Recommended Practices for Design and Use of Retarding Potential Analyzers in Electric Propulsion

Retarding potential analyzers (RPAs) are relatively simple and powerful diagnostic tools that are capable of providing ion energy distribution functions within the plume of an electric propulsion thruster. The purpose of this paper is to provide a guide to standardize RPA design, construction, operation, error analysis, and data interpretation for operation within the plasma environment produced by an electric propulsion thruster. Operational environment, correction factors, and error analysis are presented along with time-averaged and time resolved probe operation and data analysis methods. These best practices have been compiled from extensive experience using RPAs for electric propulsion thruster testing and previous publications on the design, use, and analysis of data from RPA probes. Focus in this guide is placed on the use of RPAs in the most common electric propulsion devices, namely Hall effect thrusters, where the plasma is flowing and ions have a rich variety of energy and oscillatory behaviors. The strategies presented in this guide are expected to increase the quality and repeatability of testing data obtained from electric propulsion systems, thereby allowing for improved comparisons between different thrusters and operating environments and improving the fidelity of models for on-orbit predictions.