On the role of fluctuations, cathode placement, and collisions on the transport of electrons in the near-field of Hall thrusters

Abstract

The performance of Hall thrusters can be highly sensitive to the position and operational parameters of the external cathode, hinting that the electron transport in the near-field is strongly dependent on the emitted electrons’ initial properties. In addition, the plasma plumes of Hall discharges often exhibit fluctuations which are expected to alter electron trajectories. By implementing recent near-field plasma potential measurements made on a low-power Hall thruster in 3D electron-trajectory simulations, it is shown that electron transport from the external cathode to the thruster channel is strongly sensitive to cathode parameters including position, orientation, and electron emission divergence. Periodic, low-frequency (i.e., 25 kHz) plasma potential fluctuations reduce electron transport to the channel of the thruster by more than 65% compared to the transport achieved with static 3D fields and substantially homogenize the electron density distribution. Additional gas-phase collisions are found to have only marginal effects, even when prescribed to occur at exaggerated rates (reaching 10 MHz). The three-dimensionality of the E and B fields, together with electron-wall collisions, appear to be important drivers of cross-field transport in this region of the discharge, yielding sufficient levels of electron transport to the channel without invoking plasma turbulence.