Key plasma processes in LaB6 thermionic hollow cathodes operating in He, D2, Ar, and Xe at 4–200 Pa and 0.25–5 A cm−2

Key physical processes in a thermionic hollow cathode plasma are identified using an experimental dataset and a zero-dimensional model. Model predictions for voltage are in generally good agreement with the dataset when it is assumed that the plasma is generated by collisions between the high-energy tail of a bulk thermal plasma and the fill gas, and that losses occur via ambipolar diffusion to the inner surface of the hollow cathode, modified to account for nonzero current through the sheath. Bulk thermal electrons are heated by beam electrons from the cathode surface that lose their entire energy to bulk electron heating, and this heat source is equal to the measured total heat loss of the cathode structure. Various additional and alternate processes are described, which were tested to improve model prediction accuracy where the results are at greatest variance with the experimental measurements.