MEMORANDUM
To: J. Albertine, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IRFEL Weekly Report December 2-6, 1996
Date: December 6, 1996
Management
Accelerator Division Management approved freezing the design of
the IR FEL when the current punch list on the lattice design is
completed in the next 1-2 weeks. After the design freeze, all
changes to the top level design will follow the Division's configuration
control process.
John Albertine and Fred Dylla have been invited to present an
update on the Navy High Energy Laser program including the Navy
Studies Board meeting on Dec. 10 at the National Academy of Science.
Preparations are underway for the Dec. 17-18 SURA MTAC review,
the Jan 21-22 LPC Workshop and the Feb. 24-25, DOE BES Review.
Injector Test Stand
Regarding operation of the 250 keV gun:
We finished taking the full set of 250 keV transverse-emittance
measurements that were originally planned. We then did a follow-on
experiment to acquire a set of data using a 2-mm-diameter aperture
on the laser beam and operating the gun at 300 keV. This data
set supports the contention that as the accelerating gradient
is increased the emittance decreases, particularly at high peak
charge densities. Further transverse and longitudinal data is
planned to be taken at 350 keV once the new ceramic stack is installed.
We also did another long CW run with an average charge per bunch
of 20 pC. The run extracted about 22 coulombs from the center
of the cathode and an additional 18 with the laser spot on the
cathode moved 2 mm vertically. The e-folding lifetime of this
cathode was only about 2 hours, but this is likely attributed
to an error in the cathode's fabrication process.
On 5 December we shut down the gun and began installation activities
related to the 350 keV experiments and to the full 10 MeV injector.
Specifically, we opened up the beamline to remove the harps and
install the chopper cavity, and we started work on installing
the 10 MeV stands and plumbing.
Regarding fabrication of the new high-voltage stack:
All week we have been attempting to test the resistance of the
candidate ceramics at up to 100 kV in a vacuum chamber at the
College of William and Mary. Appearance of corona from the metallization
band at the coating interface on the inner edge stopped the test
at a low voltage below the x-ray leakage threshold. The tests
continued with RADCON personnel present to monitor x-ray production
levels at higher voltages. Further tests will proceed under appropriate
safety procedures. Plans are in place to continue measurements
on Saturday with these procedures in place. In an attempt to
eliminate corona during testing, a spare mid-voltage ring was
mounted at the negative end of the pumped down ceramic.
The rotisserie for ion implantation was test mounted at Lawrence
Berkeley Lab and received that group's approval.
The new mid-voltage ring for the stack is being polished at 1micron
diamond grit on one shift and should take approximately one more
day to complete.
In a second experiment at the College of William and Mary, the
tests of the samples coated with the proprietary anti-field-emission
coating applied by FM Technologies indicated virtually no field
emission at up to 20 kV/m gradient. The result was the same for
both the polished sample and the as-machined sample, a result
so startling that the experimenter immediately verified the functioning
of his apparatus by testing the uncoated as-machined electrode.
It showed the same high levels of field emission as obtained
previously. The electron-beam polished sample showed disappointingly
high levels of field emission.
Concerning other ITS-related progress:
A waveguide aperture was built and tested to produce 10 dB of attenuation so that the buncher klystron can be used to drive the chopper cavity. This will provide up to 450 W to the cavity, which should be well above requirements.
The buncher water skid has been modified to provide a cavity temperature and improved water flow. A replacement valve for the water skid is due 12/15/96.
New downloadable code is at test for the buncher. The chopper
code will be part of the buncher. Screens will be available in
a few weeks for both the chopper and buncher.
Accelerator Systems
An updated beam-transport lattice was produced. It incorporates
a slight revision of the second recirculation bend to make it
more compatible with upgrade paths and thereby reduce the work
that will be necessary to reconfigure the machine whenever future
upgrades are done. Various updates of analyses related to particle
tracking in the revised lattice are underway. Plans are still
to freeze the top-level machine drawing not later than mid-December,
and then institute a change-control process for the overall layout.
Measurements of the as-built prototype dipole indicate that all
of the families of IRFEL dipole magnets would miss their target
specification for transverse field uniformity by factors of from
2 to 12. This week, tests of a modified prototype with high-µ
pole surface material and Percel gap increased uniformity to within
specification at all levels of excitation. At week's end, we
are removing that modification and adding cuts to the pole tips
at the inner coil edges. This technology is easier to produce
and may achieve the same improvement.
Measurements of the as-built prototype dipole also indicate a
variation in effective length of 3.6x10-3, beyond specification
over the operating range. We will test an increase in the field-clamp
thickness and perhaps a chamfer in the pole-tip edge to bring
this value within specification. As a fall-back we can have several
fixed positions of the adjustable field clamps to create identical
field integrals within specification at specific operating points.
