Highlights for the week include: recirculating up to 2.5 mA cw electron beam, lasing "easily" and stably at 100+ W cw with 2 mA recirculating beam, and putting up to 70 W cw into an optical dump in User Lab 1.

On Tuesday February 16th, Rear Admiral Paul Gaffney, Chief of Naval Research, visited Jefferson Lab for a tour and briefing on the FEL project and potential applications of interest to the Navy. Admiral Gaffney was accompanied by two officials from the Office of Naval Research, Dr. Jim DeCorpo and Dr. Eli Zimet, and the Commonwealth's Secretary of Technology, Don Upson. The technical briefing included a status report on the current FEL, the proposal to upgrade the FEL for 20 kW of IR power, and potential IR applications of interest to the Navy and two of our industrial partners, Northrop Grumman and Advanced Energy Systems. By all accounts the visit went well and we are expecting continued collaboration with the Navy.

At this writing (1015 on 19 Feb. 99) we have the FEL vault open and are installing a suite of

diagnostics on one of the cryomodule waveguides to ascertain the cause of, and fix for, the IR

detector trips. Simply screening the detector heads with a wire mesh had essentially no effect; the IR detector in cryomodule cavity #2 reproducibly trips at about 1.5 mA average current. To go to the higher currents reported under "FEL Commissioning Activities" below, we lowered the trip levels and instituted careful administrative controls. A number of other minor maintenance activities are also in progress, such as checking the sensitivity of one of the wiggler-viewer cameras, and checking cable connections to the beam scraper in the first recirculation arc. The first time the scraper was operated during commissioning was yesterday; it operated successfully (moved in and out on command), but its position readout was inconsistent with its visual movement, which led us to suspect the cable connections. Plans are to resume running at about noon today.

We are also formulating plans for the gun rebuild. Key thoughts at this time are centered on possible ion implantation of the spare cathode ball for reduced field emission, and construction of a new, focused cesiator. Plans have not yet solidified, but at this writing there appears to be the potential to do both prior to the next down-time for gun refurbishment (possibly ~2-3 weeks from now? -- we won't cease commissioning activities before it is necessary to do so).

We have run the FEL continuously since 1600 Tuesday, 16 Feb. 99. Already we have pulled ~70C off the cathode as indicated by our newly installed charge integrator, and the cathode performance has degraded very little, if at all. Improvements in machine performance have been incremental, and include: recirculating up to 2.5 mA cw beam without lasing, recirculating up to 2.3 mA beam while lasing, and devising a machine setup that quickly yields 100+ W sustained, stable output with 2 mA cw recirculating beam at 74.8 MHz repetition rate (~25 pC bunch charge). We have not attempted yet to optimize the laser output. Rather, we have concentrated on raising the cw recirculated current, which in practice means improving the match of the electron beam to the beam-transport system. Consistent with the plan mentioned in last week's report, our goal is to continue improving the match until we can recirculate ~3+ mA, then optimize lasing in an effort to establish a new power record, hopefully ~500 W. Then we will decide on a plan for achieving higher power. For example, it appears that at some point we will need to embark on a program of injector optimization to get very high currents (~5 mA), but first we wish to finish optimizing the transport and laser to see what we can produce.

Yesterday we tried using the beam scraper while running pulsed electron beam. It was encouraging in the sense that the scraper appears indeed to be useful for blocking the energy tails of the beam. In turn, it offers the potential to increase the strength of lasing (thereby generating larger energy spreads, perhaps ~10-15%) while successfully recirculating the waste beam.

Other commissioning accomplishments this week include successfully testing the Laser Safety

System in User Lab 1, putting up to 70 W cw into an optical dump in this Lab, commissioning

various laser-beam diagnostics, and in particular the autocorrelator, and making path-length

measurements around the machine using the new diagnostic rf cavities. A bunch-length measurement this morning indicated we were delivering <100 µm bunches to the wiggler.

Later today and this evening we will do some tests concerning the waveguide IR detectors in the

cryomodule (as mentioned under Installation/Maintenance above), and we will do more lasing

studies. Efforts to increase the cw power will resume next week.