Design improvements and performance evaluation of a Gridless Retarding Potential Analyzer (gRPA) for EP plume diagnostics

Abstract Faraday probes and gridded Retarding Potential Analyzers (RPAs) are staples in electric propulsion plasma diagnostics, used to evaluate thruster performance by measuring ion current density and the Ion Energy Distribution Function (IEDF), respectively. This paper evaluates a gridless RPA (gRPA) as a single, compact device capable of performing both measurements. It is shown that the gRPA’s design is functionally identical to that of a collimated Faraday probe for current density measurements, and it, therefore, inherits a characteristic filtering of low-energy particles (like CEX) and a narrow acceptance angle. Simulations were used to analyze the effect of plasma density on the repeller field to properly size the probe’s aperture. Furthermore, compact measurement electronics were developed and integrated, providing a current resolution of 1 nA at a 500 kS/s sample rate to resolve breathing mode oscillations of an Advanced Cusp Field Thruster (ACFT). Finally, through comparative measurements with a traditional nude Faraday probe, angular correction models for the gRPA were tested, thereby quantifying the strict alignment requirements of the probe and providing a method to correct for measurement errors arising from misalignment.