Advanced dispenser cathodes

Abstract

Summary form only given. New, advanced thermionic dispenser cathodes are available that provide significant improvements over previous technology. These cathodes, designated controlled porosity reservoir (CPR) cathodes provide unprecedented control of the barium diffusion rate and lifetime. Cathodes are being produced that provide emission current density of 30 A/cm2 with projected lifetime exceeding 10,000 hours. These reservoir cathodes incorporate a thin, tungsten cap over a reservoir of barium calcium aluminate. The cap is fabricated with a uniform array of pores over the emission surface. Cathode designer can control the diffusion rate by proper selection of the pore size and cap thickness. The required diffusion rate can be defined by the cathode current density specification. The current density defines the cathode temperature. From the temperature, one can calculate the monolayer evaporation rate of barium from the surface. The lifetime of the cathode can then be estimated using the reservoir volume, which also is controllable within limits. Consequently, these cathodes can provide high current density operation for significant lifetimes while avoiding excessive barium deposition on adjacent surface. Increased current density operation allows reduction or elimination of beam compression, improving beam quality and simplifying electron gun design. The current program is exploring alternative materials for work function reduction and developing larger diameter cathodes for high power and/or lower frequency devices. The program is also refining design equations to improve the accuracy and increase reliability. Calabazas Creek Research, Inc. is also working with the University of Maryland to explore the configuration for cesiated tungsten reservoir cathodes. If successful, this would dramatically extend the lifetime of photocathodes for RF guns. This presentation will review the cathode geometry, describe the design equations, present experimental results, and discuss devices currently being fabricated with CPR cathodes.