MEMORANDUM

To: J. Albertine, D. Helms, W. Skinner

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IRFEL Weekly Report, June 2-6, 1997

Date: June 6, 1997

Management

Highlights of the week include another successful week of running the photogun. Measurements of transfer and longitudinal emittance were made. The drive laser was run continuously 85 hours with stability. Details are given below in the ITS section of the report. Equipment installation in the accelerator enclosure of the User Facility is beginning to make real progress and cryogenic work is in the final stages in preparation for cooldown in July.

On Monday, June 2, Senator Warner invited representatives of SURA, Jefferson Lab, and SURA's Maritime Technical Advisory Committee (MTAC) for a briefing on the status of the FEL project and outyear funding prospects.

Final preparations were made for the second meeting of MTAC on June 9-10.

Solarex, a photovoltaic manufacturer, was visited by Jefferson Lab, Old Dominion University and William and Mary on June 3. Discussions are underway for collaborative university and joint research using the FEL and the Applied Research Center.

Injector Test Stand

Regarding the photocathode gun:

The gun continued running well this week. To lower the charge density at the cathode with the goal of reducing beam scraping at the light box, we installed a 6-mm-diameter aperture on the laser. This indeed resulted in a smaller beam at the lightbox; the beam diameter at the light-box mirror was 2.5 cm at 135 pC/bunch with no scraping, and some scraping became evident at 170 pC/bunch. The measured unnormalized transverse emittance at 135 pC was 2.6 mm-mrad. We also did some longitudinal-emittance measurements, collecting the first set of data at 60 pC/bunch. There are still some difficulties in the data-analysis software, and they should be resolved next week. We then did some experiments to mimic use of the gun during production runs in the FEL Facility. We measured transverse emittance with the laser beam oriented 3 mm off the cathode's centerline. The idea is to provide longer periods between successive recesiations of the cathode by using more of the cathode's surface area. In a cathode-lifetime test, we delivered about 20 C to the dump over a 10-hour period at bunch charges ranging from 60 to 135 pC. The cathode's quantum efficiency dropped from 7% to 0.7% during the run, which is consistent with what we had seen during the measurement program at 250 kV. We also operated the kicker cavity (that supports longitudinal emittance measurements) in pulse mode, but phase shifts show up that are not yet understood.

The drive laser performed admirably the whole time, operating continuously for 85 hours with stable power output and very good extinction ratio.

Regarding the cryounit:

The 10 kW ceramic warm windows are installed in the cryounit, and we proceeded with cooldown, reaching 4.2K without incident. However, in going to 2K our vacuum pump failed. This set the test back two days while we procured and qualified a new pump. Today (6 Jun 97) we are installing the pump. We will take the cryounit to 2K on Monday, 9 Jun, and proceed with testing the waveguides. Plans remain to move the cryounit to the FEL Facility on 16 Jun, where it is needed as a load to commission the new cryogenic system there.

The third (spare) 50 kW Klystron is on site, but the input connector is loose, and we continue to wait for instructions from the factory before tightening it. We plan to do the acceptance tests in June. The circulator for the spare klystron is also on site, and we likewise plan to do its acceptance test this month.

We are working with an experienced local company, Pollydata, Inc., to develop electronics for the magnetostrictive tuners. They will submit a proposal shortly. We hope to do a rudimentary test of the magnetostrictive tuners already installed on the cryounit prior to its move to the FEL Facility, if possible. These plans are linked to the pace of the higher-priority waveguide tests that relate to the in situ performance of the 10 kW ceramic warm windows.

Regarding development of 50 kW ceramic warm windows:

We needed to iterate Northrop Grumman's assembly drawings for their BeO windows, and the process took about two weeks. We now have final drawings and have contracted for fabrication of parts for the frames. These windows should be fully assembled and ready for testing in about six weeks.

Accelerator Systems

Regarding the beam-transport system:

Fabrication of dipole magnets continues on course. We received the second coil for the injection/extraction dipoles, and production of the 16 required coils proceeds at 1-2 per day. The iron for the 180-degree and reverse-bend dipoles is back from heat treatment to begin final machining. The first-article coils for the reverse-bend dipoles are wound and being potted. They are due for completion and inspection on 11 June 97. Production of trial pole pieces by Everson Electric to prototype the assembly process will take a week. The trials are slated for the week of 16 Jun 97.

