To: J. Cook, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IR Demo Project Weekly Report, December 14 - 18, 1998
Date: December 18, 1998
Management
Highlights for the week include attaining
300 W of cw lasing with energy recovery. This is the first time
we demonstrated delivery of more linac current (1.6 mA) than can
be sustained with the installed rf system (1.1 mA), thus demonstrating
one of the key advantages of energy recovery. Neither the laser
output power nor the accelerator operation is yet fully optimized
at these conditions, as noted below. Mixed in with this exciting
result was the premature demise of the currently installed photocathode.
It served us well for 6 weeks, delivering roughly 180 C of charge,
but died one week shy of our planned shutdown for cathode replacement
and the holidays. The FEL team wishes all our colleagues and
supporters an enjoyable Holiday Season and Happy New Year!
Preparations continued for the FEL project
presentation at next month's BESAC Panel meeting on "Novel
and Coherent Light Sources" scheduled for Jan. 18-21 at Gaithersburg,
MD. In addition to a summary on the technical capabilities of
the IR Demo FEL and Upgrade, representatives from three of the
FEL user groups will be giving presentations.
Several members of the FEL team attended a
symposium on DOD-sponsored high energy laser projects given at
Langley AFB on Tuesday, Dec. 15. On Wednesday, Dec. 16, we had
a working session with representatives from Northrop Grumman and
AES to discuss potential collaborative development of FEL technology,
as a follow-up to meetings held earlier in the month.
FEL Installation/Maintenance Activities
The following maintenance items were completed
this week:
To resolve a concern about magnet proximity,
measurements were done in the Magnet Test Stand. It was found
that, in the backleg, correctors within 1.125 inches of the quadrupole
magnets affect them only by a few parts in 10,000 when going from
0 to 10 A. At present, one backleg corrector is at 15% excitation
and the others are at 10% or less. Therefore, this is a non-problem.
The performance of three beam position monitors
(BPMs) was recovered by just tightening cables, and another BPM
had 2 cables replaced.
Rework of the gun is in progress to replace
the photocathode and repair the cesiator.
A list of tasks for the January down was compiled
and prioritized.
FEL Commissioning Activities
Commissioning of the recirculation loop in
the presence of cw lasing continued through 1600 Tuesday to the
point of reproducibly generating of order 200 W cw laser power
upstairs in the Optical Control Room at cw average currents of
order 1.5 mA. Best performance was 211 W upstairs (corresponding
to about 300 W out of the FEL proper given roughly 40% losses
in the optical transmission line) with 1.6 mA current. At that
point we were experiencing a trip of one of the beam-loss monitors
indicating beam scraping in the reinjection region, i.e., near
the end of the recirculation loop.
A second limitation also emerged this week,
namely heating of the infrared detectors in the waveguides of
the accelerating cavities that led to persistent trips in one
of the cryomodule cavities. We were gathering data to isolate
the source of heating, and a preliminary indication is that it
is due to high-frequency higher-order modes propagating through
the waveguides and onto the detectors. Consequently we are planning
to install a screen over each IR detector in the cryomodule during
the January down, a process that will involve cycling the cryomodule
to room temperature.
Unfortunately, at 1600 Tuesday we lost the
quantum efficiency of the cathode. At that time the cathode was
generating 40 pC bunch charges and we were running cw. The cryounit
rf tripped, closing the drive-laser shutter (as it should). Upon
straightforwardly resetting the cryounit rf and opening the shutter,
the cathode was "dead". This is a "new" failure
mechanism in that the gun high voltage did not trip, meaning apparently
there was no surge of vacuum pressure in the gun. Yet both visual
inspection of the cathode (to the extent it could be done) and
scans of its quantum efficiency show a large imperfection on the
cathode surface. We cannot tell conclusively if it is a large
pit or a large protrusion, or even if it is "merely"
a large inhomogeneity in the surface chemistry. We must remove
the cathode for a definitive analysis; resources for auger spectroscopy
are prepositioned.
The present cathode proved to be remarkably
resilient before its demise. During a six-week period, the cathode
supported 37 8-hour shifts of running electron beam, 12 of which
were specifically dedicated to cw runs. To roughly estimate the
total charge delivered, consider that roughly half the time during
these 12 shifts was actually spent with cw beam on (the other
half being spent on thinking, resetting trips, etc.), for a total
of ~50 hours of beam time. On average, the current during beam-on
would have been about 1 mA. Thus, roughly, a total charge of
50x3600x0.001 = 180 C was delivered, a very encouraging result
regarding prospects for further commissioning. It will be interesting
to see how future cathodes perform because our plan is to begin
running them at a macropulse rate of 75 MHz to support average
currents up to 4 mA as we endeavor to reach kW-level operations.
In view of the state of the cathode, we deemed it best to begin preparations for cathode replacement. We prepared and anodized a new GaAs wafer. Today (Friday, 18 Dec 98) we are debugging the residual gas analyzer used to monitor the composition of the gun vacuum since it, too, had failed on Tuesday, and we are high-voltage-processing the gun's electrodes to "smooth" them prior to opening the gun to air. We also prepared a detailed schedule for the refurbishment that spans the previously planned shutdown (23 Dec 98 through 19 Jan 99). If all goes well, we should be able to turn on the FEL at about the same time CEBAF is turned on.