Advances in Cesium Dispenser Photocathodes: Modeling and Experiment

Photocathodes are critical to the design of electron sources in high-power free-electron lasers but must maintain operational readiness and reliability with a long lifetime despite requisite high current density (hence high quantum efficiency), possible drive laser heating, and vacuum contamination. We at the University of Maryland have already demonstrated extended lifetime of cesiated metal photocathodes via the application of the dispenser cathode concept, a mature thermionic cathode technology, as part of our ongoing effort to develop the controlled porosity dispenser photocathode (CPD). This effort is now being extended to high-quantum-efficiency semiconductor coatings. The most efficient semiconductor coatings, notably those responsive to visible wavelengths (e.g., alkali antimonides), are prone to cesium loss in harsh operating environments; the dispenser concept promises in situ rejuvenation of cesiated surface layers by gently heating the cathode and allowing cesium to diffuse controllably to the surface through a porous substrate from a subsurface reservoir. Photocathode lifetime and robustness can be significantly enhanced. Essential to the advancement of the high-quantum-efficiency semiconductor, CPD is a comprehensive understanding of cesium’s behavior. We here discuss the use of cesium in dispenser photocathodes in three photoemission topics: lower temperature operation of a modified cesium dispenser, development of a model for the diffusion of cesium on the surface of such a dispenser, and fabrication of cesium-based semiconductor coatings on the dispenser surface (cesium antimonide; Cs3Sb) for increased quantum efficiency.