The contract for sextupole cores was placed with Process Equipment Co. They brought their price down by being able to use electro-discharge machining to obtain the tolerances we require. The design drawing for the trim quadrupoles was signed off. The contract for the three long Panofsky-style quadrupoles for the recirculation dump was awarded to New England Electric Co.

As discussed under Commissioning/Operations below, we are preparing to test the effect of fringe fields on the mu-metal correctors in our Magnet Test Stand, and we are reducing the maximum required field integral on the correctors by 44%, which makes developing the corrector magnets easier and less expensive. Engineering for phasing dipoles was completed and design started.

We completed broad-brush measurements on mock-ups of several potential configurations of an air-core corrector comprising four coils in an approximate cosine distribution. We are now analyzing the data. We also simplified the number of air-core corrector types to two styles and gave those specifications to Advanced Magnetics Laboratory (AML), an experienced "magnet factory" in Florida, so they could prepare a preliminary proposal to produce them.

The vacuum chamber for the 180-degree dipoles was sent out for bid. Welding of the body on the X-chamber connecting the injection line to the recirculation loop was nearly completed. Northrop Grumman completed details of all remaining return-bend chambers. Their design help saves us from having to design the remaining three chambers, a favorable development in view of the short schedule.

Design of the final batch of stands, those for the reverse bends, the 180-degree bends, and the sextupole/trim-quad girders, has started. Preparations were made in the FEL Facility's accelerator vault for installing the girders in the back leg next week.

Detail drawings of the insertable dumps are in checking, and design of the beam scraper started.

Regarding cryomodule fabrication:

All cavity pairs have passed qualification tests and are turned over for assembly into cryounits. One cryounit assembly is complete and turned over for assembly in the cryomodule. The other three cryounits are proceeding on track and without incident. We remain on track to meet the target date of 4 Aug 97 for installing the cryomodule in the FEL Facility. It can then be cooled quickly because the cryogenic system will have already been commissioned by using the injector cryounit as a load.

Regarding electron-beam instrumentation and controls (I&C):

The balance of the stripline beam-position monitors (BPMs) have been shipped. There are 26 total, of which 19 are already in house, with 17 turned over for girder assembly, and 2 in our Machine Shop for repair. Drawings of the interface box for the beam viewers were signed and are out for bid. One box per viewer is required; they will be installed once the units are under vacuum. Fabrication should be complete in two or three weeks. All of the chassis for stepper-motor control are complete and are being installed in the FEL Facility.

The safety system for the photocathode drive laser is being installed in the FEL Facility's clean room. The Facility's oxygen-deficiency-hazard monitoring system, radiation monitoring system, and access-control systems are 80% complete in the accelerator enclosure and 50% complete overall.

The connections between the Central Helium Liquefier (CHL) and the FEL Facility will be finished tomorrow (7 Jun 97), which will enable operation of the cryogenic input-output controller. The database for operating the cryounit and cryomodule are slated for loading next week. All cryogenic controls should be operational one week prior to the 1 Jul 97 cooldown. Installation of the server hardware continues.

Regarding rf systems:

Hipotronics, the vendor for the 225 kW variable power supply that will drive the cathodes of the 8 kW klystrons powering the cryomodule, has been working hard on fabrication and final assembly, but it looks like the delivery date will slip about two to three weeks, i.e., mid-to-late June.

FEL Systems

Wiggler

The wiggler was installed on its girder, after which the girder was turned over to our Alignment Group for alignment of the quadrupoles and wiggler on the girder. Initial test welds were done on the tubing for the wiggler vacuum chamber. More will have to be done before the final tubes can be welded.

Optics

One set of mirrors from Rocky Mountain Instrument with reflectivity R~89% was received and sent to China Lake for testing. We received the 3" molybdenum mirrors for the optical collimator this week as well. The mirror-can order was completely received. The mirror-stands order was awarded this week, and they are expected in next week. Assembly of the optical-cavity components was delayed until the parts can be cleaned. This is tentatively scheduled to begin Monday (9 Jun 97). The optical tables to support the cavity assemblies were installed in the FEL Facility. Drawings for the output Brewster windows that direct the beam to the user labs were signed today (6 Jun 97).

