MEMORANDUM

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

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IRFEL Weekly Report, May 27-30, 1997

Date: May 30, 1997

Management

The highlight of the week was a successful re-start of photogun measurements on the Injector Test Stand. Excellent progress was made on characterizing the performance of the gun at 350 kV. Details are given below in the ITS section of the report. Also of note: on the basis of further inspection and dimensional checks on the wiggler, we accepted the device for use in the IR Demo.

Lots of high level activity occurred in Washington this week concerning potential FY98 funding for the FEL program. Senator Warner requested a briefing from ONR management on FEL R&D for Naval applications. A follow-up briefing will be held on Monday, June 2, with Adm. Baciocco (the chair of MTAC), and representatives from SURA and Jefferson Lab.

F. Dylla was invited to make a presentation at the Combustion Science Workshop held this week in Chantilly, VA sponsored by DOE-BES. Several attendees expressed interest in using the IR FEL for combustion research.

Injector Test Stand

Regarding the photocathode gun:

High voltage processing of the gun at 420 kV was completed by week's end. Twenty hours of heat cleaning of the cathode was completed and a new cathode was fabricated with a measured quantum efficiency of 7.2%. The initial cathode degradation was not bad (40% in first 24 hours of operation). The dark current (field emission) was measured at 1.3 nA at 350 kV cathode voltage; however, measurements indicate the emission current is doubling in magnitude for every 10 kV increase in cathode voltage. We performed bunchlength measurements at 1, 10, 60, and 135 pC at 350 kV. Transverse emittance measurements were performed at 1, 30, 60, 100, 135 and 150 pC/bunch. We have observed some beam scraping during the transverse measurements and subsequently verified the light box as the location using Geiger Muller tube detectors. To correct the beam scraping, we are investigating possible changes to the ITS. Modelling has been started to address the issue.

Regarding development of warm ceramic windows for the cryounit:

Warm windows have been tested and qualified in the resonant ring and vacuum fixture. These windows are limited to 10 kW operation levels which will support first light operations. These will be installed on the 1/4 Cryomodule Monday 2 June. A short cold test cycle on the 1/4 Cryomodule will follow to characterize window/waveguide performance. Work continues on the production of 50 kW warm windows.

Accelerator Systems

Beam-transport system report, 5/3097:

Fabrication of dipole magnets is proceeding on course. All 16 Optical Chicane coils are on hand and most machining is done on the cores. The first article Injection Chicane coil (DU) was received and most machining is done on the cores while they await the mu metal delivery. PECO has most of the incidental parts ready for 180 degree dipoles and reverse bends and the pole pieces should be back from heat treatment next week to begin final machining. Everson Electric will glue up a few trial pieces before they retry gluing more optical-chicane dipoles and proceed with assembly.

The sextupole core bids came in high. We are clarifying our requirements to reduce the price with the responsible bidders. The Trim Quad design package is in checking. The bids for the long Panofsky-style quadrupoles for the recirculation dump are due today.

Discussions with D. Douglas indicated the most difficult high aspect ratio correctors in the arcs and the phasing dipoles may utilize the mu metal design even though hysteresis is present. Additional analysis indicated that reducing the maximum required field integral by 44% is a prudent change to the requirement for the remaining air core correctors. To home in on a cost effective air core concept that meets specified harmonics, we are measuring a mock up, made of four coils in an approximation to a cosine distribution. Early results are encouraging. The prototype air-core corrector built by Syntronics will undergo testing Monday but just from initial impressions, results may not be encouraging. We will start the TOSCA magnetic modeling once we have blocked in a direction to go. At the same time, AML a Florida company that specializes in precision field, air core magnets, is putting together a preliminary proposal to produce most of the air core correctors.

Parts for the vacuum chambers for the optical chicane are out for bid and the chamber for the 180 degree dipoles was signed off. Welding on the X Chamber for the Injection chicane started this week. Northrop Grumman completed details for the first of six Return Bed chambers. We will have to design the remaining three chambers.

Stands for the Optical Chicane and the injector were installed in the enclosure.

Regarding cryomodule fabrication:

The fourth and final cavity pair is expected to be turned over to for cryounit assembly the beginning of next week. The third cavity pair has been turned over to cryounit assembly. Tuners and instrumentation are being installed and a normal cryounit assembly is expected. The second pair turned over has been inserted in the helium vessel and assembly continues without issue. Cryomodule assembly is progressing without issue with the first cryounit. Thermal shielding has been installed and rough alignment completed. The return end can beam pipe is scheduled for installation the beginning of next week.

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

Beam Instrumentation: Documentation continues. The beam viewers and BPMs have been turned over to the installation group for installation with the beam tubes and magnets.

Safety: The cable way and conduit is complete for the personnel safety system, and the cable installation has begun. This is in preparation for the cooldown of the Quarter cryomodule July 1. Preparation continues for the Machine Protection System design review next month. The logic controllers have arrived for the laser safety systems. These will be assemble in July for installation in each of the optical labs.

Controls: The network hub and server computers are being installed in the FEL building. Also the network connection to the cryogenic control racks from the central helium liquifier (CHL) plant is in progress. These racks will be powered and controlled from the CHL for added reliability; they are an extension of the CHL systems. Software continues to be developed and debugged in the injector test stand.

DC Power: Trim rack cabling is being staged in the north linac, two of the four sets of cable are ready for installation. These cable the 32 channel power supplies to the distribution boxes in the tunnel.

