Distinct discharge modes in micro Hall thruster plasmas

Two distinct discharge modes were observed in a 50 W-class micro-Hall thruster plasma under different operating conditions. A ball-shaped plasma with a broad plume (mode A) was observed at low mass flow rates (less than 0.37 mg s−1) over the entire operational anode voltage range (160–280 V). Raising the anode voltage beyond 200 V with the mass flow rate fixed (larger than 0.37 mg s−1) produced a narrow plume and stretched jet-like structure (mode B). In mode B, the thruster showed performance improvements in terms of thrust (3.8 mN vs 3.3 mN), specific impulse (913 s vs 800 s), and anode efficiency (28% vs 22%), with only a 2 W difference in the anode power (61 W in mode B and 59 W in mode A). This suggests that operation is more advantageous in mode B than in mode A for the utilization of such low-power Hall thrusters. Unique plume properties were observed in the two modes and considerable differences were measured in the Xe II ion acceleration structure, beam angle, and ionization rate as the mode changes, which were not reported in previous studies. Mode A exhibits an axially extended ion acceleration structure outside the discharge channel, where 75% of the final ion velocity is achieved at approximately 40 mm from the thruster exit, while most of the ion acceleration occurs within 10 mm from the thruster exit in mode B. Measurements show that the full width at half maximum of the Xe II ion energy distribution function, electron temperature, and Xe II emission intensity decreased after the plasma transitioned from mode A to mode B. Based on the optical emission spectroscopy, the ionization rate in the plasma plume decreased by 30%–41% after the mode change, which is likely related to the reduction of the beam angle and electron current by 24% and 30%, respectively.