A New Method for Analyzing Near-Field Faraday Probe Data of Hall Thrusters

Abstract

This paper presents a new method for analyzing near-field Faraday probe data obtained from Hall thrusters. Traditional methods spawned from far-field Faraday probe analysis rely on assumptions that are not applicable to near-field Faraday probe data. In particular, arbitrary choices for the point of origin and limits of integration have made interpretation of the results difficult. The new method, called iterative pathfinding, uses the evolution of the near-field plume with distance to provide feedback for determining the location of the point of origin. Although still susceptible to the choice of integration limits, this method presents a systematic approach to determining the origin point for calculating the divergence angle. The iterative pathfinding method is applied to near-field Faraday probe data taken in a previous study from the NASA-300M and NASA-457Mv2 Hall thrusters. Since these two thrusters use centrally mounted cathodes, the current density associated with the cathode plume is removed before applying iterative pathfinding. A procedure is presented for removing the cathode plume. The results of the analysis are compared to far-field probe analysis results. This paper ends with checks on the validity of the new method and discussions on the implications of the results. Abbreviations

Recommended Practice for Use of Faraday Probes in Electric Propulsion Testing

“…The probe area and the residence time in the plasma are proportional to the total energy transferred to the probe as heat. Past near-field studies of a 20 kW HET plasma demonstrated a probe residence time in the plasma of ∼1 s was acceptable for a probe with frontal area of ∼10 mm 2 at 0.05 TDD from the exit plane [45]. During these studies, the probe translation speed ranged from 150 to 500 mm∕s.…”

“…Spatial variations of plasma properties in EP plumes span orders of magnitude, and they necessitate consideration of the facility interactions with the thruster plasma and probe ion collection. The time-averaged ion current density in the plume of HETs and GITs typically ranges from 1 to 100 mA∕cm 2 in the near-field to less than 0.001 mA∕cm 2 in they periphery of the far-field plume [21,45]. Over this region, the time-averaged electron temperature and ion density range from approximately 10 eV and greater than 10 18 m −3 in the near-field (Z > 0.2D T ) to less than 1 eVand 10 13 m −3 in the far-field plume periphery.…”

“…In near-field measurements, there can be a large variation in bias voltage required for ion saturation, and interactions between the plasma and thruster surfaces can introduce large variations in floating potential. A Faraday probe bias voltage as low as −100 V may be necessary to enter the ion saturation regime at the closest approach to the thruster [45]. Since the bias voltage can vary greatly depending on distance from the thruster and thruster operating condition, a variable bias voltage may be used.…”

“…The radial measurement range should extend to the location where measured ion current density is less than 0.2% of the maximum current density along the radial profile at fixed Z. This range enables a 0.2% threshold-based integration limit, and it is recommended based on past experiments [45]. Faraday probe sweeps should also be conducted at fixed Φ for the opposite side of the plume (i.e., Φ 180 deg), such that a continuous radial sweep is taken to less than 0.2% of the maximum current density value for the left side (r > 0) and right side (r < 0), as shown in Fig.…”

“…The near-field HET plume should be analyzed at axial distances upstream of the plume merging region, termed the "transitional region" in [54], where ion beams from opposing sides of the channel merge. Past near-field studies of multiple HET designs found this transitional region was approximately 0.5 to 1.0 TDD [44,45,54]. Axial distances less than 0.2 TDD are not recommended due to ionization and acceleration processes that may occur downstream of the HET exit plane, and they have been shown to be influenced by background pressure [68].…”