Experimental Prediction of Performance Degradation During Space Operation in Microwave Discharge Ion Thruster µ10

抄録

The microwave discharge ion thruster μ10 contributed to the success of the Japanese asteroid exploration missions Hayabusa and Hayabusa2. During these space missions, the performance degradation, i.e., the decrease of propellant utilization efficiency, was observed to be approximately 16% after an operation time of 9,000 h. In contrast, no degradation was observed in the ground test. Previous work has indicated that the main cause of the degradation is carbon contamination from ion sputtering in the accelerator grid. This work also suggested that a facility effect occurred, i.e., back-sputtered materials at the wall of the vacuum chamber canceled out the performance degradation. In this study, we propose a time-saving method by increasing the accelerator current, for use in efforts to reproduce the carbon contamination without the facility effect. The approach was successful in reducing the operation time by a factor of 90–200, while the degradation observed during space operation was reproduced. Finally, we attempted to recover the thruster performance by offsetting the microwave frequency by −0.06 to +0.01 GHz from the conventional microwave frequency of 4.25 GHz. The experimental results indicate that the performance degradation can be mitigated using a lower microwave frequency, which provide some ideas to suppress performance degradation in space missions.