Effects of negative ions on discharge characteristics of water plasma source for a miniature microwave discharge ion thruster

This work analyzes the discharge characteristics of water plasma for a miniature microwave discharge ion thruster via three-dimensional particle-in-cell simulations with Monte Carlo collisions. It incorporates the negative ions (H−, O−, and OH−) into the simulation code, aside from the three major positive ion species (H2O+, OH+, and H+), and investigates their effects on the discharge characteristics. On the one hand, the simulation results indicate that H2O+ is the dominant species and the negative ions have little effect on the positive ion density and transport. On the other hand, this study confirms the axial oscillation of the OH− negative ion density, in addition to azimuthal rotation, causing plasma instability with a periodic low-frequency anomalous diffusion. The azimuthal rotation is similar to the phenomena observed in other E × B devices, where the difference in spatial distributions of magnetized electrons and unmagnetized ions causes the local potential hump and the resultant instability. M...