Plasma Potential and Langmuir Probe Measurements in the Near-field Plume of the NASA-457Mv2 Hall Thruster

Abstract

In order to further the design of future high-power Hall thrusters and provide experimental validation for ongoing modeling efforts, plasma potential and Langmuir probe measurements were performed on the 50-kW NASA-457Mv2. An electrostatic probe array comprised of a near-field Faraday probe, single Langmuir probe, and emissive probe was used to interrogate the near-field plume from approximately 0.1 -2.0 mean thruster diameters downstream of the thruster exit plane at the following operating conditions: 300 V, 400 V and 500 V at 30 kW and 500 V at 50 kW. Results have shown that the acceleration zone is limited to within 0.4 mean thruster diameters of the exit plane while the hightemperature region is limited to 0.25 mean thruster diameters from the exit plane at all four operating conditions. Maximum plasma potentials in the near-field at 300 and 400 V were approximately 50 V with respect to cathode potential, while maximum electron temperatures varied from 24 -32 eV, depending on operating condition. Isothermal lines at all operating conditions were found to strongly resemble the magnetic field topology in the high-temperature regions. This distribution was found to create regions of high temperature and low density near the magnetic poles, indicating strong, thick sheath formation along these surfaces. The data taken from this study are considered valuable for future design as well as modeling validation. Nomenclature= mean thruster diameter I cap = capacitive current = anode mass flow rate P d = discharge power T = thrust T e = electron temperature η a = anode efficiency

Plasma Potential and Langmuir Probe Measurements in the Near-field Plume of the NASA-300M Hall Thruster

“…Due to interest and funding from OCT, these thrusters, in particular the 300M and 457Mv2, are now being revisited and a number of measurements are being taken in order to better understand highpower Hall thruster physics and characterize performance. [4][5][6][7] The purpose of these measurements is three-fold. The first is to provide data that will help characterize thruster performance and efficiency that will ultimately lead to the design of a high-performance, high-power Hall thruster with long lifetime.…”

Ion Current Density Study of the NASA-300M and NASA-457Mv2 Hall Thrusters

“…Plasma potential data was taken using the emissive probe on the probe array, which is described in a companion paper. 19 In this figure, negative potential created by the centrally-mounted cathode appears as deep blue along the thruster centerline. The radial extend of this deep blue region is about ±0.05 in normalized radial coordinate.…”

“…Floating potential was measured via a Langmuir probe in the probe array and is described in a companion paper. 19 While an even greater negative bias can potentially remove these regions of negative current density, it will also cause a greater sheath expansion effect. No attempt was made to determine a proper trade-off between less negative-currentdensity data and less sheath expansion effect.…”

“…The Langmuir-and emissive-probe results are reported in companion papers. 18,19 Other diagnostics used during the tests include thrust stand, 8 optical emission spectroscopy, ExB probes, retarding potential analyzer, and thermocouples. The results from these diagnostics will not be reported in this paper.…”

Overview of the Development of the Solar Electric Propulsion Technology Demonstration Mission 12.5-kW Hall Thruster

“…24,25 Since then NASA GRC has been involved with testing and/or design of numerous high power Hall thrusters that include the NASA-457Mv1 (50 kW), NASA-400M (40 kW), NASA-300M (20 kW), NASA-457Mv2 (50 kW), Busek's BHT-20k, and Aerojet-Rockedyne's XR-12. 26,27,28,29,3031,32,33,34,35,36 NASA GRC's and JPL's extensive experience with performance testing, and ion (gridded and non-gridded) thrusters near and far-field plasma measurements is being leveraged in support of the development of a long-life high-performing Hall thruster for NASA's SEP TDM.…”