Very high voltage photoemission electron guns

There are a growing number of applications for CW electron accelerators, many requiring high average current and small transverse and longitudinal emittances. Very high voltage DC electron guns with photoemission cathodes are a natural choice for generating the beams for these machines. High average current applications require high quantum efficiency photocathodes at practical wavelengths. The necessary lasers are state-of-the-art systems that must be considered in choosing a photocathode. Field emission from the electrode structures limits the operating voltage and cathode field gradient in these guns. The photocathode operational lifetime is limited by the gun vacuum and by ion back bombardment. Recent developments in areas as diverse as vacuum technology, CW lasers with RF time structure, and the reduction of field emission from large area electrodes show promise for the development of a new generation of DC photoemission electron guns, operating at very high voltage and cathode field strengths well above those obtainable in the past. These developments, and various designs for very high voltage photoemission electron guns, will be reviewed.