MEMORANDUM

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

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IRFEL Weekly Report, June 23-27, 1997

Date: June 27, 1997

Management

A successful Laser Processing Workshop was held on June 25-26. Approximately 50 attendees representing most of the LPC institutions participated in the workshop. Accomplishments at the workshop included initiation of a series of proposals to be developed over the summer for initial R&D projects with the FEL and planning by three of the working groups (polymers, metals and microfabrication) for fit-up of the User Laboratory.

The week's highlights included considerable installation activity including moving the gun into the FEL building. Details are discussed below.

Injector Test Stand

The gun was moved to the FEL Facility this week, and the cryounit is there, as well. Accordingly, this will be the last weekly report on the Injector Test Stand.

Regarding the photocathode gun:

Prior to moving the gun, we completed the planned measurements of bunch length and longitudinal emittance at high bunch charges, obtaining a complete set of "good" data for 135 pC bunch charge. Although we still need to analyze the totality of the data carefully, preliminary indications are that they agree very closely with the predictions of simulations (PARMELA).

We also assessed the results of last week's run to measure the influence of dark current on photocathode lifetime. The cathode quantum efficiency was measured for eight six hour periods, divided into four separate conditions, each repeated for two six hour periods. These conditions were: 1. HV off, 2. HV on, solenoid unpowered, 3. HV on, solenoid powered to nominal correct strength, 4. HV on, solenoid powered to about double strength, to try to capture electrons emitted from the edges of the cathode, nearby areas of the ball, etc. The net result is that there is no obvious dependence of the cathode lifetime on HV on or off, nor on the solenoid strength. For the conditions of this measurement, whatever field-emission currents were present did not materially affect the photocathode lifetime, no matter where the solenoid was used to put the beam, nor did the fact that the high voltage was on have any discernible effect. A rough estimate of the cathode e-folding lifetime under these conditions is 220 hours. This isn't great, but it does imply that the photocathode should stay alive long enough to conduct operations over some reasonable period of time. Typical initial quantum efficiencies (QEs) are in the neighborhood of 7% or better, and if there were 2.5 W of green light incident on the cathode, (corresponding to having a nominal 5 W of green at 74.85 MHz from the laser and operating at a bunch repetition rate of 37.425 MHz) we could maintain 5 mA of beam current with cathode QEs down to 0.5%. If the cathode decay were truly exponential, started from 7% initially, and had a 220 hour 1/e lifetime, this would imply operation for almost 600 hours. This means that during our experiments something other than the bad effects of field emission or the simple presence of high voltage on the cathode was shortening the photocathode lifetime. Whatever is doing the damage is associated with electrons being emitted, and the decay is likely determined by where these electrons go. It is conceivable that things will be better in the FEL injector, i.e., when the beam goes through the cryounit as opposed to going directly into a nearby dump, as long as there is no significant scraping along the way.

The drive laser was operated this week to make some light box transmission measurements, and some high dynamic range autocorrelation measurements. We will move the drive laser to the FEL building next Monday (6/30), with the rest of the optics following the next day. We plan on having the drive laser operational by the end of the week. In preparation for the move, the optical table was set in the Clean Room, and we cleaned the room. Plumbing for the laser's chiller will be completed when the laser is moved.

Accelerator Systems

Regarding the beam-transport system:

Everson Electric glued mu metal on one of optical chicane dipole successfully and are continuing with the assembly, anticipating completion Monday.

Reverse bend cores continue in final machining at Process Equipment Co. while core slabs for the 180 degree bends are in heat treat and its coils may start winding next week. At Master Machine, they are performing the last operation (grinding to height) on the last parts (back legs) required for the injection/extraction dipoles. All remaining injection/extraction dipole coils were shipped Wednesday.

We signed-off of the Panofsky trim quad drawing set. Sextupole cores and coils remain on track for a delivery on July 8.

In the corrector area, all three mu metal correctors (4 each required) are being designed. Steve Lassiter is using TOSCA to analyze a constant perimeter design, including end effects. So far he has achieved a magnet good to 2.7 % using crude angle changes in the coil shells and is confident that he can improve on this to the required 1 % level. This sets the stage for designing the basic coil of the 15 nested and 11 single pole air core correctors that we need.

More formalized welding and cleaning specifications were signed off at the start of the week to aid in making the fabrication and welding of chambers a more defined process. Welding on the extraction dump Y Chamber body was completed while parts for the optical chicane chambers were being readied for welding. The 60-inch test piece chamber was deliberately contaminated with machining oil and was cleaned with our steam cleaner. It will be capped and pumped down to establish a base pressure for that level of treatment. This will help us assess if we can use the Injection X Chamber.

