Rotating Spoke Instabilities in Hall Thrusters

High-speed imaging of a Hall thruster plume reveals near-omnipresent rotating regions of elevated light emission, dubbed rotating spokes, in the annular thruster discharge channel. Azimuthal oscillations have long been suggested to induce cross-field electron transport in Hall thrusters, but conclusive experimental identification of such oscillations with probes is often challenging. However, simple processing of high-speed images taken at a few tens of thousands of frames per second clearly reveals long-wavelength rotating spokes at very low frequencies, corresponding to velocities of only a few hundred meters per second.