Cross-field electron transport induced by a rotating spoke in a cylindrical Hall thruster

TLDR

Rotating spoke phenomena are observed in Hall thrusters and other E×B devices and are thought to enhance electron cross‑field transport. The spoke current was measured with a segmented anode, synchronized with high‑speed camera imaging, and complemented by emissive and Langmuir probes to capture plasma potential, density, and temperature fluctuations. The rotating spoke carries more than half of the total current, occupying a quarter of the channel, and its in‑phase oscillations of azimuthal electric field and density produce a drift current that matches the spoke current, confirming enhanced electron transport.

Abstract

Rotating spoke phenomena have been observed in a variety of Hall thruster and other E × B devices. It has been suggested that the spoke may be associated with the enhancement of the electron cross-field transport. In this paper, the current conducted across the magnetic field via a rotating spoke has been directly measured for the first time in the E × B discharge of a cylindrical Hall thruster. The spoke current was measured using a segmented anode. Synchronized measurements with a high speed camera and a four-segment anode allow observation of the current as a function of time and azimuthal position. Upwards of 50% of the total current is conducted through the spoke, which occupies a quarter of the Hall thruster channel area. To determine the transport mechanism, emissive and Langmuir probes were installed to measure fluctuating plasma potential, electron density, and temperature. A perturbed, azimuthal electric field and density are observed to oscillate in-phase with the rotating spoke. The resulting drift current is found to enhance electron transport with a magnitude equal to the spoke current to within margins of error.