A study of tungstate formation in dispenser cathodes

L-3 Communications ETI has been involved in the manufacture of Vacuum Electron Devices (VEDs) for over 50 years. The cathode is a critical component of the VED and L-3 ETI has developed in-house facilities for the research, development, manufacture, and test of the Space TWT cathode [1, 2]. In addition to cathodes used in VEDs, the Electric Propulsion Product Line at L-3 ETI uses hollow dispenser cathodes. In this area as well, L-3 ETI has invested resources in research and test. The performance of the M-coated dispenser cathode over time depends primarily on the diffusion of the M-coat layer and the depletion of barium. For the hollow cathode, the depletion of barium is considered the primary life-limiting factor. The depletion of barium is dominated by evaporation but the chemical reactions that occur inside the dispenser pellet can produce barium tungstates that effectively prevent the barium from migrating to the surface. This presentation examines the role of barium tungstate formation on the life of the cathode.