MEMORANDUM
To: J. Albertine, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IRFEL Weekly Report December 9-13, 1996
Date: December 13, 1996
Management
John Albertine and Fred Dylla presented a requested briefing to
the Navy Studies Board on Dec. 10 on the Navy High Energy
Laser Program and the status of the kilowatt Demo.
Briefing material and the final agenda for next week's SURA Maritime
Technical Advisory Committee were prepared and distributed to
the Committee members by Dec.11.
A preliminary agenda and suggested list of referees were prepared
for the Feb. DOE-BES review. The information was distributed
to DOE-BES and the Navy HEL Office for comments.
A presentation was given to a Metals Fabrication working group
at Newport News Shipbuilding on Dec. 9 about possible interactions
with the Jefferson Lab FEL program.
Injector Test Stand
Regarding fabrication of the new high-voltage stack:
Resistance measurements of the new ceramics that had initially
been done at HiTemp in California were done under a nitrogen purge.
They indicated success in that the required resistance values
were obtained after application of a second spray-and-bake coating
of chromium sesquioxide and vanadium pentoxide on the new batch
of ceramics containing 3% glass in the aluminum oxide. However,
subsequent readings were erratic. None of the resistances matched
within 50%. This lead us to make measurements using a vacuum
chamber at the College of William and Mary. After taking data
last weekend, and upon confirming our measurements during the
week, we are convinced that we are not obtaining the desired 10^9
to 10^10 ohm from the coating on the new ceramic insulators. Rather,
the ceramics exhibited resistances on the order of 10^13 to 10^14
ohm, approximately the measured bulk resistance of an uncoated
ceramic.
To compare, we made measurements in vacuum on a spare coated ceramic
from the original batch, i.e., one from the same generation as
the ceramics in the 250 keV gun, and obtained mixed results.
The measurement yielded resistances of 6x10^10 ohm up to 40 kV.
Thereafter, up to the test limit of 50 kV, the reading changed
to the 10^13 ohm value of the new ceramics. The reading then
reversed itself at 10 kV, and these observations held under three
repetitions. We plan to measure one of the ceramics in the gun
when we open it up to see if we can reproduce these measurements.
In summary, the data on the new coated ceramics unambiguously
indicate that the coating offers no significant improvement in
electrical properties, with the exception that both the new and
original ceramic coatings still provide for significant suppression
of secondary electron emission from the ceramic surfaces by lowering
the secondary emission coefficient of pure alumina. Thus, the
chief benefits of the new ceramics are restricted to a lower propensity
to crack or puncture, properties that arise as a consequence of
the higher percentage of glass phase.
In view of our findings, we decided that it is not worth further
delaying the gun schedule to accommodate a new ceramic stack.
Rather, we are reverting to our back-up plan of commissioning
FM Technologies to apply their coating, known to suppress field
emission significantly, to the entire cathode support tube. We
will then use it to replace the tube presently in the gun, and
we will keep the existing stack in the gun. This strategy should
enable completion of the "new" gun by mid-January.
In parallel, we will construct two new high-voltage ceramic stacks,
one using two of the new ceramics, and another using two new ceramics
with an ion-implantation coating done at UC Berkeley. These stacks
will then be stored should they be needed in the future.
Concerning other ITS-related progress:
Installation activities in the ITS are proceeding on schedule,
and are being closely coordinated to keep them on schedule. The
most noteworthy accomplishment of the week was removing the roof,
inserting large components including the cryounit, beam dump,
and assorted stands and girders, and then replacing the roof.
Work is also proceeding on upgrades to the photocathode drive
laser and its optical transport system.
Accelerator Systems
The top-level machine drawing that includes the updated beam-transport
lattice is in production. Once it is reviewed, which should be
next week, plans are to freeze the top-level design and institute
a change-control process.
After adding cuts to the pole tips at the inner coil edges in
an attempt to improve field uniformity, we again tested the prototype
dipole magnet. The slotted core was insufficient to meet the
specification for transverse-field uniformity. We therefore decided,
based on last week's successful tests, to use high-µ pole
surface material coupled with a Purcell gap to do the job. We
are also taking data on the utility of field clamps in adjusting
the effective length of the prototype dipole. The totality of
our data gathering and analysis is nearing completion, and we
will close out the magnet review on the recirculation dipoles
as soon as a complete set of successful tests are available, which
should be sometime next week.
