Compatibility of SC cavity and "foreigner" photocathodes in SRF gun

More and more electron accelerator projects ask for “super” electron beams with high brightness, low emittance, and high average current. Under this background, much attention is paid on the research and development of new electron sources potentially providing electron beams with better quality. Superconducting RF photoinjectors allow CW operation and meanwhile provide high E-field on cathode to generator high bunch charge and low emittance beam, thus it is a promising candidate for such kind of high current and high brightness electron source.

However, because Nb itself has too low quantum efficiency, the normal conducting photocathodes, such as alkali photocathodes, are the best photoemitter with high quantum efficiency for SRF gun. The compatibility of photocathodes made of ‘foreigner materials’ inside the sensitive Nb cavity is of the biggest difficulty for the designer of SRF gun. One has to solve a lot of unwanted problems like compatibility, particle contamination, multipacting, dark current etc.

In this presentation we collect the running results of SRF guns with normal conducting photocathodes at HZDR and BNL, share the experience gained during the long term development experiments, and discuss the relative problems in the future development.