Performance of a Magnetically Shielded Hall Thruster Operating on Krypton at High Powers

Abstract

Hall thrusters are increasingly being used for a variety of applications, including deep space missions, requiring higher powers than ever before. As their role in the world of space propulsion continues to grow, so too does the need for using non-traditional propellants. One of the most appealing is krypton, a noble gas with a lighter mass and higher ionization energy compared to the more traditional xenon. Traditionally, krypton has a lower performance than xenon. Additionally, the advent of magnetic shielding has enabled longer lifetimes and higher power operation on Hall thrusters, opening up a new regime of operation while altering the internal plasma properties. There is an apparent need to characterize the performance differences between xenon and krypton on these shielded Hall thrusters. With a framework for characterizing performance equipped, including a 0D model for predicting mass utilization in terms of plasma parameters, this work sets out to investigate how xenon and krypton efficiencies scale at high powers and the underlying physics behind these trends.

First, the efficiency of a shielded Hall thruster operating across a standard throttling table is measured and explored. It is found that with increasing discharge voltage, the efficiency gap between xenon and krypton does not close as it does for unshielded thrusters, a phenomenon attributed to different trends in mass utilization. Next, a modified shielded Hall thruster is operated at current densities an order of magnitude higher than the typical 100--200 mA/cm^2. At these conditions, the efficiency gap between propellants not only closes but also reverses. The closing of this gap is attributed to krypton's more rapid increases in mass utilization given its lower initial plasma density; the reversal is attributed to the weaker dependence of krypton's electron confinement on current density. Finally, the derived 0D model of mass utilization is leveraged with surrogate measurements of internal plasma properties to elucidate why the performance gap scales differently with increasing voltage and increasing current. It is found that increases in plasma density are primarily what drive the increases in mass utilization and the more rapid gains for krypton with increasing power. These findings represent a marked step in our understanding of Hall thruster operation at high power on alternate propellants.