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

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IR Demo Project Weekly Report, December 14 - 18, 1998

Date: December 18, 1998

Management

Highlights for the week include attaining 300 W of cw lasing with energy recovery. This is the first time we demonstrated delivery of more linac current (1.6 mA) than can be sustained with the installed rf system (1.1 mA), thus demonstrating one of the key advantages of energy recovery. Neither the laser output power nor the accelerator operation is yet fully optimized at these conditions, as noted below. Mixed in with this exciting result was the premature demise of the currently installed photocathode. It served us well for 6 weeks, delivering roughly 180 C of charge, but died one week shy of our planned shutdown for cathode replacement and the holidays. The FEL team wishes all our colleagues and supporters an enjoyable Holiday Season and Happy New Year!

Preparations continued for the FEL project presentation at next month's BESAC Panel meeting on "Novel and Coherent Light Sources" scheduled for Jan. 18-21 at Gaithersburg, MD. In addition to a summary on the technical capabilities of the IR Demo FEL and Upgrade, representatives from three of the FEL user groups will be giving presentations.

Several members of the FEL team attended a symposium on DOD-sponsored high energy laser projects given at Langley AFB on Tuesday, Dec. 15. On Wednesday, Dec. 16, we had a working session with representatives from Northrop Grumman and AES to discuss potential collaborative development of FEL technology, as a follow-up to meetings held earlier in the month.

FEL Installation/Maintenance Activities

The following maintenance items were completed this week:

To resolve a concern about magnet proximity, measurements were done in the Magnet Test Stand. It was found that, in the backleg, correctors within 1.125 inches of the quadrupole magnets affect them only by a few parts in 10,000 when going from 0 to 10 A. At present, one backleg corrector is at 15% excitation and the others are at 10% or less. Therefore, this is a non-problem.

The performance of three beam position monitors (BPMs) was recovered by just tightening cables, and another BPM had 2 cables replaced.

Rework of the gun is in progress to replace the photocathode and repair the cesiator.

A list of tasks for the January down was compiled and prioritized.

FEL Commissioning Activities

Commissioning of the recirculation loop in the presence of cw lasing continued through 1600 Tuesday to the point of reproducibly generating of order 200 W cw laser power upstairs in the Optical Control Room at cw average currents of order 1.5 mA. Best performance was 211 W upstairs (corresponding to about 300 W out of the FEL proper given roughly 40% losses in the optical transmission line) with 1.6 mA current. At that point we were experiencing a trip of one of the beam-loss monitors indicating beam scraping in the reinjection region, i.e., near the end of the recirculation loop.

A second limitation also emerged this week, namely heating of the infrared detectors in the waveguides of the accelerating cavities that led to persistent trips in one of the cryomodule cavities. We were gathering data to isolate the source of heating, and a preliminary indication is that it is due to high-frequency higher-order modes propagating through the waveguides and onto the detectors. Consequently we are planning to install a screen over each IR detector in the cryomodule during the January down, a process that will involve cycling the cryomodule to room temperature.

Unfortunately, at 1600 Tuesday we lost the quantum efficiency of the cathode. At that time the cathode was generating 40 pC bunch charges and we were running cw. The cryounit rf tripped, closing the drive-laser shutter (as it should). Upon straightforwardly resetting the cryounit rf and opening the shutter, the cathode was "dead". This is a "new" failure mechanism in that the gun high voltage did not trip, meaning apparently there was no surge of vacuum pressure in the gun. Yet both visual inspection of the cathode (to the extent it could be done) and scans of its quantum efficiency show a large imperfection on the cathode surface. We cannot tell conclusively if it is a large pit or a large protrusion, or even if it is "merely" a large inhomogeneity in the surface chemistry. We must remove the cathode for a definitive analysis; resources for auger spectroscopy are prepositioned.

The present cathode proved to be remarkably resilient before its demise. During a six-week period, the cathode supported 37 8-hour shifts of running electron beam, 12 of which were specifically dedicated to cw runs. To roughly estimate the total charge delivered, consider that roughly half the time during these 12 shifts was actually spent with cw beam on (the other half being spent on thinking, resetting trips, etc.), for a total of ~50 hours of beam time. On average, the current during beam-on would have been about 1 mA. Thus, roughly, a total charge of 50x3600x0.001 = 180 C was delivered, a very encouraging result regarding prospects for further commissioning. It will be interesting to see how future cathodes perform because our plan is to begin running them at a macropulse rate of 75 MHz to support average currents up to 4 mA as we endeavor to reach kW-level operations.

In view of the state of the cathode, we deemed it best to begin preparations for cathode replacement. We prepared and anodized a new GaAs wafer. Today (Friday, 18 Dec 98) we are debugging the residual gas analyzer used to monitor the composition of the gun vacuum since it, too, had failed on Tuesday, and we are high-voltage-processing the gun's electrodes to "smooth" them prior to opening the gun to air. We also prepared a detailed schedule for the refurbishment that spans the previously planned shutdown (23 Dec 98 through 19 Jan 99). If all goes well, we should be able to turn on the FEL at about the same time CEBAF is turned on.