Operation Mode Transitions in a Heaterless Hollow Cathode

Abstract

Hollow cathodes are used to neutralize ions in electric propulsion systems, and are a lifetime-limiting component due to the presence of high-energy ions generated in the plasma plume. These ions impact the cathode's keeper electrode destroying it over a period of tens of thousands of hours. The increased interest in high-power electric propulsion systems has been motivated by proposed missions with long required lifetimes, longer than cathode erosion rates would allow in these high-power systems. In the so-called plume mode, high-energy ions are produced at an increased rate but it is unclear how this operation mode comes about from physics in the plume and it is unclear how oscillations in the plume produce large populations of high-energy ions. Experimental research of a heaterless hollow cathode is presented with a specific focus on operation mode transitions in hollow cathodes. The first experimental campaign highlights a unique behavior where two mode transitions, instead of the typically reported one, occur at high flow rates across the cathode's operational parameter space. In this unique behavior two spot mode regimes are found with the high-current spot mode still exhibiting oscillation characteristics of plume mode. Wavenumber-frequency spectrograms corroborated this with multiple features suggesting waves present in the plume related to the plume mode instability and ion acoustic turbulence modulation.