Development of a Single-orifice Retarding Potential Analyzer for Hall Thruster Plume Characterization

A retarding potential analyzer (RPA) was carefully designed to investigate the high energy tail often observed in the ion energy distribution measurements in the plumes of Hall thrusters. The probe was designed to operate in regions of the plume where the Debye length was 100 pm or greater. Probe design was assisted by the PIC code MAGIC to maximize the ratio of the effective ion retarding potential to the applied potential to ensure that ions were not being detected at artificially high ion energies. The probe was built and tested. The results verify the existence of ions that have been effectively accelerated through voltages in excess of the thruster discharge voltage. This paper thoroughly discusses the design of the probe and presents preliminary measurements in comparison with the ion energy distribution measurements taken with two other diagnostics.