Effect of Background Pressure on the Plasma Oscillation Characteristics of the HiVHAc Hall Thruster

Abstract

During a component compatibility test of the NASA HiVHAc Hall thruster, a number of plasma diagnostics were implemented to study the effect of varying facility background pressure on thruster operation. These diagnostics characterized the thruster performance, the plume, and the plasma oscillations in the thruster. Thruster performance and plume characteristics as functions of background pressure were previously published. This paper focuses on changes in the plasma oscillation characteristics with changing background pressure. The diagnostics used to study plasma oscillations include a high-speed camera and a set of high-speed Langmuir probes. The results show a rise in the oscillation frequency of the "breathing" mode with rising background pressure, which is hypothesized to be due to a shortening acceleration/ionization zone. An attempt is made to apply a simplified ingestion model to the data. The combined results are used to estimate the maximum acceptable background pressure for performance and wear testing.

Effect of Background Pressure on the Plasma Oscillation Characteristics of the HiVHAc Hall Thruster

“…At a given throttle point, retarding potential analyzer data previously reported shows that the ion velocity remains roughly constant as background pressure varies. 1 The neutral velocity scales with the square root of the anode temperature and is expected to remain roughly constant as background pressure varies. The implication is that the breathing mode frequency must be rising with background pressure because the characteristic length is decreasing.…”

“…The experimental setup was previously reported 1 and is repeated here with new details about the high-speed diagnostics for convenience. To simplify plot labeling, operating conditions will be labeled as vvv-k.k-Pnx, where vvv is the discharge voltage in volts, k.k is the discharge power in kilowatts, and n is the normalized background pressure, defined as the number of multiples of the lowest achievable operating background pressure for the throttle point vvv-k.k.…”

“…1 The part of the model that predicts the properties of the external stream turned out to yield unphysical results leading to the conclusion that linearly super-imposing the internal (from propellant manifold) and external (from facility) streams on top of each other does not accurately model the ingestion physics. Nevertheless, calculating the ingestion rate using the change in discharge current should be a valid approach since every ingested neutral will produce an electron upon ionization.…”

“…For reference, the criterion on background pressure based on a maximum of 2% increase in thrust due to ingestion is 5e-7 Torr/(mg/s). 1 16…”

“…1 While the previous paper focused on the breakdown of performance into various efficiency factors and far-field time-averaged diagnostics, this paper will focus on plasma oscillation characteristics and time-resolved diagnostics.…”