The 350 kV DC GaAs photoemission electron gun for the Jefferson Laboratory 10 kW IR Upgrade FEL is the highest average current source of its type in operation today, delivering up to 8 mA of high brightness electron beam. Creating and maintaining ultra high vacuum conditions in the gun chamber is essential for achieving both high yield from the photocathode and long photocathode lifetime performance at high average currents. With a background pressure dominated by hydrogen in the order of 5×10-11 Torr and operating at over 5 mA average current, the photocathode lifetime is 60 hours or 550 Coulombs. Presently, higher average current is limited by electron beam halo from the photocathode and longer lifetime by ion back bombardment, while higher voltages are limited by field emission from electrode structures in the gun. Photocathode lifetimes over 100 hours are required for the next generation photoemission electron gun currently under construction at Jefferson Lab, which is designed to operate at 500 kV DC and 100 mA average current. To meet these specifications, research is focused in three areas; techniques for reducing hydrogen outgassing, reduction of field emission from electrodes coated with an oxy-nitride film, and halo reduction through surface analysis of photocathode roughening during heat processing. Addressing these issues is of vital importance for the continuing development of high average current electron sources for Energy-Recovery-Linac-based machines such as next generation FELs and proposed electron-ion colliders.
This work supported by the Office of Naval Research, the Joint Technology Office, the Commonwealth of Virginia, the Air Force Research Laboratory, Army Night Vision Lab, and by DOE Contract DE-AC05-060R23177.
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