105 / 2021-06-05 23:45:31

THE MODE TRANSITION MECHANISM BASED ON THE NEAR-ANODE CUSP MAGNETIC FIELD IN STATIONARY PLASMA DISCHARGE THRUSTER

Two distinct discharge modes were observed in a 200 W cylindrical stationary plasma discharge thruster with near-anode cusp magnetic field. Since a certain operation parameter (discharge voltage, anode flow rate, electromagnetic coil current) varies consecutively, an apparent discharge mode transition occurred along with the sharp decreasing of discharge current (by 11%) and apparent narrowing of plume angle (by 15%). In model I, a cylindrical-shaped plasma with a large plume angle (> 58°) occurred over the entire operational discharge voltage range (240～440 V) at high anode flow rate (>0.5mg/s ), low middle coil current (<3.5A ). In model II, raising the discharge voltage beyond 340 V with a smaller anode flow rate (< 0.5 mg/s) and a high coil current (>3.5 A) produced a cone-shaped plasma with an apparent narrowing plume angle (<51°). During the mode transition, the ionization rate could be hardly changed due to its constant propellant utilization efficiency. Whereas the electron current decreased sharply along with the raising of discharge voltage. This abnormal phenomenon predicts that a new electron confinement type should play a critical role in ionization and acceleration process. Moreover, the test results of ion energy distribution function indicate that the voltage loss was decreased remarkably, which means that the main ionization region should shift towards upstream. Combining these test results, it is speculated that a high-speed electron Hall current could be established at the annular-cylindrical boundary during the mode transition, resulting in the avalanche ionization here, whereas the ionization in the original magnetic-mirror field region could be inhibit. Furthermore, the test results from the variation tendency of ion energy distribution function with anode flow rate and coil current confirm this speculation. Ultimately, the mode transition should be attributed to the dynamic change of ionization region induced by the transition of electron confinement mechanism. This work opens up a new perspective in comprehensive adjustment of ionization and acceleration process for miniaturized Hall thruster.