Influence of the upstream axial magnetic mirror field on the plume characteristics in the full cylindrical Hall thruster

Abstract

Full cylindrical Hall thruster holds considerable promise in terms of its low wall erosion rate and long-lifetime performance advantages. As far as we know, its upstream magnetic mirror field plays a fundamental role in the determination of plasma behavior and hence the thruster performance. In this paper, a series of upstream axial magnetic mirror fields with different axial gradient values were generated by adjusting the inverse-electromagnetic coil currents near the upstream channel and its effect on the plume characteristics were investigated. The experiment results indicate that under a low magnetic mirror field with 44.8 G/mm gradient value, a bulb-like plasma discharge region emitting a visible high-intensity light is discovered near the thruster exhaust, its plume manifests a hollow profile with a plume angle of 77°. In contrast, as the axial gradient value of magnetic mirror field increases from 44.8 G/mm to 72.5 G/mm, this bulb-like plasma region near exhaust shrink gradually to disappear along with the plume angle decreasing to 65°. Meanwhile, the anode efficiency enhances significantly (from 16% to 28%) correspondingly. According to the analysis of ion current, electron current and ion energy distribution, the observed strong plasma discharge near the thruster exhaust should be intimately related with the extension of the ionization region towards exit due to the weak electron confinement under the low magnetic mirror field. However, the larger upstream magnetic mirror field could diminish the leaking of local electrons towards the anode efficiently, which could not only intensify the ionization, but also induce the shrink of the ionization region axially towards the upstream channel and the clearer separation of acceleration region from the ionization region. Ultimately, the enhanced propellant utilization and current utilization, higher ion energy and the collimated plume are achieved. This work provides a feasible way to recognize the ionization and acceleration process under the mirror-like magnetic field of the miniaturized full cylindrical Hall thruster.