MEMORANDUM
To: J. Albertine, D. Helms, W. Skinner
cc: Division (M7), FEL Coordination Group
From: F. Dylla
Subject: IRFEL Weekly Report Oct. 28-Nov. 1, 1996
Date: November 1, 1996
Management
SURA held a review of the institutional management of Jefferson
Lab on Oct. 28-29. An FEL program overview was presented at the
review in addition to "customer" statements by John
Albertine from the Navy FEL and Mike Kelley, chair of the LPC.
A technical, cost and schedule progress review of the IR FEL Demo
project was held on Oct. 29 by the Navy and DOE.
A meeting was held with Northrop Grumman management on Oct. 31
to discuss continuing technical support the IRFEL Demo project
by Northrop Grumman engineering staff. A draft agenda was circulated
to the chairman of SURA's Maritime Technical Advisory Committee
(MTAC) for the committee's first meeting on Dec. 17-18.
Injector Test Stand
Regarding operation of the 250 keV gun:
The troublesome electro-optic modulator was repaired and reinstalled.
Its extinction ratio was remeasured and found to be 2.7e-5.
This is sufficiently low and allows experiments to proceed.
The distribution of the laser spot was remeasured with the new
modulator and found to be gaussian, as desired, but a drift on
a time scale of minutes caused the spot to wander across the copper
aperture which is imaged on the cathode. To correct this, two
mirror mounts on the optical drive line were replaced. Final
measurements of the laser beam spot size and distribution are
in progress prior to starting the production runs that will provide
publishable transverse-emittance data.
The laser-beam pulse length was optimized using the autocorrelator,
resulting in an rms value of 17 ps. This will be used for the
experiment.
The dispersion at the harp used for momentum-spread measurements
was characterized. All of the components on the experimental
beamline are now commissioned.
Three operators from the nuclear-physics machine were trained
on the setup of the present machine configuration. We are now
running two-shift operations in the ITS.
Regarding fabrication of the new high-voltage stack:
A day-by-day schedule for the high-voltage (HV) stack was compiled.
Completion date is projected to be 26 November, provided all
processes go well.
The bulk resistivity of the second ceramic insulator of the new
batch was measured before coating it. Assuming this difficult
measurement was done with sufficient accuracy, we believe the
bulk conductivity is large enough to dissipate the buried charge
anticipated from the field-emitted electrons. A build-up of these
electrons in the ceramic is the suspected cause of the vacuum
leaks in the present HV stack. This second ceramic was then coated
with the standard spray and fired. The coating's resistivity
was measured and found to have a value about 20 times higher than
the coatings on the ceramics of the previous batch, a variability
that motivates continued work on ion implantation. We anticipate
that, though higher than in previous successful ceramics, this
value still allows drainage of field-emitted electrons and establishment
of the required uniform field gradient. The first ceramic insulator
of this batch is being prepared for a spray and firing on Monday,
November 4. If this insulator's coating matches the resistivity
of the previous one, remaining work at the vendor is to plate
the metalized ends of both with nickel, braze on the kovar rings,
leak-test it, and ship.
The new mid-voltage ring for the new stack is being polished on
two shifts and is now at 600 grit, silicone carbide.
The rotisserie for the alternate ion-implantation charge-bleed
treatment is not yet shipped. The contact surface of the ceramic
insulator's rolling rings was wearing against the rotisserie rollers.
We installed a ball-bearing contact to take thrust loads and
coated all contact surfaces with Rulon.
Testing of the anti-field-emission-coated sample electrodes started
at the College of William and Mary.
Regarding testing of the injector cryounit:
Cryounit tests are almost done. The performance of the magnetostrictive
tuner meets its frequency-range specification. Microphonics data
were taken and passed to the RF Modeling Group for interpretation.
The initial assessment is that the microphonics are not a problem.
Low-gradient unloaded Qs were measured and were favorable. On
one of the cavities, the unloaded Q was measured to a cw gradient
of 9 MV/m, at which point the rf power available in the Cryomodule
Test Facility was exhausted. We are in process of adding to the
available power. Plans are to finish all measurements of unloaded
Q for both cavities early next week, at which time acceptance
tests will be complete.
