To: J. Cook, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IR Demo Project Weekly Report, November 2-6, 1998
Date: November 6, 1998
Management
Highlights for the week include: achievement of quiet electrodes
in the photocathode gun at 350 kV, achievement of sufficient cathode
quantum efficiency to run electron beam, and checkout of the machine
with electron beam (proceeding today).
The group had several presentations at external meetings this
week. At the Denton Conference on Applications of Small Accelerators,
Steve Benson presented a talk on the IR Demo FEL and Jim Boyce
gave a talk on the use of the IR Demo Linac for Electron Beam
exposure of materials. Jim also presented a second talk on the
production of x-rays by Compton scattering. Fred Dylla presented
a talk on the Design and Conditioning of the demo vacuum system
at the AVS symposium in Baltimore and a FEL program update for
the SURA Board of Trustees fall meeting at George Washington University.
FEL Installation/Maintenance Activities
Apart from the gun activities summarized in the next section,
this week's maintenance/installation activities resulted in the
following accomplishments:
The cathode-photoresponse scanner was made operational and used
this week after the last heat treatment. Both hardware and software
are working.
The beam-scraper hardware was commissioned without electron beam
this week and appears to be functioning per design.
The 1497 MHz RF noise on the drive-laser fast photodiode was finally
eliminated. The problem was found to be a poorly shielded cable
from the patch panel to the spectrum analyzer. Installation of
a new 1/4" heliax cable solved the problem
FEL Commissioning Activities
At this writing (0945 Friday, 6 November 1998), we are running
the FEL. Translated, that means the gun is presently operational.
However, we may need to replace the cathode wafer starting early
next week. A detailed summary follows:
At week's beginning, after finishing the rework of the cesiator,
we still had achieved neither a good cathode photoresponse nor
a gun that behaved quietly at 350 kV. Consequently, we did two
things. First, we high-voltage processed the gun to 500 kV in
an effort to suppress what appeared to have been two field-emitting
sites (a soft statement, given the grossly coarse resolution of
the geiger-tube array, though the emission was observed unambiguously
to be bimodal). Then we heat-cleaned the cathode at a high temperature
(675 C) for a long time (8 hours), an action that was motivated
by vacuum data from previous heat cleanings. Finally, we recesiated
the cathode yesterday morning. The resultant photoresponse appeared
to be about a factor three better than previously with this cathode,
but still poor. Basically, the cathode exhibited a photoresponse
like this summer's cathodes after they had been well used and
were in need of replacement. We then did a high-voltage check
with the now-cesiated electrodes and found a quiet gun (no field
emission) at 350 kV -- finally achieving at least one success.
Given the state of the gun yesterday afternoon, we decided to
try pulling pulsed electron beam off of the cathode, and we found
we could do it. We are presently unsure of the calibration of
our beam-current-monitoring (BCM) cavity and will need to check
it, but taken at face value, it indicates we are achieving 60
pC bunch charge with modest power (a 25% polarizer setting) from
the drive laser. Consequently, we embarked on a 32-hour program
(through Swing Shift tonight) of commissioning with the fundamental
goal of taking pulsed beam through the machine and putting together
a punch list of hardware/software problems; one would expect problems
given the gun has been down for 12 weeks and considerable work
has been done on the machine in the interim. At this writing,
we have already put the beam into the straight-ahead dump and
were making some steering adjustments in the wiggler region in
preparation for pulsed lasing to check the FEL systems. The electron
beam looks quite reasonable, again given the machine has sat dormant
for so long. Plans are to take the beam all the way around, check
the new diagnostics in the recirculation loop, and then go back
to the straight-ahead dump for a brief cw run to calibrate the
BCM cavity. Presently the conditioning resistor is in place,
meaning the running resistor still needs to be put in prior to
going cw. The new resistor-change hardware should make it possible
to do this in just a few minutes, versus the two hours it used
to take.
Presently we cannot know whether the cathode will need changing
next week. The photoresponse measurement suggests that we should
expect a very short lifetime. We shall find out soon, perhaps
by trying to turn on again Monday morning and seeing what is left
of the cathode. We will run the machine as long as is sensible
to do so before replacing the wafer.
Another way to view the developments is as follows: The exorcism
our gun owner, Tim Siggins, undertook on Halloween night turned
out to be a protracted one. Relics of the exorcism are still
present and remain to be purged from the FEL vault. We gave Tim
the day off today. He has a new twitch, but we don't think he
needs the straight jacket yet.
The two new M_56 cavities are ready, and they will be installed in parallel with the next cathode replacement. These cavities, along with the available software, will provide data on path length and phase transfer. An off-line calculation will be needed to translate the data into M_56 values because the software has not yet been upgraded to do this. That should not pose a big problem since the calculation is straightforward.