Migration and escape of barium atoms in a thermionic cathode

A quantitative study of the physical processes on a thermionic cathode that are crucial to both cathode life and noise are investigated. Of particular interest are the transport of the barium atoms from the cathode interior to the cathode surface and beyond. To investigate these physical processes, a model of barium migration to the surface and its subsequent removal by bulk and monolayer evaporation and ion spattering is presented. This model incorporates material and tube environment parameters for the calculation of key terms governing diffusion, evaporation, and sputtering. Estimates of coverage, adsorbate work function values, lifetime, and current density are made, which are in qualitative agreement with the experiment. The model is based on a hopping model of barium diffusion to determine barium concentration within the cathode and the surface coverage ratio. Insofar as possible, all parameters are determined from simple models of the underlying physical processes.