Distribution of high-energy backflow ions near cathode plume in microwave discharge ion thruster

Backflowing ions generated by electric thrusters are known to cause sputtering and surface degradation on spacecraft, which may ultimately shorten mission lifetimes. In this study, velocity measurements for metastable xenon ions were conducted near the cathode plume of the microwave discharge ion thruster μ10 using the laser-induced fluorescence technique. In addition, a spatial distribution of plasma potential was measured by an emissive probe. The ion velocity distribution functions showed different features depending on the measurement position. The mean velocity of the drifting population at each measurement position was visualized as a velocity field. A comparison of the velocity field with the plasma potential distribution or ion number density distribution revealed a diverging center that coincided with the cathode plume, suggesting that a certain number of backflow ions originated from the cathode plume region. In particular, high-energy ions were observed between the cathode plume and the spacecraft surface. Asymmetric features were found in both the plasma potential distribution and the ion number density distribution, which implies that the effect of the magnetic field is not negligible in this region.