Design considerations for a high voltage DC photoemission electron gun at Cornell University

At Cornell University, we have designed and built a DC photoemission electron gun that is capable of operating above 500 kV and up to 100 mA average current. Such a gun is the best choice as a high average power driver for an RF linear accelerator. High brightness photoemission guns must fulfill a difficult set of requirements in order to provide the types of beams needed for new x-ray light sources, including XHV vacuum properties, robust HV insulators, stable power supplies, electrode materials and preparation to reduce dark current, and electrode conditioning. In this paper, we will discuss these requirements, how to meet them, and the difficulties encountered along the way.