The close-out of the Magnet Review of the recirculation dipoles,
scheduled for December 6, was delayed a week as the prototype
tests continued. The only issues remaining are those discussed
here, and they are being handled on a one-to-one basis with the
reviewers. The detail design can continue because all anticipated
modifications are easily retrofitted to the drawing package.
To save time, the 95% drawing package will be reviewed early next
week with our principal magnet-physicist reviewer.
Regarding other magnet developments: Northrop Grumman is continuing
to draw the details of the optical-chicane dipole magnets with
check prints due early next week. Layout of the injection/extraction
dipoles started. The bids on the weak quadrupole were evaluated,
and placement of the contract is imminent.
Detail drawings of the injector vacuum pipe continued. Cable
tray and utility layout is ready for sign-off.
Having completed the design of the cryomodule based on the test results of the cryounit, we are preparing for its fabrication. We received the first-article stainless steel subassembly for the higher-order-mode (HOM) loads and are evaluating it now. The prototype HOM assembly was prepared for vacuum evaluation. We welded the dished head assembly for two cavity pairs and are waiting for final dimensions on the valves. We had an on-the-shelf spare cavity pair for the injector cryounit. Because it was not needed, it is in process of being reworked for use in the cryomodule. Redesign of end can pump drop started to reduce cost and improve performance. We began to plan in detail for the manpower that will be required during assembly, to include cross-training.
Production of warm windows continues. Two windows were produced
and one has been tested. It failed in the resonant ring facility
at 25 kw cw rf power, possibly due to inadequacies in the fabrication
process. The Window Team continues to evaluate assembly procedures
and make allowances for potential deficiencies. Additional parts
and materials are in fabrication now to support continued production
of warm windows.
An internal design review of the beam-position monitors (BPMs)
for the IRFEL was held on 4 December. There are a number of design
details and scheduling matters that need to be resolved. Generally,
plans are to refine the various BPM designs upon resolving an
unwanted resonance in the prototype BPM. Designs of the BPMs
that will be located at the centers of the 180-degree bend magnets
are currently only rudimentary. Plans are to resolve the open
issues over the next two weeks, with the goal of closing the review
this month.
The proposed computer network needed to control the IRFEL was
augmented to include specific provision for linking the Cryogenic
Helium Facility to the FEL Facility.
FEL Systems
Wiggler
Piece part manufacture continues at STI according to schedule.
The optical bench for mounting the wiggler was ordered. Work
on analysis of the wiggler beamline vacuum indicated that 20 l/s
pumps would be required. Work is underway with the NG designer
to incorporate the pumps into the vacuum line design. Work continued
on detailing the approach to constructing the wiggler vacuum tube
and incorporate the screens with the required reproducibility
tolerances.
Optics
Minor mods to the resonator Brewster window sealing were done
on the drawing based on suggestions from fabricators to simplify
and improve the manufacturing. A number of quotes were elicited
for windows and transport mirrors in preparation for procurement
release of catalog transport components. Several other purchase
requests were submitted for piece components. A list of optical
software tasks and priorities for them was developed for coordination
with other priorities of the software group. An initial layout
of the FEL diagnostics was circulated for comment in the group.
A paper for the SPIE conference modeling session was drafted.
A set of mods to the ITS laser to be performed during the December
shutdown to provide remote control and monitoring as well as more
stable operation was initiated.
Operations/Commissioning
A first draft of the procedures for first-light turn-on of the
FEL was written. In addition, several sets of candidate accelerator
settings to support generation of first light were proposed based
on numerical simulations of the electron-beam dynamics. We have
begun the process of iterating the development of the turn-on
procedure against the numerical simulations to refine the overall
process for first-light commissioning.
Facility
The facility continued to progress this week. Significant amounts
of sand were transported to the interfloor area. The penetrations
(which have to be placed in the sand) were > 1/2 installed.
The retaining walls to the truck ramp were poured. Preparations
were made to clean up the tunnel area in preparation for first
painting and survey for as builts to begin in 1 week. Fill has
been laid in around most of the base to ground level and footings
poured for the entrance and loading dock area. The first two
sections of helium transfer line from the CHL were laid into place
permanently blocking the center road on site. The pipes are protected
from accident by massive I beam barriers. A coordination meeting
on installation activities was held. Plans are going per schedule
for support of the survey activity. The requisitions for cable
tray was signed over to procurement this week, deliver in 3 weeks
supports our installation schedule. The cable datatbase is being
assembled. All groups are working to have their inputs complete
by 12/16. Most of the cables will be ordered assembled (and in
some cases bundled) simplifying the installation process. A detail
task list with milestones has been produced and at present all
groups appear to be able to meet the initial three months of installation
activities (our current focus). An approach for cryo control
power and robust and reliable communications with the CHL has
been established which all agree to. A primary consideration
is that perturbations caused by the FEL building activities do
not cause the CHL to crash the main machine. Markups of the sprinkler
system plans so as not to interfere with optical transport lines,
cable tray, and injector crane systems were initiated.