A model for hollow cathode surface work function evolution

Description/Abstract

In this paper a model to predict the evolution with time of the surface work function of an hollow cathode insert will be presented. The processes involved in BaO deposition and desorption on the surface is described and quantified using chemical data present in the literature. The model is compared with test results from the ELT NSTAR cathode. Predictions obtained from the model have been found to be qualitatively in agreement with the expectations and conservative, model predictions always overestimating the increases in work function with time. An explanation of this has been given noting that all the assumption made in the model tend to overestimate the desorption rates hence producing conservative estimate of the cathode lifetime. Possible ways to improve the model accuracy will be the development of an ignition model and the coupling of the model presented in this paper with a plasma model to take into account the effect that a change in the emission characteristics of the insert surface will produce on the cathode plasma.