Commissioning/Operations

Simulations of the electron beam from the photocathode to the wiggler proceeded, in which the gun includes the spacer for lower electric field at the cathode (as is the case in the ITS now). The goal was to find machine settings that generated bunch lengths that were short enough to be in the range of the interferometric bunch-length monitors that we are procuring from the University of Georgia. These devices are limited by diffraction at long wavelengths. The outcome was that minimum bunch lengths of about 0.5 mm could be produced, and we are in the process of assessing the consequence for the bunch-length monitors. If this turns out to be a problem, we can use cross phasing in the dispersed region of the 10 MeV line, a technique that works well at longer bunch lengths. A side comment is that longer bunch lengths are desirable from the point of view of reducing wakefields and their effects back on the beam, so there is a need to achieve a good balance between these competing factors. Simulations with the "long" gun are continuing toward determining sensitivities to machine settings.

One byproduct of comparing the simulations to data from gun tests in the ITS is that there is some indication the code (PARMELA) misses certain details about the transverse phase space at low energies. Thus, harp scans from the ITS showing real beam envelopes at known solenoid settings are now being passed to the modeler (Byung Yunn) for cross-correlation.

After careful consideration, we decided that we could relax the strengths (field integrals) of the corrector magnets by 44% without undue risk to the commissioning effort. This should make them easier to design without impacting setup of the machine. To compensate, we are presently considering a slightly tighter tolerance on vertical alignment.

We are planning to use mu-metal correctors in the eight high-aspect-ratio positions in the recirculation arcs as well as for the four phasing positions near the 180-degree bend magnets. However, this is subject to verification that the mu-metal does not saturate in the presence of fringe fields from the adjacent magnets. An experimental technique was devised to measure crosstalk between sextupole, trim quad, and mu-metal corrector magnets, and the experiments are planned into the overall program in the Magnet Test Stand.

Facility

A lot of finish work was done this week. Tile was completed in all labs and the rf gallery except for a few small finish areas that need work. All labs are now essentially complete. Ceramic tile was placed in the bathroom walls and one floor. Drywall was hung in the break area and upper stairwell, then painted. Electricians continued hooking up wall circuits and preparing to energize the HVAC. HVAC folks are in the process of finishing up all the return headers in the equipment room. The elevator hydraulics were tested and the car floor was assembled and tested. The platform rode to all floors. Everything works and should proceed rapidly to certification and operation in approximately two weeks.

Grading was performed outside over the low-conductivity-water trenches. The clean room assembly was completed in preparation for flooring, electrical, and HVAC hookup this week. The clean room is still scheduled for completion on 16 Jun 97. Tables and some power supplies were mounted in the control room. The optical tables for the FEL optical cavity were mounted on stands in the accelerator vault. Several other stands were mounted, surveyed, and prepared for grouting.

All stands for the injector are now in place. Preparations were made for mounting the injector crane rail. The support plates for the injector crane rail are installed. All components for the crane-rail systems except for the cryounit crane are in house. The crane itself is due next week.

Safety-system hardware was mounted in many areas and cables pulled to the switches from the communications room. The high-power rf amplifier for the cryomodule was placed and connected to the waveguides which are now at operating pressure. Jefferson Lab electricians began their hookup of equipment, an activity that will continue for several weeks.

The high-voltage tank for the photocathode gun's power supply was installed in its final location in the FEL Facility. Its legs were cut by a couple of inches to align the output flange with the beam centerline. The components of the elbow of the gas-filled transmission line that connects the tank to the gun are out for bid.

Work on the cryogenic system progresses nicely in preparation for hooking up the injector cryounit and cooling it down in the FEL Facility on 1 Jul 97. All vacuum jackets are being pumped, and vacuum readings are all below 100 microns at this writing (1330, 6 Jun 97). All U-tubes for connecting to the CHL are complete. U-tube design drawings for the cryounit in the FEL Facility were released. The shield return U-tube is 100% complete, and shield supply is 50% complete. Gas lines are 99% complete, needing only a couple more welds and pressure tests. Tests of the transfer lines and gas lines are planned for next week. In short, we are on schedule for 1 Jul 97 cooldown, although detailed plans still need to be made.