In situ annealing of photocathodes using backside electron-beam irradiation

Metal and metal-compound photocathodes are generally used as electron sources in radio frequency (RF) guns. These photocathodes are robust against laser irradiation and RF fields, are easy to handle, and have longevity. The surface cleaning of photocathodes is essential to improve and maintain their quantum efficiency (QE) in an RF gun cavity. In this paper, we propose a backside electron-beam irradiation heating technique as an in situ surface-cleaning procedure for photocathodes in an RF gun cavity. In the proposed technique, thermal electrons accelerated by a DC electric field are irradiated from the back of the photocathode, and their beam power is converted to heat, thereby heating the photocathode. Unlike other surface-cleaning techniques, surfaces of any cathode shape can be cleaned uniformly using this heating method. We designed and developed a cathode plug that includes the proposed heating system for large-diameter (similar to 8 mm) photocathodes. Thermal simulation of the entire system, including the photocathode and its holding structure, and RF simulation of the choke structure for preventing the leakage of RF power were performed. The developed heating system was installed in an RF gun in High Energy Accelerator Research Organization KEK and subjected to RF aging and beam commissioning. From the results of beam commissioning, it was found that the QE of the photocathode increased to approximately 2.5 times the initial value due to the in situ annealing in the RF gun cavity.