Evaluation of Plume Divergence and Facility Effects on Far-Field Faraday Probe Current Density Profiles

A Faraday probe with three concentric rings was designed and fabricated to assess the effects of gap width and probe geometry in a systematic study of the Hall thruster plume. The nested Faraday probe consisted of two concentric collector rings and an outer guard ring, which enabled simultaneous current density measurements on the inner and outer collector. Two versions of the outer collector were fabricated to create 0.5 mm or 1.5 mm gaps between the rings. Current density distribution of a Hall thruster ion source were studied at 8, 12, 16, and 20 thruster diameters downstream of the exit plane with four probe configurations at background pressures of 3x10, 1x10, and 3x10 torr. Several correction factors are proposed to account for the effective probe collection area and systematic measurement error associated with measuring an annular device as a point source. These corrections enable the investigation of beam expansion and ion migration from the central core, and result in a highly accurate estimate of divergence in the Hall thruster plume. Application of the correction factors and characterization of facility effects are shown to decrease the calculated beam current by 10-20% compared to conventional analysis and reduce measurement uncertainty to ±3%. DISTRIBUTION STATEMENT A. Approved for public release; distribution is unlimited. 1 Research Scientist, Spacecraft Branch, Propulsion Directorate, AFRL/RZSS, [email protected]. 2 Professor, Department of Aerospace Engineering, Associate Dean, Horace H. Rackham School of Graduate Studies, Director, Plasmadynamics and Electric Propulsion Laboratory, [email protected].