Effect of spacecraft surface conductivity on voltage of microwave neutralizer in gridded ion thrusters

The μ10 gridded ion thruster, designed and developed at the Institute of Space and Astronautical Science, Japan, is used in the asteroid explorer Hayabusa2. In this propulsion system, the cathode is negatively biased with a power supply. The potential difference between the cathode and the system ground is regarded as an essential indicator of the cathode condition. In the operation of μ10 during the Hayabusa2 mission, it was found that the potential difference increased faster than that measured in a durability test on the ground. In this study, we performed a fault tree analysis of the on-orbit increase in the potential difference between the cathode and the system ground considering the total current balance (including electrons that flow back to the spacecraft). In addition, to understand the effect of the conductive surface around the thruster on the potential difference, we experimentally simulated the change in conductive area. It was found that the potential difference increased under a certain conductive surface condition. The results suggest that the on-orbit increase in the potential difference between the cathode and the system ground is caused by a loss of surface conductivity.