Ion Emission From a Positively Biased Hollow Cathode Plasma

Hollow cathode plasma contactors have an extensive history as spacecraft neutralizers, but questions remain on whether the technology can neutralize significant positive charging in tenuous space plasmas. Simulations and a representative semianalytical model (which we term the ion emission model) predict effective neutralization in these scenarios as ion current is emitted from the surface of the quasi-neutral contactor plasma according to the space-charge limit. This article focuses on experimental plasma measurements of a hollow cathode biased to several positive voltages with respect to a surrounding vacuum chamber in order to validate the ion emission model. Particular attention is paid to the emitted ions (those reaching the chamber wall) and to the ion-rich sheath region where the ion emission process takes place. Retarding potential analyzer (RPA) measurements were performed to understand ion flow velocity and direction as these parameters relate directly to the current predicted by the ion emission model. Planar probe measurements were performed at the chamber wall to compare local emission currents to model predictions. Evidence of collisions within the plasma (particularly charge exchange collisions) and simple models predicting ion energetics are presented. These experiments suggest that ion emission from the contactor plasma acts according to the ion emission model and adds to the physical understanding of spacecraft neutralization using hollow cathode plasma contactors as it may occur in tenuous space plasmas.