Regarding other ITS progress:
RF testing of chopper cavity, which will be needed for measurements
of bunch length as part of the reconfiguration for 350 keV operation,
was completed successfully. The cavity is now in the machine
shop to have its solenoid and associated flange removed and replaced
with a new flange on which will be mounted a new solenoid. These
design changes are part of the reconfiguration of the beamline
necessary to permit the longitudinal and transverse 350 keV experiments
to be run in parallel.
Accelerator Systems
The prototype dipole magnet with the saddle coil was modified
to include a prototype field clamp and then tested. The field
clamp was found to be successful in suppressing the extent of
the protrusion of the fringe field from the aperture of the magnet.
However, it also lowered the strength of the integrated fringe
fields below the value assumed in designing the transport lattice.
Initial indications are that this has no significant effect on
the lattice. In any event, we have decided to include, in the
magnet design, provision for mounting field clamps should we later
decide to use them. This provides time (if needed) to experiment
with field clamps without further delaying procurement.
Software development for "10 MeV" transverse-emittance
measurements with the multislit diagnostic is almost complete.
Progress has been much more rapid than originally anticipated.
Various mechanical parts for this diagnostic are on order. All
of the parts should be in hand by mid-December.
An internal review of the design of the vacuum system for the
IRFEL was held. It included impedance analyses, vacuum hardware,
and vacuum control/interlock electronics. Conclusions of the
review panel were that impedance issues are within budget for
this machine but that some hardware should be made with less impedance,
if easily done, so that an upgrade would also fall comfortably
below its impedance budget. There was agreement that the system
pumping and design concepts are sound and that ordering standard
parts, pumps, valves and bellows could proceed. Principal recommendations
were that both the method of interfacing the baked system to the
unbaked system and the methods and emphasis on the handling of
particulates needs greater attention. Some of this study can
take place in the Injector Test Stand. Additional recommendations
were given concerning details of the vacuum-system design, to
include assorted components and procedures.
FEL Systems
Wiggler
STI Optronics continues to receive parts as the wiggler continues
on schedule.
We integrated the STI Optronics top-level drawings into the assembly
drawings for the wiggler girder. The design should be mostly
done by next week, after which we will be able to order the optical
bench. We also plan to order a smaller optical bench for holding
the wiggler when it is not on the beamline.
Optics
Procurement of the optical components for the resonator cavity
continues. The drawings were modified to include the final adjustments
to specifications for vacuum compatibility and machining techniques.
One drawing remains to be modified for the inclusion of LVDT
position sensors. The others are in preparation for sending out
for quotes.
Operations/Commissioning
A proposed procedure for commissioning the injection line of the
IRFEL was written.
We held the first meeting of the Diagnostic Review Committee to
consider nominal changes to the baseline diagnostics suite that
were proposed as a result of further considerations of commissioning
procedures, such as that of the injection line. This first trial
of our "change-order process" went smoothly.
Preparation of a Users' Manual designed to define the user interactions,
facility capability, and instrument availability has begun.
This will be a living document that will be continuously updated
to include new capabilities and new requirements from the users
as they occur. It will incorporate the outputs of the working-group
sessions from the last meeting of the Laser Processing Consortium.
Facility
Laying in of the rebar for the final third pour of the ceiling
is nearly complete, and construction continues on schedule. It
was discovered that the placement of the waveguide penetrations
in one zone was off by 9 feet. The contractor intends to fill
in that hole and add a new penetration by cutting the first 6"
to clear the rebar and then jackhammering a large enough hole
that a new properly positioned and sized hole can be cast in the
3-foot slab. A schedule for this activity is being developed.
In principle, it does not hold up any other work. A major pour
of wall areas and shielding around the entrance labyrinth was
completed this week.
Specifications for the new clean room to be installed in the FEL
Facility were revised. Although we expect that the new room will
cost somewhat more than moving the old one, we believe that it
would improve the schedule for moving the injector laser by about
2 months.
We laid out the optical-transport line for the FEL Facility and
resolved issues concerning interference with cable trays and the
clean hood for the gun.