Optical tomographic reconstruction and electrical diagnostic characterization of two mode-transitions in a low-power planar Hall thruster

Abstract The planar Hall thruster is believed to have an extended lifetime due to eliminating the discharge channel, which has received significant attention. A low-power planar Hall thruster incorporating a vortex inlet structure has been developed to enhance propulsion performance. Three distinct discharge modes were observed under different anode voltages. In the anode voltage range of 200-275 V, mode I exhibits a spherical shape plume. In the range of 300-350 V, mode II features a more concentrated plume. In these two modes, the propulsion performance and efficiencies increase with the anode voltage rising. In mode III, as the anode voltage rises from 350 V to 375 V, the thrust decreases by 41%, and the plume divergence efficiency drops notably by 14%. The combination of optical and electrical diagnostics, including a newly developed three-dimensional optical diagnostic method, reveals that the mode differences stem from the variations in the ionization and acceleration region. A direct comparison of the normalized ionization rate distribution in the plume region near the thruster outlet was made across the three discharge modes. Mode I and mode II are speculated to have a near-axis center ionization region and a near-anode annular ionization region, respectively, while mode III with high anode voltage exhibits the coexistence of dual ionization regions. The primary driver of mode transition is identified as the changes in the electron transport mechanism. The first type of transition should be attributed to the fact that increased anode voltage enhances electron group energy and facilitates their cross-field transport in the near-anode region. The second type of transition involves the establishment and dominance of a new path for high-energy electrons to traverse toward the anode through the plume region. This work can be beneficial for selecting the operating mode and optimizing the design of the low-power planar Hall thruster.