Preliminary Assessment of the Role of a Conducting Vacuum Chamber in the Hall Effect Thruster Electrical Circuit

Abstract

The role of the electrically conductive vacuum chamber wall in the completion of the discharge circuit of a Hall effect thruster (HET) is experimentally investigated. The AerojetRocketdyne T-140 laboratory-model HET operating at a discharge voltage of 300 V, discharge current of 5.16 A, and anode flow rate of 5.80 mg/s serves as a representative HET test bed. The nominal facility operating pressure during thruster firings is 4.9 × 10 -6 Torr corrected for xenon. Two 0.91 m x 0.91 m square aluminum plates are placed adjacent to, but electrically isolated from, the walls of the stainless steel vacuum chamber at two locations with respect to the center of the thruster exit plane: 4.3 m axially downstream along thruster centerline and 2.3 m radially outward centered on the exit plane. The plates are configured in three distinct electrical configurations with corresponding measurements: a) electrically grounded plates with measurements of currents to ground, b) electrically isolated plates with measurements of floating voltages, and c) isolated but electrically connected plates with measurements of the current conducted between them. The measurements are all taken simultaneously with the discharge current oscillations of the thruster at a sampling frequency of 100 MHz. Measurements of the current conducted to ground in the electrically grounded configuration reveal that the axial and radial plates collect ion currents that are 13.6% and 10.7% of the discharge current, respectively; the collected current is coupled to the discharge current oscillations but is smaller in magnitude and phase-delayed. In the electrically connected plate configuration, 5.5% of the average discharge current is observed to flow from the axial plate to the radial plate driven by a floating voltage difference of 7.6 V; this current is uncorrelated in time with the discharge current oscillations. These results indicate that the vacuum chamber conducts current and is a recombination site for a significant number of plume ions during HET operation. NomenclatureI = current, A 2 Pb = vacuum chamber base pressure, Torr Pc = corrected vacuum chamber background pressure, Torr Pi = indicated vacuum chamber background pressure, Torr V = voltage, V Vc-g = cathode-to-ground voltage, V Vd = discharge voltage, V Vp = plasma potential with respect to ground, V Vpa = axial plate voltage with respect to ground, V Vpr = radial plate voltage with respect to ground, V ΔV = acceleration voltage, V ΔVcath = cathode coupling voltage, V

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

“…HET test facilities are also almost ubiquitously metallic and, as such, have finite conductivity. Recent work has indicated that chamber conductivity plays a significant role in the HET electrical circuit, and consequently represents a previously unexplored topic of electrical facility effects [18,19]. Specifically, this work has shown that the facility walls collect a significant fraction of the discharge current and, as such, act as an alternate recombination site for plume ions and electrons that have not undergone recombination before reaching the facility walls.…”

“…The magnetic circuit configuration of the T-140 HET (two concentric coils centered on the thruster centerline) restricts the position of the magnetic field separatrix to the thruster centerline and precludes the T-140 HET from exhibiting the off-centerline separatrix surfaces shown in HETs with magnetic coils centered off axis [18][19][20][21]33]. This magnetic field topology eliminates any concerns about the changing nature of the near-field plume properties and cathode coupling as a function of cathode position relative to the absent off-centerline separatrix surface [20].…”

“…This alternate pathway is an artificial effect introduced by the presence of the vacuum facility and is expected to be absent on orbit. Thus, any process dependent upon the path of the electrons through the plasma (including plasma reactance and resistance) may be significantly different between ground operation and operation on orbit [18][19][20][21].…”

Background Flow Model of Hall Thruster Neutral Ingestion

“…A low-density plasma flow (i.e., the HET plume) is then exhausted from the thruster in the axial direction toward the downstream end of the facility. Although some of the emitted ions and electrons recombine before striking the downstream facility surfaces, the ion-electron recombination mean free path is generally longer than the characteristic axial facility dimensions; thus, most of these charged particles strike the downstream facility surfaces, recombine, and reflect as neutrals with a thermal speed characterized by the chamber wall temperature [22,25,28]. Upon reflection, these neutrals are then able to traverse the vacuum facility until they strike and are condensed on one of the cryopumps.…”

“…Figure 1a shows a schematic of a typical vacuum test facility used for HET testing. These test facilities are usually stainless-steel cylindrical vacuum chambers measuring 4 m or more in length and 2 m or more in diameter, equipped with cryopumps in order to achieve and maintain operating pressures on the order of 10 −5 Torr or less [6,22,[25][26][27]. The cryopumps operate by cooling a plate or series of plates to an operating temperature of approximately 15 K using gaseous helium [22].…”