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.