Performance of Oscillating Plasma Thrusters

TL;DR

This paper investigates how oscillations in plasma parameters affect the performance metrics of plasma thrusters, proposing revised formulas to account for time-varying behavior and optimizing thrust through modulation.

Contribution

It introduces a new formulation for thrust and efficiency that incorporates plasma oscillations and phase shifts, advancing understanding of unstable plasma thruster performance.

Findings

Thrust can vary more than 20% due to plasma oscillations.

Modulating ion energy can maximize thrust in oscillating regimes.

Derived an expression for thruster efficiency considering modulation effects.

Abstract

Traditional expressions and definitions describing performance of plasma thrusters, including the thrust, specific impulse, and the thruster efficiency, assume a steady state plasma flow with a constant flow velocity. However, it is very common for these thrusters that the plasma exhibits unstable behavior resulting in time-variations of the thrust and the exhaust velocity. For example, in Hall thrusters, the ionization instability leads to strong oscillations of the discharge current (so-called breathing oscillations), plasma density, ion energy, and as a result the ion flow. In this paper, we revisit the formulation of the thrust and the thrust efficiency to account for time variations of the ion parameters including the phase shift between the ion energy and the ion flow. For sinusoidal oscillations it was found thrust can potentially change more than 20%. It is shown that by modulating ion energy at specific amplitudes, thrust can be maximized in such regimes. Finally, an expression for the thruster efficiency of the modulating thruster is derived to show a mechanism for inefficiencies in such thrusters.