Ionic and neutral particle transport property measurements in the plume of an SPT-100

In order to understand the interaction between the SPT-100 thruster and the spacecraft accurate knowledge of exhaust plume properties must be obtained. This investigation utilized a suite of inter-related diagnostic probes in an effort to provide detailed knowledge of the exhaust plume on a molecular kinetic level. Probes used for the investigation included a Retarding Potential Analyzer, (RPA), a planar Faraday probe, both total and radiant heat flux probes, as well as a newly developed Neutral Particle Flux probe. Combining knowledge gained from these probes it was possible to quantify the transport of mass, energy, and charge within the plume at 0.5m and 1.0m radius from thruster exit. Among the phenomena uncovered were a high energy ionic annulus surrounding a narrow low energy core within the plume at 0.5m. This core was still incompletely filled in at 1.0m radius. Additionally, a population of energetic charge exchange neutrals was found along with the corresponding low energy charge exchange ions due to plume/background gas interactions.