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

cc: Division (M7), FEL Coordination Group

From: F. Dylla

Subject: IR Demo Project Weekly Report, November 9-13, 1998

Date: November 13, 1998

Management

Highlights for the week: we are back on the air with FEL lasing at the 300 watt level with 1 mA of straight ahead current. Commissioning of the recirculation loop has re-commenced.

To make the most of our last two months of experience with turning the photogun around, we have commissioned a gun study team that will analyze the gun performance since its first operation in the Injector Test Stand until the most recent conditioning exercise. The first set of tasks for the team will involve analysis of installation, maintenance and conditioning procedures and recommendations for incremental improvements that would improve the performance and reliability of the gun. A second set of tasks will involve recommendations for the most effective rebuild or redesign of the gun that would be included in the 20kW upgrade proposal.

A meeting was held this week with Northrop Grumman and Advanced Energy Systems (AES) about possible collaboration in the development and presentation of a proposal to the Navy for the 20 kW Upgrade. Northrop Grumman and AES provided essential technical and programmatic support to Jefferson Lab during the original IR Demo construction project.

The program for a SURA sponsored FEL workshop on short wavelength FEL technology and applications has been finalized. The program was organized by us and the Duke, Vanderbilt, and BNL FEL projects. The one day workshop will be held next Friday (Nov. 20th) at the AAAS Building in downtown Washington. Full information about the workshop is posted on our WEB site at http://www.jlab.org/intralab/calendar/sura98agenda.html

FEL Installation/Maintenance Activities

Apart from the gun activities summarized in the next section, this week's maintenance/installation activities resulted in the following accomplishments:

The power supply for the recirculation dipole string failed and was repaired: a SCR pulse firing board needed to be replaced; the Be type SCRs and an unmodified phase imbalance relay were found and also replaced.

Assorted hardware checks were accomplished. For example, a trim winding on the injection dipole magnet was suspect but was found to operate properly, and the mirrors in the Happek #2 interferometer needed to be realigned as a result of difficulties with remote operation of their picomotors.

FEL Commissioning Activities

We are all really quite excited. After losing the cathode due to an arc Friday afternoon, the gun was successfully resurrected late Wednesday after careful processing up to 510 kV, a long heat clean of the cathode, and careful cesiation. Within 14 hours after turning on electron beam (during which 60 pC gun performance at 350 kV was established and the injector setup was iterated), the machine was lasing, and that is after being down nearly 13 weeks, save for the 2-day run last week. After another 14 hours (during which the injector and accelerator setups were further refined) the machine had delivered 235 W of cw IR laser power upstairs in the Optical Control Room! We have yet to calibrate accurately the transmission losses in the optical transport line, but from what we know now, the delivered power would correspond to at least 300 W, and perhaps more like 350 W, out of the FEL itself. During the lasing runs, the accelerator delivered uninterrupted 1.1 mA cw beam to the straight-ahead dump for several hours. Bottom line is, within 28 hours after pulling electrons from the cathode, the straight-ahead machine performed at least as well as it ever did.

At this writing (0945 Friday, 13 November 1998), we are running the FEL in the recirculation mode and beginning to work through the High-Power Setup Procedure that should ultimately lead to high-power-cw lasing with energy recovery. The quantum efficiency of the cathode has been dropping off during the cw runs, not surprisingly, and we anticipate needing to rejuvenate it sometime next week, perhaps midweek. Some combination of heat cleaning and recesiation will likely be used; however, we are planning to do a cathode scan with the new scanner early Monday morning to ascertain whether the "poor" quantum efficiency is localized coincident with the drive- laser spot on the cathode. If so, we may be able to proceed by simply moving the drive-laser spot to a new location on the cathode. We will soon know; it will be a contribution to our learning curve. Of course, eventually the cathode wafer will need replacing, but right now we don't know when that will be.

We believe the problems we were having with the gun are traceable to a persistent and robust field emission site that took much time and high voltage to process away. Profuse field emission during high-voltage turn-on kept killing the cathode photoresponse after cesiation. Processing late Friday and during the first half of this week ultimately (and finally!) resulted in quiet electrodes before and after cesiation. Another contributor may have been a malfunctioning cesiator which we rebuilt last week. We feel that the gun availability can be greatly improved with some straightforward upgrades, such as the addition of a load lock to enable processing the cathode without affecting the electrodes, but our penurious condition prohibits their incorporation. It continues to puzzle us why we are still indigent.