Controlled porosity reservoir cathode and photocathode research

Summary form only given. Calabazas Creek Research, Inc. (CCR) is continuing research on controlled porosity reservoir (CPR) cathodes [Ives, R.L., et al., 2010]. In addition to thermionic dispenser cathodes, CCR is teamed with the Institute for Research in Electronics and Applied Physics (IREAP) at the University of Maryland to apply this technology to cesiated dispenser photocathodes. CPR technology provides increased emission uniformity with longer lifetime. Uniform distribution of work function material over the cathode surface provides improved emission. For thermionic cathodes, this is material is barium; for photocathodes, it is cesium. The cathode includes a hexagonal pattern of pores through sintered tungsten. The barium/cesium diffusion rate can be controlled by the pore size and cathode thickness. For thermionic cathodes, the goal is to provide a barium diffusion rate that matches the evaporation rate from the surface, which depends on the emitter temperature. The temperature is defined by the required emission current density. For photocathodes, cesium is periodically deposited on the surface whenever the quantum efficiency drops below a minimum value. Normally, the cathode operates cold, but it is heated whenever cesium is required to rejuvenate the surface. The heating initiates cesium diffusion to the cathode surface. Lifetime is defined by the rate at which barium/cesium is used and the volume of the reservoir. The reservoir is located between the cathode and the heater. For thermionic cathode, lifetime can be increased by approximately a factor of four over impregnated porous tungsten cathodes with the same volume. Lifetime for dispenser photocathodes depends on operating conditions, and early experiments demonstrated improvements of well over an order of magnitude [Moody, N., et al, 2007].