Three-dimensional inhomogeneous characteristics of low-frequency plasma oscillations in wall-less Hall thrusters

The low-frequency plasma oscillations in a wall-less Hall thruster are investigated via three-dimensional particle-in-cell-Monte Carlo collision simulations. The breathing oscillation exhibits three-dimensional azimuthal non-uniformity, initiating from a localized ionization onset region. Ionization subsequently propagates in the −E × B direction and, more rapidly, in the E × B direction due to electron drift, covering the anode surface in an extremely short time. Azimuthally, rotating spokes show structural variation with axial position: near the anode, ionization stably rotates toward regions of higher neutral density, while farther axially, spokes connect radially to the central ionization zone and rotate synchronously. These results underscore the intrinsic three-dimensionality of both axial and azimuthal oscillations in E × B devices with three-dimensional magnetic fields.