Current pathways model for hall thruster plumes in ground-based vacuum test facilities: measurements and observations

Abstract

A previous companion paper introduced a current pathways model that represents the electrical coupling between the Hall effect thruster (HET) and the ground-based vacuum test facility operational environment. In this work, we operated a 7-kW class HET at 4.5 kW, 15 A and 6 kW, 20 A on krypton to quantify aspects of the current pathways model to characterize the role metal vacuum chambers play in the thruster's discharge circuit as a function of discharge current. During HET operation, far-field ion and electron saturation currents at 47 near-facility wall locations were measured using an array of 5.08 cm diameter, stainless-steel planar electrodes. In addition, the plasma properties at three distinct locations within the facility, 25 cm from the facility wall, were obtained using Langmuir probes. Experimental results show that thruster beam ions do not readily neutralize with cathode electrons and instead neutralize with the free electrons provided by the metallic chamber wall. In addition, significant charge-exchange (CEX) ion current was measured in the background plasma environment and constituted about 23% of the total ion current measured by the planar electrode array. Thus, the metallic vacuum chamber surfaces facilitate charge neutralization for both ion populations. Additionally, the plasma environment near the facility walls was characterized to be non-uniform with an estimated plasma sheath capacitance ranging between 0.45 µF and 1.79 µF. Further analysis shows that the plasma sheath at the facility wall behaves like a parallel RC circuit, potentially concealing the thruster's AC characteristics. Inherent plasma oscillations give rise to inductive effects with inductances that varied between 76.5 nH and 101.4 nH. Hence, the dynamic characteristics of the HET's discharge are influenced by the capacitive and inductive effects introduced by the vacuum test facility operational environment.

Keywords: Electrical facility effects; Hall effect thruster; Plasma plume; Plume facility interaction.

Fig. 1

HET Discharge-Facility Current Pathways Model

Fig. 2

The BHT-7000 Hall effect thruster used for this work

Fig. 3

Schematic of the location of all 47 witness plates along VTF-2’s walls

Fig. 4

Relay switchboard circuit system to operate the 47-witness plate array

Fig. 5

Sample trace of all 47 I-V curves for the BHT-7000 operating at the 6-kW discharge operating condition

Fig. 6

Location of the three Langmuir probes and Faraday probe placed inside VTF-2

Fig. 7

I_{i, sat} and I_{e, sat} profiles at the 47 near-wall locations for the 4.5 kW, 15 A and 6 kW, 20 A test conditions. Note: The thruster orientation shown here is with respect to the WPs, in accordance with Fig. 3, to depict which WPs lie in front of and behind the thruster exit plane

Fig. 8

Parallel RC circuit analysis: (a) electrical circuit (b) current as a function of time

Fig. 9

Current magnitude and phase as a function of frequency