Discharge mode transition of a 100-W class external discharge Hall thruster

Miniaturizing Hall thrusters to sub-kilowatt levels intensifies plasma-wall interactions through increased surface-to-volume ratios, accelerating erosion and degrading longevity/performance. The wall-less external discharge Hall thruster (XHT) eliminates channels by externalizing ionization/acceleration, resolving erosion to enable applications like Very Low Earth Orbit (VLEO) spacecraft. Its configuration—negative magnetic gradient, small anode (15 mm), strong B-field (>0.1 T)—may alter discharge modes and ionization. Experiments reveal mode transitions under varying voltage and magnetic flux, showing an optimal B-field. Voltage changes induce performance transitions (rise, saturation, decline) at 180 V and 220 V thresholds. XHT's design enabled unprecedented full-region spectral imaging: luminous zone thickness decreased from 4 mm (140 V) to 2 mm (180 V; 50%) then 1.8 mm (240 V; 10%), correlating spatially with mode shifts. A dynamic-statistical model predicted ionization boundaries using electron behavior in E×B fields. The mode locus method mapped plume evolution, validated experimentally. This framework extends to mode transitions in conventional Hall thrusters and other E×B discharges.