Operation mode transition in a low-current krypton-fed heaterless hollow cathode

Hollow cathodes used in electric propulsion systems exhibit two modes of steady-state operation, a noisy lifetime-limiting plume mode, and a quiet spot mode. Plume mode is characterized by large fluctuations in discharge current and plasma parameters due to instabilities in the plasma plume. At a given gas flow rate, spot mode is present at low discharge currents while plume mode occurs at large discharge currents. An independently-developed low-current krypton-fed heaterless hollow cathode exhibits both modes determined by widely-used plume mode onset criteria. Spot mode is observed from 0.4-0.6A until reaching plume mode at 0.7A for two krytpon flow rates. Further increase in discharge current results in a reduction of voltage/current oscillations suggesting the existence of another low-oscillatory regime similar to classic spot mode for this cathode. We present our investigation of the dampening of plume mode oscillations in this higher-current regime.