Vacuum controls: The control chassis are being fabricated. The four equipment racks are progressing well, they should be powered up next week. The control cards for the ion pump power supplies are being fabricated now.

Regarding the cryogenic distribution system:

All major transfer line welding is complete. All vacuum jackets are being pumped. I&C engineering and fabrication are both complete. Installation will be completed after 1/4 cryomodule is in the FEL building. 4 of 5 U-tubes for CHL connection are complete. Number 5 is 75% complete U-tube designs for the 1/4 cryomodule in the FEL building were released. The shield return U-tube is 60% complete but on hold for transfer line installation. Final connections of gas lines at Flex can are being worked. Gas lines in FEL User Facility Accelerator enclosure are complete. We are on schedule for July 1 cooldown. Detail plans still need to be made.

FEL Systems

Injector Reinstallation

The high voltage tank had to have its legs shortened to correct a design error. This was completed by weeks end. Continuing delays with the delivery of the air handling unit for the clean room have moved the completion date to the week of June 16th. We have obtained permission to install the laser safety system and the new optical table in parallel with the final installation. This will minimize the overall schedule slip. The room itself was installed this week and looks quite good.

Wiggler

The wiggler has been accepted from the vendor STI Optronics. Measurements indicate no damage to the wiggler from shipment. The machine shop is making progress on the wide wiggler tubing and is planning for a test weld next week. The wiggler vacuum chamber support has been ordered and is due in mid-June.

Optics

FEL

Inspection of the transmission curves for the mirrors from Rocky Mountain Instrument indicates that two of the three sets most likely will not meet specifications (the R=99.7% and R=98% mirrors), so they were returned to the vendor. The optical collimator has been awarded to a local machine shop. Designs for some mirror stands will be signed today (5/30). The bellows assemblies are brazed to the cooled mirror holders, and we will begin assembly in a clean room next week. The stands for the optical tables were grouted, and the survey team has alignment fiducials for the upstairs optical transport line.

ITS

The laser was operated for over 60 hours in support of gun experiments. The power output was quite stable over this period. We received some new information on modulator biasing that raised the overall extinction ratio of the two variable-speed modulators by over a factor of two. The second "dry" EO modulator was also installed in the beamline. The divide-by-two EO modulator suddenly quit responding to changes in the DC bias, the unit was returned to the vendor.

Commissioning/Operations

The main consequence of the experimental finding in the ITS that there is scraping of the outer portions of high-charge bunches inside the light box is a reduction in the measured transverse emittance of these bunches. The outer parts of the beam are weighted heavily in the coordinate and velocity moments that comprise the transverse emittance. Consequently it is more difficult to compare the experimental results with scraping to numerical simulations wherein scraping is absent. In view of this development, we embarked on a series of numerical simulations to assess whether scraping can be significantly reduced or eliminated by varying the laser spot size at the photocathode. For instance, one would expect space charge to have less influence in a larger beam, so perhaps increasing the spot size will reduce scraping. At this writing (noon Friday, 30 May 97), the investigation is in progress.

Simulations from cathode to wiggler were also done by including the cathode-anode spacer for lower electric field at the cathode, as is now the case in the ITS. The present status is that it has been easy to find settings that deliver the required beam at the wiggler, but the bunch lengths in the injection line are longer than desired given the performance specifications of the interferometric bunch-length diagnostics that will be installed there. These simulations are continuing in an effort to find solutions with acceptably short bunches in the injection line.

We made considerable progress establishing locations and types of weak-field corrector magnets based on the measured performance of various prototypes constructed to date. This activity is continuing, and our goal is to finish it very soon, pending the outcome of additional measurements in our Magnet Test Stand of existing prototypes that have been slightly modified for what hopefully will be improved field quality.

Facility

A small army labored through the week trying to bring the upstairs to a finished state. Despite several days delay in getting started on floor tiling, two labs and a large fraction of the RF gallery were tiled by week's end. Several other areas are prepared and will be done shortly. Electricians spent the week pulling cable to outlet boxes in all the labs from the breaker panels. Drywall was completed in the break area and the stairwell. Only the downstairs lobby and touch-ups remain to be drywalled. Glass is placed in the stairwell skylight and double doors are on the front of the building. The facility is fully enclosed now. Tiling has begun in the bathrooms. The hydraulic piping was lowered into the main elevator shaft. That work is 50% complete. We expect the elevator to be complete in 2 weeks and ready for inspection. Despite having to co-exist around tilers, electricians, etc., the photo-injector clean room was delivered, assembled, and installed except for electrical hookup, and installation of the air handler. The air handler is still about two weeks from being installed. The safety system box duct was installed in the upstairs and cables are being pulled to the downstairs duct from the communications room racks. The second (and final) line of cable tray was 20% complete upstairs by the end of the week. Berming is 80% complete outside. Downstairs grouting is underway on the magnet stands previously installed. More stands are due today (5/30/97). The gun HV tank was discovered to have been constructed 2" too high so welders cut the legs and reinstalled the tank. Installation of the power supply should get underway next week. Both the supply and return cryogenic transfer lines were welded to the lines installed last week in the sleeves and are undergoing leak check. A power outage is scheduled for Saturday to energize the upstairs electrical panels. Over the holiday weekend the low conductivity water circuit was hooked up to the CEBAF main system and pressurized. No problems. We now have cooling water available in the building. Since the tiling is nearly complete in the RF gallery, the LCW lines can be finished in the coming weeks as welders are available.