Anatomy of cross-field electron transport by steady and unsteady plasma structures in Hall thrusters

Self-organizing plasma structures and their effects on the cross-field electron transport in Hall thrusters have been investigated. It is hypothesized that 1) an azimuthal plasma oscillation is induced by a gradient-drift instability (GDI), and 2) the azimuthal plasma structure enhances the axial electron transport by an E_θ×B_r drift. In this study, a hybrid particle-fluid model was developed to analyze the evolution process and characteristics of the GDI in the Hall thruster discharge. Our simulation results showed complex vortex-like structures with frequencies of 100 kHz to 1 MHz in the electron flow field. The observed oscillation exhibited a consistency with the perturbation theory of GDI. Further, to experimentally investigate the relationship between azimuthal plasma structures and cross-field electron transport, we operated a Hall thruster with artificial disturbances, such as azimuthally nonuniform propellant supply and nonuniform magnetic field. Because equilibrium plasma structures were attained in azimuth during the operations with artificial inhomogeneities, detailed probe diagnostics were performend to obtain the azimuthal plasma properties. The measurement results clearly showed that the electron density and plasma potential were azimuthally out of phase, which further indicated that the azimuthal plasma structure enhanced the cross-field electron transport toward the anode direction.