MEMORANDUM
To: J. Albertine, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IRFEL Weekly Report November 11-15, 1996
Date: November 15, 1996
Management
A meeting was held with DOE (Dean Helms, R. Marianelli, D. Lehman,
and J. Kearns) on Nov. 14 to discuss the planning of the February
1997 DOE Review of the FEL project. The review will focus on
building DOE mission support of the project through an emphasis
on planning for the commissioning phase and first operational
period.
Discussions were held this week with groups from the Vanderbilt
IRFEL facility, the Oak Ridge National Laboratory Solid State
Division, and the NASA Langley Research Center about possible
collaborations on FEL applications.
Injector Test Stand
Regarding operation of the 250 keV gun:
We found that we can reliably generate a top-hat distribution of laser light on the gun's photocathode, and that this distribution is very stable. The existence of the fresnel pattern has no significant effect on the emittance. Thus, using the top-hat distribution, we took two sets of publication quality data with the 2-mm-diameter and 1-mm-diameter apertures on the laser beam. The data looks good and the 2-mm data reproduces the 2-mm data taken a week earlier quite well.
We tested an alternative scheme, based on a wire scan of the beam
without the slit, to measure the transverse emittance for very
low charge beams. This serves as a cross-calibration of the slit
measurement.
We then embarked on some hardware improvements prior to starting
another round of data collection These included recesiating the
photocathode, and then achieving 4% quantum efficiency; replacing
the drive-laser lamps; and adjusting the laser-pulse width for
a longer pulse.
Today (Friday, 15 November) we started another round of data taking.
Regarding fabrication of the new high-voltage stack:
The Serial #1 ceramic was sprayed and fired once again in an attempt
to get a better match to the resistivity of the Serial #2 ceramic.
The outcome was marginal in our judgment, so we had a third ceramic
coated, and it was fired on Thursday, 14 November. Measurement
of its resistivity is slated to take place imminently. In view
of these developments, fabrication of the new stack is about one
week behind schedule. However, we are compensating by resequencing
the December schedule for installation activities in the Injector
Test Stand.
The rotisserie for ion implantation is complete and tested. We
will take it to Berkeley during the first week of December. This
is coincident with the scheduled brazing of the kovar rings, and
we will be able to monitor the implementation of both processes.
Regarding testing of the injector cryounit:
The cryounit is now warming up after completion of its acceptance
tests. Fine tuning of the data reduction is still ongoing; however,
qualitative indications are that the cryounit meets all of its
specifications.
Regarding other ITS progress:
The solenoid was removed from the chopper cavity, and the cavity
was reconfigured for use in taking longitudinal-phase-space measurements
in upcoming 350 keV gun experiments. The cavity was also prepared
for additional testing prior to installation.
The test plan for the 350 keV gun experiments is now posted on
the WWW.
The buncher cavity, needed for the 10 MeV injector, was operated
to test its control system. So far, the only problem to surface
with respect to the buncher cavity was a sticky valve on its water
skid, and the valve is being replaced.
Accelerator Systems
Northrop-Grumman continued to develop the engineering drawings
of the optical-chicane dipole magnets with the goal of producing
a set for production in two weeks. We have also started, in house,
the engineering design of the injection-chicane dipoles. Based
on extensive analysis, the configuration of the shunts for adjusting
the fields in the recirculation dipoles was defined.
The procurement package for the weak quadrupole magnets remains
out for bid and is due in next week. The coils are finished for
the prototype trim quadrupole, but work on the core is running
several days late. A contract for fabrication of the coils for
the prototype sextupole magnet was awarded, with receipt by mid-December.
All of these developments contribute to preparation for an internal
magnet design review scheduled for 18 November.
Detail drawings of the injector vacuum pipe started, and detailing
of the cable-tray and utility layouts continued.
The 3-inch beamline valves for the cavity pairs of the cryomodule
were all received, marking completion of one of our major procurements.
Testing is going well on the new low-level RF racks, and we have
found very few wiring errors.
Progress toward development of the 2-inch-diameter beam-position
monitor (BPM) for the IRFEL has been oriented toward preparing
for an internal review of the BPM design. The review, originally
slated for 15 November, will be held on 22 November.
A concerted effort is underway to study, by way of computer simulations,
the bunch dynamics from the photocathode through the optical chicane,
for all operational modes of interest, i.e., both for first light
and for full-current operation. The accelerator layout used in
the code was updated to reflect the latest configuration since
there have been numerous changes in detail over the past few months.
First results are indicating favorable bunch profiles for all
peak currents of interest. Disagreements between earlier tabulated
simulation results and analytic calculations were traced partly
to typographical errors; however, work is continuing toward a
complete understanding.
Work on the influence of coherent synchrotron radiation (CSR)
on the electron-bunch dynamics in magnetic bends has continued.
We discovered that localized radiation and coulombic fields in
the bunch have the potential to make important contributions to
the transient bunch dynamics. Previously we had ignored these
fields believing that they were negligible compared to the long-
range influence of CSR. It turns out that this statement is true
after long times, but the local fields can be more important on
short, but still macroscopic, time scales. We are now upgrading
our analytical and numerical analyses to include all of the local
fields. The radiative transients have proved to be much more
intricate than we originally suspected.
We completed an inventory of trim cards and found that we have
just enough on hand for the IRFEL.
FEL Systems
Wiggler
STI Optronics has sent us the detailed drawings for the wiggler.
ProEngineer CAD files have been successfully transferred and
translated between Northrop Grumman and Jefferson Lab, so work
on the vacuum chamber can begin. The engineering design of the
girder will be reviewed next week at the 80%-complete level.
Optics
This week's efforts revolved around the optical transport system.
We ordered the tubing, so it will be ready for installation when
we are allowed into the ground floor of the FEL Facility. A
meeting was held to discuss changes in the optical transport collimator,
so we can better adjust for differences in the thermal distortion
from different output couplers. We inspected the ground floor
HVAC penetration and determined it will necessitate a change in
our optical transport layout. We are considering two different
schemes and should be ready to incorporate them in early December.
The LVDT setup for the cavity mirror gimbal mounts is being incorporated
into the drawings. The first test set of 3-mm HR coatings on
calcium fluoride substrates is being shipped to China Lake for
testing.
Operations/Commissioning
Requirements for the rf-cavity-based path-length monitor were
solidified. Questions concerning the potential importance of
higher-order modes in the cavity were answered. At the full 5
mA IRFEL current, the predicted temperature rise in the (normal-conducting)
cavity should be modest, about 10 degrees Celsius, which means
power loading and phase stability are nonissues.
A standardized set of IRFEL operating modes was tabulated, circulated,
and posted on the WWW. The operating modes are compatible with
present plans for the machine protection system. They also constitute
a set of basic requirements for subsystems of the accelerator.
Facility
New rf waveguide feedthroughs were cut out of the 3-foot ceiling
in the correct location and the misplaced ones were filled. Scaffolding
came down in most of the interior, and the linac vault was fully
enclosed except for the open truck ramp door. An initial pour
on the truck ramp was performed, and most of the interlevel walls
were also poured. The process of filling the interlevel with
sand was initiated. Additional fill was placed around the outside
in the front of the Facility. HVAC duct mounting was initiated
in the lower level. The Facility remains on schedule.
We are negotiating with potential vendors over specifications
and costing for the laser clean room. We continue to work on
the injector layout. The sketch for the tank that will house
the 600 kV power supply for the IRFEL photocathode gun was sent
out for budgetary quotes.