Electrical Facility Effects on Hall-Effect-Thruster Cathode Coupling: Discharge Oscillations and Facility Coupling

Abstract

The physical mechanisms that govern the electrical interaction between the Hall-effect-thruster electrical circuit and the conductive vacuum-facility walls are not fully understood. As a representative test bed, an Aerojet Rocketdyne T-140 Hall-effect thruster is operated at 3.05 kW and a xenon mass flow rate of 11.6 mg∕s with a vacuum facility operating neutral pressure of 7.3 × 10 −6 torr, corrected for xenon. Two electrical witness plates, representative of the facility chamber walls, are placed 2.3 m radially outward from thruster centerline and 4.3 m axially downstream from the thruster exit plane. The cathode is radially translated from 18.1 to 77.8 cm away from the thruster centerline. At each cathode position, the discharge current and the electrical waveform of the radial and axial plates are simultaneously measured. As the cathode radial position changes from 18.1 to 77.8 cm from the thruster centerline, the discharge-current oscillation frequency decreases between 17 and 35% for the electrically grounded thruster-body case, and between 15 and 23% for the electrically floating thruster-body case. The analysis of the electron current collected by the radial plate suggests that electrons directly sourced from the cathode impinge on the radial plate at large cathode positions. Overall, the results of this work show that the chamber walls act as an artificial electrical boundary condition that keeps the Hall-effect-thruster plume plasma potential to within certain bounds.

Impact of Propellant Species on Hall Effect Thruster Electrical Facility Effects

“…Matching volumetric flow rates results in the injection of the same number of neutral particles during both xenon and krypton operations, thereby yielding equivalent near-field pressures for similar neutral temperatures [45]. This pressure equivalency is important because previous work indicated that electrical facility effects may be sensitive to the neutral pressure distribution in the facility [28,48]. The choice to match either mass flow rates or discharge powers (which are the two other common methods for krypton operation) was avoided because it would have yielded an increase in number density of 25-60%, and therefore could have resulted in the conflation of electrical and backpressure effects and altered the relative current collected by the axial and radial plates [28,45,48].…”

“…Although the current collection by the facility walls is controlled by the wall sheath and does not impact quasi-neutrality in the plume, this alternate electron recombination pathway is an artificial effect introduced by the presence of the vacuum facility that is expected to be absent on orbit. Furthermore, previous work has confirmed that the presence of this alternate pathway can alter processes dependent on the electron path through the plasma, such as cathode coupling and plasma reactance, [26][27][28][29].…”

“…Figure 1 shows the physical location of the plates with respect to the T-40 HET. Identical plates have been used in previous studies of electrical facility effects [26][27][28][29]. The surface area of the plates represents approximately 2% of the total facility wall area.…”

“…HET test facility walls are also almost ubiquitously metallic and, as such, have finite electrical conductivity. Recent work has indicated that the electrical conductivity of the chamber plays a significant role in the HETelectrical circuit, and consequently represents an electrical facility effect [23,[26][27][28][29]. Specifically, this work has shown that the facility walls collect a significant fraction of the discharge current, thereby acting as an alternate recombination site for plume ions and electrons that have not undergone recombination before reaching the facility walls.…”

“…The majority of the archival work on facility effects (both electrical and pressure) has been conducted using HETs operating with xenon propellant [26][27][28][29]. Although xenon is the most common choice for HET propellant, the scarce quantity and increasing demand for xenon has sparked interest in other potential HET propellant options.…”