Angularly-Resolved ExB Probe Spectra in the Plume of a 6-kW Hall Thruster

Abstract

An E×B probe was used to characterize the angular distribution of multiply-charged ions in the plume of a 6-kW Hall thruster operating at discharge voltages of 150-600 V, and anode mass flow rates of 10-30 mg/s. The local ion current fractions were measured in conjunction with ion current density at several locations from 0-30º from thruster centerline, and axial locations of 8, 10, and 12 thruster diameters. Typically the fraction of Xe 2+ increased by the drop in Xe 1+ , while Xe 3+ remained approximately constant at all angles. The current fraction of Xe 1+ decreased with increased discharge voltage, having values of 0.92, 0.87, and 0.70 at 150, 300, and 600 V, respectively. The plume-averaged Xe 1+ current fraction also decreased with increased flow rate, having fractions of 0.87, 0.75, and 0.60 at 10, 20, and 30 mg/s, respectively. The increasing fraction of multiply-charged ions with discharge voltage was attributed to the increase in electron temperature. The increasing fraction of multiply-charged ions with anode flow rate was explained by the increasing ratio of Xe 1+ to neutral Xe found by plasma simulations in HPHall. The results were corrected for the loss of main beam ions due to charge-exchange collisions between the thruster exit and probe location. The correction method performed well, producing plume-averaged correction factors that were within 0.5% of each other with the probe positioned at 8, 10, and 12 thruster diameters downstream. The correction due to charge-exchange collisions was on the order of 1-5%, depending on operating condition, exceeding the errors introduced by other parameters used in performance models. The plume-averaged correction for multiply-charged ions deviated from the discharge channel centerline value by approximately 1.5% over a range of discharge powers from 1 to 10 kW, with the maximum deviation of 5% occurring at the 600 V, 10 mg/s condition. The results indicate that a single measurement of the local ion current fractions near discharge channel centerline is sufficient to accurately gauge the overall correction for multiply-charged ion species. While this is true for studies that are concerned with the behavior of the thruster over large throttling ranges, plume-averaged quantities are likely to be a necessity for studies focused on fine changes in thruster performance.

Methodology and Historical Perspective of a Hall Thruster Efficiency Analysis

“…Species current fractions were corrected for the attenuation of beam ions due to CEX collisions using the procedure outlined by Shastry et al [63]. Conclusions from a recent study of angularly resolved ExB probe spectra using this thruster model asserted that a measurement on channel centerline was representative of the species fractions throughout the plume and deviated from plume averaged results by less than 1.5% over a wide range of thruster operating conditions [64]. In this case study, ExB and RPA results are from channel centerline measurements, and variations in the plume are accounted for in the specified uncertainty.…”

“…C2, this range is approximately 3 to 21 deg. Near-and far-field plume measurements of the 6 kW Hall thruster [64], the SPT-100 [45,66], and the BHT-200 [73] Hall thruster found that the ion species populations did not significantly change within 15 deg from thruster centerline, and the fraction of Xe 2 increased sharply by approximately 5% at 20 deg. These changes in ion species composition have a negligible effect on the value of hcosi mv , thus enabling the approximation of equivalence between momentumweighted divergence and charge-weighted plume divergence as measured with a Faraday probe.…”

The Effects of Background Pressure on Hall Thruster Operation

“…This was a surprising result (though one that has also been reported by Reid et al 13 ) since in that orientation the probe has a direct line-of-sight only to the ceramic cover on the inner magnetic pole. It is therefore not obvious from which source volume the ions collected by the ExB probe originated.…”

“…It is therefore not obvious from which source volume the ions collected by the ExB probe originated. Reid et al 13 recommended weighting current fractions measured with the ExB probe with current density measurements when integrating over angles to determine plume-averaged current fractions. Adoption of this procedure implies that the fraction of ions detectable by the highly collimating ExB probe at a given angle varies with angle, thereby requiring a second probe with wider angular acceptance to produce the weighting factors.…”

Recommended Practice for Use of Faraday Probes in Electric Propulsion Testing

“…Thus, not correcting measurements with κ SEE may lead to overprediction of local ion current density by 5 to 10%, and it should be accounted for in the data analysis and/or error assessment. However, the spatial variation of ion species composition in the central plume is not expected to have a significant effect on the value of κ SEE ; therefore, a single measurement of ion species current fraction in the central core is sufficient to calculate κ SEE [82] for evaluation of ion beam current and divergence. The population of multiply charged ions may increase beyond the central core; thus, it is advised to incorporate local values of ion species current composition in evaluations of local spacecraft plume interactions on the plume periphery.…”