Mode transition and ionization region variation of the external discharge Hall thruster

Abstract Mode transition phenomenon in an external discharge Hall thruster (XHT) has been reported under different anode voltage and magnetic flux density. Experimental data reveal a transition point indicating an optimal magnetic field for the thruster. Mode transition with varying anode voltages is more pronounced, showing performance trends of rise, saturation, and decline, with transition thresholds at 180 V and 220 V. Moreover, benefiting from the channel-less discharge of the XHT, the emission spectral image of the entire discharge region was acquired for the first time. The thickness of the luminous region from 140 to 180 V changed from 4 mm to 2 mm (50.0% decrease), and from 180 to 240 V changed from 2.0 mm to 1.8 mm (10.0% decrease). The finding provides new insights and a valuable dataset for the investigation of related thrusters and the understanding of discharge mechanisms. It showed that a strong correlation between the mode transition process and the spatial variation of the ionization region. A combined dynamical and statistical model was developed to predict the steady ionization region boundaries, focusing on the dynamical behavior of electrons and the magnetized electrons region in E × B field. The ionization region formation requires magnetized electrons, gas concentration, and sufficient electron energy, making the varying definitions of its boundary the primary cause of mode transition. Furthermore, a mode locus plot method was developed to exactly reveal the plume structure and parameters change, which was verified by the experimental observation. It is worth noting that although the model is based on the of an XHT, the analytical methods therein can be extended to explain the widespread mode transition phenomenon in conventional Hall thrusters, as well as to E × B field discharges at vacuum or low atmospheric pressure.