The viewer flag for one of the wiggler viewers was changed from graphite to beryllium for increased sensitivity.

Alignment was completed of all installed elements on the two recirculation arcs.

The buncher cavity was run stably at lower power levels in its heater mode to check its cooling loop and associated phase stability. It was also run stably with an additional 1 kW heater load that opened its water control valve 1 turn.

FEL Commissioning Activities

We again did not run electron beam this week, although we are attempting a short-duration run in the injector at this writing (1610, 2 Jul 98) to check the gun. After processing the cathode in situ over the weekend, its photoresponse was marginal but probably satisfactory for sustaining electron-beam operations for some amount of time. This processing was noteworthy because we subjected the gun to its highest voltage to date, 494 kV. However, the beamline vacuum along the machine was generally poor, too poor to open many of the vacuum valves. The sources of vacuum contamination were traced to the newly installed valves that replace the eight defective valves. Thus, considerable effort was spent through the week toward improving the vacuum, as is reported in the Installation Section above.

As the week progressed, we noted that the vacuum in the gun stabilized at a value somewhat higher than normal, a circumstance detrimental to cathode performance (and unrelated to the vacuum situation just mentioned). Remeasurement of the cathode's photoresponse showed a 40% drop over a 48-hour period without any electron beam. Consequently, we processed again, this time by raising the voltage to 500 kV for several hours and then doing a heat clean at 600 C. Unfortunately the photoresponse did not appreciably change. At week's end we were in process of checking out the gun with electron beam to determine definitively if it can be used for pulsed beam operation next week. If it can, a detailed program is planned for the week for which pulsed beam suffices. In particular, we can do hardware checks with electron beam, including running the beam at higher energy that would aid in lasing near 3 µm in the straight-ahead machine, as well as electron-beam diagnostics, reestablishing lasing, and working toward optimizing the lasing process. Otherwise we will have to replace the cathode wafer (which, on the positive side, would provide more time for additional installation and development of detailed procedures for kilowatt lasing).

Additional progress was made toward developing procedures that support recirculation and energy recovery. Thanks to the efforts of Dave Douglas, we now have a "High-Power Setup Procedure" that incorporates the thinking the commissioning team has done thus far.