Electrical Facility Effects on Hall Thruster Cathode Coupling: Performance and Plume Properties

Abstract

The impact of facility conductivity on Hall effect thruster cathode coupling is experimentally investigated. The 3.4 kW Aerojet Rocketdyne T-140 Hall effect thruster operating at a discharge voltage of 300 V, a discharge current of 10.3 A, and an anode flow rate of 11.6 mg∕s serves as a representative Hall effect thruster test bed. The nominal facility operating pressure during thruster operation is 7.3 × 10 −6 Torr corrected for xenon. Two 0.91 × 0.91 m square aluminum plates are placed adjacent to, but electrically isolated from, the walls of the conductive vacuum chamber at two locations with respect to the center of the thruster exit plane: 4.3 m axially downstream along the thruster centerline, and 2.3 m radially outward centered on the exit plane. The plates and body of the Hall effect thruster are configured in three distinct electrical configurations with corresponding measurements: 1) electrically grounded with measurements of currents to ground, 2) electrically isolated with measurements of floating voltages, and 3) isolated from ground but electrically connected with measurements of the current conducted between the plates. Measurements are taken as the radial position of the cathode is varied from 0 to 129 cm with respect to the nominal cathode location. Measurements of the current collected by the plates and thruster body indicate that cathode electrons preferentially travel to the thruster body, Hall effect thruster plume, and radial facility surfaces for cathode locations in the near field, midfield, and far field, respectively. These results indicate that cathode position alters the recombination pathways taken by cathode electrons in the Hall effect thruster circuit.

Impact of Propellant Species on Hall Effect Thruster Electrical Facility Effects

“…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.…”

Simple Model for Cathode Coupling Voltage Versus Background Pressure in a Hall Thruster

“…To this end, multiple theories behind the ground facility-thruster interactions have emerged. Frieman et al 15,16 have looked at the role of the conducting wall in facility effects and Crofton suggested that the increase in thrust could be due to charge exchange. 17 However, the fundamental manifestation of these effects still remains unidentified.…”