To: J. Cook, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IR Demo Project Weekly Report, October 12-16, 1998
Date: October 16, 1998
Management
This week was spent baking the photogun after replacement of the
cathode last Friday. High voltage conditioning and reforming of
the cathode are planned for early next week followed by start-up
of operations if the cathode is responsive.
An internal review of the Laser Saftey System was held this week.
Next week, a DOE/SURA review of the the laboratory's "institutional
management" will be held on Oct. 19-20. Presentations to
the committee will include the FEL program and the associated
activities in the Applied Research Center.
Plans were made to hold two planning sessions next week to brainstorm
on the subject of the 20kW IR FEL upgrade. System requirements,
gun designs, FEL design issues and beam transport lattice issues
will be examined.
A draft announcement was prepared for a workshop on UV and x-ray
FEL applications that will be held on Nov. 20th at SURA headquarters.
IRFEL Installation and Maintenance
The positions of the wiggler and its associated quadrupole magnets
were adjusted based on last week's alignment check.
A weld on the tank surrounding the gun was redone, and the tank
was successfully pressure tested. Also, an additional valve was
added to the SF6 control panel for this tank.
The Personnel Safety System (PSS) was recertified following some
hardware improvements. One of the drive-laser shutters that is
an important indicator for the PSS was found to stick in the open
position. It was therefore modified and is now being tested.
The Thomson backscattering X-ray detection chamber has successfully
passed vacuum tests and is now being assembled for calibration
tests prior to installation in the beam line. After calibration,
the chamber will undergo a final cleaning, assembly, and installation
into the beam line.
IRFEL Commissioning
Regarding the photocathode gun, the cathode wafer was replaced
early this week and verified to yield a photoresponse. The vacuum
bake was readied and executed; tear-down of the bake is slated
for this afternoon. The whole process has thus far progressed
well. Plans are to do careful high-voltage processing of the
gun starting Monday morning and ending Wednesday morning, followed
by heat cleaning the cathode wafer, and then cesiating it after
it has cooled down. We should be able to pull electron beam off
of the gun starting Thursday morning, 22 Oct 98. As a related
aside: the previous cathode wafer is undergoing a barrage of surface
studies at William & Mary which thus far has yielded no surprises.
The cathode has a crack that may have been the source of field
emission during the last attempt to refurbish the gun.
The internal Laser Safety System Review was held 16 Oct 98, and
it was well received by the review committee. The committee is
submitting its findings and recommendations this afternoon, and
they will be used to strengthen the LSS as well as the documentation
being prepared for the external reviewer, Anteon Corporation,
that is slated to be sent by the end of next week.
Efforts to organize a set of beam-breakup (BBU) experiments on
the FEL continued. The fundamental motivation is to validate
the code TDBBU that calculates BBU in a recirculating accelerator.
Once benchmarked, the code can be used with more confidence to
establish BBU threshold currents in FEL upgrades or alternative
designs. It also could be used to establish with more confidence
whether HOM couplers are required on the new 7-cell cavities envisioned
for the 12 GeV upgrade of CEBAF. Considerable money and effort
would be saved were the answer to be "No".
TDBBU predicts the threshold current in the IR Demo to be 27 mA,
well below its attainable current. Thus, the strategy is to force
the BBU instability in the FEL by, for example, driving the dominant
deflecting mode with an external rf source and possibly lowering
the electron-beam energy. Stripline kicker and pickup cavities
are envisioned. Those used in a previous energy-recovery experiment
done parasitically on the CEBAF accelerator have been found and
examined. Their apertures don't match those of the FEL vacuum
chambers, so new designs are presently under consideration. The
kickers will go in the injection line (one horizontal and one
vertical). There is an issue concerning the lowest attainable
electron-beam energy, particularly if it involves lowering the
energy of the injected beam since the "injector aperture"
is presently undetermined.
A final plan for the BBU experiments is anticipated to be available
by 15 Nov 98. If the plan were implemented, the experiments would
likely be conducted next summer.
Plans to support the "stent irradiation" for Duke continued to be developed. As it stands now, Duke would build the holders to mount the stents on the straight-ahead dump line, and Jefferson Laboratory would build the target and shielding. The purpose of the target is to generate gamma-rays from the (nominally 2 kW) electron beam; the gamma-rays serve as the radiation for the stents.