Reliability assessment of dispenser‐type thermionic cathodes

Abstract

An extensive analysis has been performed of life test results obtained in the long‐term (15,000 h) operation of both tungsten matrix and tungsten‐osmium matrix dispenser cathodes. Sensitive indications of potential life capability for these cathodes are shown to be available via systematic interpretation of temperature‐limited emission characteristics over time. These characteristics follow Arrhenius law temperature dependencies, with activation energies either identical with or closely similar to those determined separately for active material dispensation from these cathodes. Optimum temperature settings are determined, for each cathode type, at which projected life is maximized. Only modestly accelerated constant stress and step‐stress results yield credible reliability projections at these optimum operational temperatures. Furthermore, there remains some uncertainty concerning the application of a (time)½ dependence to all data generated in testing of the tungsten‐osmium, mixed metal matrix cathodes. Nevertheless, it is demonstrated that such cathodes can have mean lifetimes of at least 12 years, and may reach 20 years of serviceable life, sustaining only minor levels of degradation.