To: J. Cook, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IR Demo Project Weekly Report, November 9-13, 1998
Date: November 13, 1998
Management
Highlights for the week: we are back on the air with FEL lasing
at the 300 watt level with 1 mA of straight ahead current. Commissioning
of the recirculation loop has re-commenced.
To make the most of our last two months of experience with turning
the photogun around, we have commissioned a gun study team that
will analyze the gun performance since its first operation in
the Injector Test Stand until the most recent conditioning exercise.
The first set of tasks for the team will involve analysis of
installation, maintenance and conditioning procedures and recommendations
for incremental improvements that would improve the performance
and reliability of the gun. A second set of tasks will involve
recommendations for the most effective rebuild or redesign of
the gun that would be included in the 20kW upgrade proposal.
A meeting was held this week with Northrop Grumman and Advanced
Energy Systems (AES) about possible collaboration in the development
and presentation of a proposal to the Navy for the 20 kW Upgrade.
Northrop Grumman and AES provided essential technical and programmatic
support to Jefferson Lab during the original IR Demo construction
project.
The program for a SURA sponsored FEL workshop on short wavelength
FEL technology and applications has been finalized. The program
was organized by us and the Duke, Vanderbilt, and BNL FEL projects.
The one day workshop will be held next Friday (Nov. 20th) at
the AAAS Building in downtown Washington. Full information about
the workshop is posted on our WEB site at http://www.jlab.org/intralab/calendar/sura98agenda.html
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 power supply for the recirculation dipole string failed and
was repaired: a SCR pulse firing board needed to be replaced;
the Be type SCRs and an unmodified phase imbalance relay were
found and also replaced.
Assorted hardware checks were accomplished. For example, a trim
winding on the injection dipole magnet was suspect but was found
to operate properly, and the mirrors in the Happek #2 interferometer
needed to be realigned as a result of difficulties with remote
operation of their picomotors.
FEL Commissioning Activities
We are all really quite excited. After losing the cathode due
to an arc Friday afternoon, the gun was successfully resurrected
late Wednesday after careful processing up to 510 kV, a long heat
clean of the cathode, and careful cesiation. Within 14 hours
after turning on electron beam (during which 60 pC gun performance
at 350 kV was established and the injector setup was iterated),
the machine was lasing, and that is after being down nearly 13
weeks, save for the 2-day run last week. After another 14 hours
(during which the injector and accelerator setups were further
refined) the machine had delivered 235 W of cw IR laser power
upstairs in the Optical Control Room! We have yet to calibrate
accurately the transmission losses in the optical transport line,
but from what we know now, the delivered power would correspond
to at least 300 W, and perhaps more like 350 W, out of the FEL
itself. During the lasing runs, the accelerator delivered uninterrupted
1.1 mA cw beam to the straight-ahead dump for several hours. Bottom
line is, within 28 hours after pulling electrons from the cathode,
the straight-ahead machine performed at least as well as it ever
did.
At this writing (0945 Friday, 13 November 1998), we are running
the FEL in the recirculation mode and beginning to work through
the High-Power Setup Procedure that should ultimately lead to
high-power-cw lasing with energy recovery. The quantum efficiency
of the cathode has been dropping off during the cw runs, not surprisingly,
and we anticipate needing to rejuvenate it sometime next week,
perhaps midweek. Some combination of heat cleaning and recesiation
will likely be used; however, we are planning to do a cathode
scan with the new scanner early Monday morning to ascertain whether
the "poor" quantum efficiency is localized coincident
with the drive- laser spot on the cathode. If so, we may be able
to proceed by simply moving the drive-laser spot to a new location
on the cathode. We will soon know; it will be a contribution
to our learning curve. Of course, eventually the cathode wafer
will need replacing, but right now we don't know when that will
be.
We believe the problems we were having with the gun are traceable to a persistent and robust field emission site that took much time and high voltage to process away. Profuse field emission during high-voltage turn-on kept killing the cathode photoresponse after cesiation. Processing late Friday and during the first half of this week ultimately (and finally!) resulted in quiet electrodes before and after cesiation. Another contributor may have been a malfunctioning cesiator which we rebuilt last week. We feel that the gun availability can be greatly improved with some straightforward upgrades, such as the addition of a load lock to enable processing the cathode without affecting the electrodes, but our penurious condition prohibits their incorporation. It continues to puzzle us why we are still indigent.