An ultrahigh-vacuum S-band photocathode radio-frequency electron gun

High gradient photocathode radio-frequency guns have enabled a broad range of modern accelerator-based scientific instruments. To further enhance the capabilities of these instruments by generating brighter beams, it is highly desirable to the synergies this type of electron sources with advanced photocathodes. However, various photocathodes with low intrinsic emittance and high quantum efficiency require ultrahigh vacuum condition which is far beyond available in existing high-gradient RF guns. Therefore, the physics, performance and feasibility of utilizing these advanced photocathodes under high gradient essentially remain unexplored. In this paper, we report on the concept and comprehensive physical design of a novel S-band photocathode gun that is compatible with advanced photocathodes. The new design features a more than ten-fold improvement in the vacuum level, supported by detailed vacuum modeling. This new gun will enable studies of advanced photocathodes under high acceleration gradient, and eventually be capable of delivering significantly brighter electron beams.