Experimental Investigations on the Influence of the Facility Background Pressure on the Plume of the RIT-4 Ion Engine

It is known that the presence of residual gases during on-ground testing of electric propulsion thrusters can significantly influence the plasma parameters and the quantity of charge-exchange ions obtained in the backflow of the ion source. In the frame of an ESA study dedicated to the "Assessment of Interactions between Spacecraft and Electric Propulsion Systems" (AISEPS) a miniaturized μN-RIT thruster (RIT-4), developed by Giessen University, was tested in the Corona vacuum facility at the ESA Propulsion Laboratory (EPL), ESA-ESTEC. A single filament neutralizer was used for beam neutralization. The main ion beam and backflow properties of the thruster were investigated by means of Faraday probes (FP) and retarding potential analyzers (RPA). The Cathode Reference Potential (CRP) was also investigated. The background pressure in the vacuum facility was increased to assess the influence of the xenon residual neutral density on the plume of the thruster. Different electrical coupling configurations between the thruster and the ground were also studied. The μN-RIT was operated at three different thrust levels ranging from 100 to 500 μN while the neutralizer was operated with constant heating voltage during the entire test campaign to allow emission up to 12 mA depending on the electrical grounding configuration. The background pressure was increased in the main vessel of the Corona facility by injecting an auxiliary xenon flow ranging from 10 to 50 sccm. The influence of the neutralizer was clearly observed on the RPA measurements in the main ion beam. The divergence was also clearly correlated to the grounding configuration of the thruster. However, even if the xenon background pressure increased the backflow ion current it did not have a clear influence on the divergence of the thruster.