was also
measured and found acceptable.
G. Neil followed with a presentation of a draft plan to investigate the
sensitivity of the FEL to off-normal machine settings, a post-lasing
activity. The basic idea is to identify the most important parameters to
monitor and adjust during user experiments with the laser. Neil proposed
investigating lasing sensitivity with respect to a number of parameters,
including: bunch charge (which will surely drop as cathode quantum
efficiency drops), solenoid focusing in the gun (which Yunn has already
identified as sensitively driving beam properties at the wiggler), buncher
cavity settings, cryounit cavity settings, cryomodule cavity settings,
mismatch of electron beam to wiggler, missteering of electron beam through
wiggler, missteering of optical-cavity mode, detuning of optical cavity,
and variations in electron-beam energy. He anticipates the totality of the
study will require about four months. Of course, we will learn much during
prelasing commissioning about the sensitivity of electron-beam properties
at the wiggler location vs. machine settings. Neil is going to provide D.
Oepts of FOM, Netherlands, a copy of the draft plan to solicit his input.
Oepts already has a copy of the existing lasing procedures. (Note: Oepts,
a seasoned FEL expert, is scheduled to arrive in mid-January 1998 for a
one-year sabbatical.)
New Issues
As mentioned above, measured electron-beam momentum out of the injector is
lower than was anticipated based on srf-measured cavity gradients. We need
to understand why. Is it due to, e.g., miscalibration of the cavities
and/or the field map of the (first) DU magnet that was used to make the
measurement?
New Action Items
- Preble: Check gradient calibration of cryounit cavities. (due 24 Nov
97) Note: Preble is planning to look at this over swing shift this evening.
- Douglas/Karn: Check field-map calibration of first DU magnet in
injection line. (due 24 Nov 97)
- Legg: Follow up to determine detailed requirements for FEL Maintenance
On-Call List. (due 24 Nov 97)
- Krafft: In conjunction with P. Piot and J. Song, decide whether
phase-space tomography should be pursued, and if so, on what schedule.
(due 26 Nov 97)
- Biallas/Douglas/Karn: Determine whether the AT correctors are suitable
for temporary use in the straight-ahead machine. If so, install three
immediately for use in running beam to the energy- recovery dump. (due 24
Nov 97)
Old Action Items
None.
Procedures in Work
First Light Setup
- Thread to Straight-Ahead Dump Douglas, finalize by 1 Dec 97
- Dump Setup Legg, finalize by 1 Dec 97
- CW Setup Legg, 1 Dec 97
Emittance Growth from CSR
- Tomographic Phase-Space Map Piot, 1 Dec 97?
- Back Leg Thread Douglas, 1 Dec 97
Thread Beam around Machine, Top-Level Douglas, 1 Dec 97