Azimuthal plasma inhomogeneity in planar Hall thrusters: spatiotemporal evolution and its impacts on performance
Wei Liu, Weizong Wang, Chen Lingxi, Peiran Li, Yifei Li, Guangchuan Zhang
Abstract
Abstract Planar Hall thrusters (PHTs) offer a promising solution for miniaturized electric propulsion by eliminating the discharge channel. The PHT exhibits complex plasma oscillations that significantly influence electron transport, ion acceleration, and thruster performance, which remain insufficiently explored. This study uses high-speed imaging to investigate the spatiotemporal evolution of azimuthal plasma inhomogeneity in a permanent-magnet PHT across a wide range of operating conditions. Proper orthogonal decomposition (POD) is employed to extract the dominant modes and analyze their transitions. Results reveal three distinct discharge modes: the rotating spoke mode, characterized by a stably rotating luminous plasma region at low discharge voltages; the breathing mode, dominated by global low-frequency oscillations at high discharge voltages and low neutral flow rates; and the transition mode, exhibiting more uniform azimuthal ionization distributions and reduced fluctuations. The POD spatial modes are obtained and ranked by their energy contributions, identifying the dominant structures that govern the plasma oscillations. A minimum relative discharge current amplitude of ∼6.2% corresponds to the threshold for the transition from rotating spoke to transition mode, with the threshold anode potential ranging from 150 to 200 V and increasing with propellant flow rate. Further experiments establish correlations between discharge modes and thruster performance using electrostatic probes and a torsional thrust stand. Both rotating spoke mode and breathing mode lead to a reduction in plume divergence efficiency. The Pearson correlation coefficient between the relative amplitude of light intensity and the reciprocal of the beam divergence efficiency reaches 0.9, indicating a strong inverse relationship between the plasma inhomogeneity and beam divergence efficiency. The transition mode is the most favorable for optimal thruster performance, as it strikes a favorable balance between key performance parameters. This study provides deeper insights into PHT discharge instabilities and their impact on performance, offering a strategy for optimizing the thruster’s operating parameters.