Regarding the cryomodule, new design drawings of the pump drop
that will support lower cost and delivery time were signed off.
We received the first-article higher-order-mode (HOM) flange
assembly and sent comments to the vendor. We do not anticipate
any problems procuring these assemblies. HOM-load fabrication
started with prototype parts. We are producing the schedule for
cavity-pair assembly, and anticipate starting in mid-January.
The schedule is in keeping with the delivery schedule for the
cryounit shields. Presently we believe we can keep fabrication
of the cryomodule on track. Problems in fabricating the warm
windows are still being worked.
We received from the University of Virginia a prototype diode
for use in a prototype coherent (or diffraction) transition radiation
detector. Plans are to test the detector in the nuclear-physics
machine, where it will be used to observe diffraction radiation.
Later, it will be tested at 10 MeV in the ITS, where it will
be used to observe coherent transition radiation. If successful,
the detector then becomes a candidate for use as a fast bunch-length
monitor in the IRFEL.
The data-reduction algorithm and software screen for the multslit
transverse-emittance diagnostic were refined, tested thoroughly,
and are now complete. The Lab's machine shop tried to produce
the multislit geometry but could not achieve the required tolerance,
so we contracted with an outside shop that will make the pattern
using electron-discharge machining.
FEL Systems
Wiggler
The wiggler poles are complete and ready for installation at STI
Optonics. All other parts are being fabricated. A tentative
date of 18 February 1996 has been set for initial magnet measurements.
The rates for the work being contracted out to Northrop Grumman
are being negotiated before we commission them to work on the
vacuum chamber design, but the conceptual design is complete.
Design of the wiggler supports is nearly complete and should
be done before the end of the year.
Optics
We have been getting responses on our quotes for windows and turning
mirrors for the optical transport system and will begin placing
orders. We are still awaiting budgetary estimates on the first
turning mirror in the transport, which is dielectrically coated
on a silicon substrate. We've had discussions on the laser safety
system (LSS) and its interfaces to the accelerator's personnel
safety system, and are drafting an outline of the LSS states and
the associated logic. We have also been testing a cooled CCD
camera that may be used for synchrotron radiation measurements.
Operations/Commissioning
We compiled a comprehensive list of software-development tasks
needed to support every phase of the IRFEL, from the upcoming
350 keV experiments in the ITS all the way through a complete
machine operating in support of the user facility. The list was
then presented to, and coordinated with, the Accelerator Division's
software group to aid their resource planning.
An analysis was done of the droop in, and stability of, the rf system that could be expected from beam loading associated with pulsed 1.1 mA operation with the 8 kW klystrons. Both the rf and beam should be stable. However, there will be some energy spread and phase slip imparted to the electron bunches that must be considered in developing the technique for measuring the momentum spread.
Further simulations were done to explore the influence of accelerator
settings on beam properties at the wiggler entrance. There is
not a problem in finding settings to achieve desired combinations
of bunch length and momentum spread at the wiggler. In the coming
months, we plan to use a combination of analytic techniques and
simulations to devise suites of settings that minimize complications
in commissioning the machine, i.e., transitioning from first-light
current to full-power operation.
We started the process of planning specific CSR experiments using
the IRFEL. The plans, as they are developed, will be folded into
the overall commissioning plan for the machine, and emittance
diagnostics for these studies are already part of the diagnostic
suite under development.
Facility
The first coat of paint was put on the accelerator enclosure this
week. This sets the stage for mounting the box duct for the safety
system, as well as the U channel for the air and LCW pipes on
the wall. Surveyors will be inside the room next week putting
in their monuments for establishing the building grid. There
is still substantial contractor work going on inside. HVAC duct
work is finishing up, and piping for compressed air feed has been
installed. Outside, the fill was going in at a rapid pace in
the interfloor area. Penetrations were then positioned and prepared
for hooking to the upper level in most of the floor. That effort
is about 80% complete. Simultaneously, the foundations were being
prepared for pouring of the floor in the loading dock and entrance
lobby and finishing up the retaining walls for the truck ramp.
The contractor-proposed placement drawing of the sprinklers was
marked up to accommodate the actual placement of FEL hardware
and cable trays and returned to the contractor for construction.
Work continued on the layout of the injector in the FEL Facility.
Interferences with the sprinkler system were resolved. The design
for the high-voltage tank is being checked before final signoff.
We expect to receive a new bid for the new clean room early next
week.