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Faraday Probe Analysis, Part 2: Evaluation of Facility Effects on Ion Migration in a Hall Thruster Plume (Preprint)
AIR FORCE RESEARCH LAB EDWARDS AFB CA PROPULSION DIRECTORATE
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A nested Faraday probe was designed and fabricated to assess facility effects in a systematic study of ion migration in a Hall thruster plume. Current density distributions were studied at 8, 12, 16, and 20 thruster diameters downstream of the Hall thruster exit plane with four probe configurations at background pressures of 3x10-6, 1x10-5, and 3x10-5 torr. Several correction factors are applied to account for the effective probe collection area and systematic measurement error associated with measuring an annular device as an axisymmetric point source. These corrections enable the investigation of facility effects on beam expansion and ion migration in the plume. The effects of background facility neutrals are isolated, which enables precise and accurate estimate of thruster ion beam current and plume divergence. A set of guidelines are recommended for Faraday probe design, experimental approach, and data analysis of results that are aimed at minimizing measurement error of far-field Faraday probe measurements. These guidelines are shown to reduce the calculated ion beam current by 10-20 compared to conventional analysis techniques and to reduce measurement uncertainty to approximately -3. The reductions in measurement uncertainty and the increased capability to approximate the on-orbit plume expansion from ground-based measurements are a significant improvement for comparisons with numerical simulations and investigations of Hall thruster performance.
APPROVED FOR PUBLIC RELEASE