Design continued on the Scraper while the insertable dumps are out for bid.

Regarding cryomodule fabrication:

Cryounit assembly production is on accelerated schedule providing small amount of slack to support on time delivery of the cryomodule.

Cryomodule assembly is on schedule with no issues. The cryomodule should be delivered on schedule.

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

The network for CEBAF connections are 80% installed. 50% of telephone cables are in place. Computer server racks have begun to arrive, with the balance arriving next week. The network hub has been set up and will be installed and commissioned late next week. The ITS equipment will begin to move in on Monday.

The ODH system has been certified in preparation for next week's cooldown. The cryogenic controls are fully operational. Trim magnet trunk cables were installed. The control room tables are complete.

FEL Systems

Wiggler

The vacuum chamber supports were delivered this week. The e-beam welder is continuing to refine the welding technique on the vacuum chamber parts. The long tubes being made outside should be complete next week. The quadrupole girders which mount on the wiggler are being modified and should be reassembled next week.

Optics

The Brewster window pickoffs (one plus a spare) passed inspection and are being submitted for cleaning. All optical cavity components, save the mirror holders and bellows, have been cleaned. This will begin on 7/7, and then we will reassemble the cavities in the FEL building. The mirror can stands were received; the installation team is beginning to mark positions to set the anchors for them. They are also marking the position for the optical transport line through the User Labs. The Mach-Zehnder interferometer measurements in the FEL building indicate a relative change in separation between the two optical tables of about +/- 60nm. The fundamental frequency of the oscillation is ~28-29 Hz. While the level of oscillation shouldn't impact us, we'd like to identify the source of the vibration, so tests will be conducted next Monday evening, where we shut down LCW and the HVAC to the building for a brief time. We twice attempted to braze calcium fluoride mirrors into their copper holders, both times they cracked during cooled down. We believe we are cooling too quickly and will try again next week, once we receive another mirror.

During the LPC meeting, we met with Dr. Sadegh Siahatgar and updated him on optics results from China Lake and gave him a tour of the optics lab and the FEL Building.

Commissioning/Operations

Some effort was spent to ascertain whether the tight alignment tolerances on the quadrupoles immediately preceding the wiggler could be met by way of the conventional Jefferson Lab alignment procedures. Specifically, residual twist, yaw, and pitch in these magnets are potentially of concern. We are in process of modelling the effects of these possible alignment "errors" on the beam leading to the wiggler, and this will lead to a more refined specification on alignment tolerances.

Follow-up studies with PARMELA (mentioned in last week's report) were done to investigate whether ghost pulses during the 135 pC runs of the photocathode gun were scraping in the light box. The results indicate tighter confinement of the ghost pulses than of the bunches, indicating no significant scraping. We therefore are investigating where on the ball of the cathode-support assembly would field-emission sites likely lead to scraping in the light box. We know scraping was occurring in the light box during the high-bunch-charge experiments because the resultant radiation saturated Geiger-Muller tubes located there. So, it is clearly of interest to ascertain the source of the scraped electrons.

Facility

Finish work and punch lists are the order of the day for the rest of the contractor effort. Partitions went up in the men's bathroom. The toilets now flush. Vinyl tile is in the lower lobby. The kitchenette cabinets were installed but incorrectly and will require rework. Site work is heavily underway to put in the road to the loading dock, the west loading ramp, and curbs and parking lots in front. By weeks end the areas had been graded and the curbs poured. West emergency exit stairs were also being cast. They were about 50% done at weeks end. The elevator doors are installed and the controls are being hooked up. The plan is for certification next week. Controls people were balancing the HVAC units through the week. It was a good test case for high temperature excursions. No problem keeping things cool. Chilled water was plumbed to the clean room and set up for the photo-injector laser. Safety system equipment was installed by Jefferson Lab folks in preparation for the turning on of liquid helium in two weeks. ODH sensors were successfully tested and are on line. Helium gas began circulating in the cryo transfer lines. RF crews are heavily into hooking up the injector cryounit and wiring up the cryomodule low level controls. The gun was moved into the injector pit. Survey crews are checking grids and we continued measuring vibrations and length excursions on the optical cavity tables. So far it looks very good. Controls are starting to go in to the control room. Fiber and cable was pulled for the control network and phones throughout the upstairs.

The gun is moved to the FEL facility and is in the injector pit. The drawings for the new optical transport mirror cans are being checked. The clean room is awaiting certification from Clean Air Technology. The laser optical bench and the plumbing for the laser chilled water were installed this week.