Ion Energy and Wave Propagation in a Hollow Cathode Plume

Hollow cathodes used as electron sources for long duration and high power electric propulsion missions exhibit a poorly understood phenomenon that can lead to anomalously energetic ion bombardment of cathode surfaces. An understanding of this mechanism is therefore needed to prevent failure of high current cathodes and to enable predictive modeling of cathode lifetime. Recent experimental and theoretical work has shown ion acoustic turbulence (IAT) can in some operating conditions lead to energetic ion formation and subsequent erosion of the keeper. The primary goal of this research is to understand the relationship between IAT and ions in the plume of a high current hollow cathode. Additionally, understanding this relationship will provide insight into the overall importance of IAT to cathode plume behavior such as anomalous resistivity caused by the interaction of IAT with electrons. The experimental effort used a combination of plasma probes and laser-induced fluorescence (LIF) diagnostics. Parametric studies of ion heating as a function of mass flow rate and discharge current showed an increase of both wave energy and ion heating with discharge current, and a decrease in both with higher mass flow rates. The first study was conducted along the plume centerline, focusing on the scaling of IAT wave energy with ion temperature. For the first time it was shown that ion heating does in fact qualitatively scale with IAT energy locally in the near plume. The scaling parameter for ion heating to wave energy was obtained for a range of operating conditions and locations in the plume, and was compared to a first-principles-derived fluid formulation for ion heating. These results and accompanying analyses showed that near the cathode the scaling parameter was higher than predicted but converged toward the predicted values downstream. To delve further into the plume behavior, a second set of experiments investigated the correlation between IAT wave propagation, ion drift, and ion heating in the plume of the same cathode for similar operating conditions. An array of plasma probes was used to measure the IAT wave vectors using correlations between plasma fluctuations, and LIF was used for obtaining the axial and radial ion velocity distribution function. It was also found that variations of discharge current and flow rate resulted in consistent ion velocity and ion acoustic wave vector propagation. It was also found that for the high current to flow rate condition that IAT wave energy and ion heating was much more significant, again providing evidence that there is a scaling between these two properties