To: J. Cook, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IR Demo Project Weekly Report, September 28-October 2,
1998
Date: October 2, 1998
Management
This week was devoted primarily to photogun conditioning activities.
Highlights for the week include: (1) use of a helium glow discharge
procedure for in-situ processing and cleaning of the photogun
electrode surfaces; (2) continued check-out of the Machine Protection
System (MPS) for high power operation.
On Thursday, Oct. 1, the Dept. of Navy (SPAWAR) approved a 6-month
extension of the Navy/DOE Memorandum of Agreement which encompasses
the FEL project.
A Dedication event for the FEL User Facility has been tentatively
scheduled for Nov. 4, 1998. We are hoping for confirmation of
the date by DOE officials in the next week.
The third Virginia Technology Summit was held in Richmond on Monday,
Sept. 28. A report written by the Virginia Technology Council
entitled "Leveraging Virginia's Federal R&D Assets"
was released at the summit. The report highlighted the FEL project
as a positive example of a jointly funded project.
A meeting was held at Duke University of Monday, Sept. 28 to discuss
collaboration between the Duke and JLab FEL programs. The two
institutions will sponsor a workshop on UV and x-ray applications
in the next few months.
On Thursday, Oct. 1, a pre-proposal was submitted to the DOE Office
of Energy Research, Laboratories Technology Research Program.
The pre-proposal was in response to a solicitation for applied
research topics which could attract 50% matching funds from industry.
Our pre-proposal was concerned with supporting the user lab equipment
which is being installed by the three industry led LPC working
groups on metals processing, polymer processing and microfabrication.
FEL Installation/Maintenance Activities
Installation maintenance activities:
Upgraded software on one of our workstations (felsrv1) to HP-UX
10.20. Modified the SF6 line orifice and removed a relief valve.
Replaced a broken SF6 pump in the SF6 recovery system. Recertified
the Drive Laser enclosure. Leaked check the Compton x-ray detector
diagnostic off-line. Peformed an audit of Machine Protection
System. There are still a few open items to be corrected before
the MPS is fully certified for all operational modes.
FEL Commissioning Activities
On Monday the gun's cathode was cesiated after last weekend's
heat cleaning. Unfortunately, field emission again showed up
at about 300 kV, thereby preventing resumption of commissioning.
Aided by PARMELA simulations combined with observations of the
field emission and a careful electrostatic field map of the cathode
ball, a diagnosis of the location of the field-emission site was
attempted. It was inconclusive. The best guess is that the rim
of the hole in the ball in which the cathode is inserted field-emitted
with the cathode retracted (inferred from last Friday's observations)
due to an accentuated surface electric field, while the cathode
itself field-emitted when the cathode was inserted in place (inferred
from Monday's observation). We decided to attempt helium-glow
discharge processing rather than go directly to opening the gun.
The processing was done Wednesday.
The aim of the helium-glow discharge processing was two-fold --
to smooth out point sources of field emission, and to remove monolayer
quantities of carbon and oxygen (and whatever else) contaminants
from the surface of the electrodes, especially that of the cathode
for improved quantum efficiency. The evolution of the arc rate
during processing suggests that the processing evolved through
two stages, the first being the processing of the cathode ball
and anode plate where the field was highest, and the second being
the processing of the remaining structures, e.g., the cathode-support
stem. The procedure lasted for about five hours, during which
a discharge current of nominally 50 mA was sustained at a voltage
of nominally 500 V. Based on prior studies of helium processing
in connection with improving vacuum conditions, the goal was to
deposit 0.5 C/cm^2 over the electrodes under these conditions,
and the target was nearly achieved by the time the process halted
due to exhaustion of the source of purified helium gas.
On Thursday, after processing, the high voltage was ramped up
to 420 kV on the cathode. Ramping was slow because its consequence
was to blow off a monolayer quantity of helium that had been implanted
to the electrode surfaces. Blowoff started at the low voltage
of 200 kV, resulting in vacuum spikes. The ramp-up took about
16 hours principally because the gun's ion pump was slow at removing
helium. It was encouraging that the conditioning was characterized
by primarily gas emission and very modest field emission.
With helium processing and high-voltage ramp-up now complete, the "gun plan" is as follows:
- Activate the NEG pumps; purge gas contaminants to achieve ultra-high vacuum.
- Heat-treat the cathode (675 C for ~2 hrs) late today, let it cool over the weekend.
- Cesiate cathode Monday morning and check its photoresponse (PR).
- Run high voltage to 350 kV on the cathode; check for post-cesiation field emission (FE).
-- If PR and/or FE are poor, process the gun to 500 kV, open it, and refurbish it.
-- If both PR and FE are good, resume commissioning.
We will know the outcome sometime Monday afternoon. If we can
run, we will then resume commissioning per plans already in place.
We also worked this week toward certifying the Machine Protection
System. About 80-90% of the MPS is now certified, but there is
a four-item punch list awaiting resolution. Certification should
be complete by noon Monday given the hope of resuming beam operations
Monday afternoon.
Two additional M_56 cavities, useful for measurements of path
lengths and phases, should be available within 1-2 weeks. They
already exist, and the flanges and beam tube of one of them are
being reworked to fit in the reinjection region. The other will
be installed near the beginning of the back-leg region. Their
installation will be timed compatibly with the commissioning activities.
No software additions are needed for these cavities.
We now have a PARMELA deck for the entire FEL machine including sextupoles. The purpose for developing the deck was to ascertain more fully the character of the longitudinal phase space of the beam as it propagates through the machine. For example, less detailed transport simulations indicate that the beam profile at the energy-recovery dump is very sensitive to both the cryomodule gang phase and quadratic nonlinearities in the lattice. It is of interest to explore more carefully, e.g., how to counter the quadratic nonlinearities with the sextupoles and trim quads. Now that the tool is available, we are putting together a plan for the PARMELA simulations.