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J. Cook, D. Helms, W. Skinner

Division (M7), FEL Coordination Group

F. Dylla

IR Demo Project Weekly Report, July 6-10, 1998

July 10, 1998

Management

Highlights for the week include beginning the process of replacing the cathode wafer in the gun, and replacing the 98% reflective outcouplingmirror in the optical cavity with a 90% reflective mirror to extract higher power.

FEL Installation Activities

We installed and hooked up the air-core correctors in the back leg, and we began to wrap mu-metal where possible through the recirculation loop to screen the Earth's magnetic field. We installed beam-loss monitors along the recirculation loop and hooked up much of the back-leg diagnostics. We also replaced the outcoupling mirror in the optical cavity with a 90% reflective mirror. Both the optical transport line and the Laser SafetySystem for Lab 1 (Polymer Lab) are nearing completion.

FEL Commissioning Activities

Yesterday (9 July 98) we lost the photocathode due to arcing and vacuum activity near 350 kV. Recesiation did not help; apparently a field-emitting site came into being that emits after cesiation at the voltage indicated. Consequently, we processed the gun at 500 kV overnight and through this morning to try to burn away field-emission sites. At 1245 today we decided to pull the cathode since it exhibited observable field emission at 440 kV coincident with the location of the drive-laser spot, and since we had some evidence of hydrocarbon contamination duringhigh-voltage processing that would make recovery improbable. The processof replacing the cathode and reconstituting the gun will take about ten days, but the time will be well spent toward completing more installation tasks and working on additional detailed procedures for kW lasing.

Prior to losing the cathode we accomplished several commissioning tasksthis week, including checking the magnetostrictive tuner for one of the cryounit cavities (its current drive circuit needs more work); running the cryomodule cavities at their peak gradients (for 1.5 hours with only one rf trip, indicating we should be able to produce and sustain 47 MeV beam for straight-ahead lasing