The effects of antenna configuration on the thrust performance of a 30-W-class miniature microwave discharge ion engine were investigated. The ion beam currents were measured for three different antenna configurations such as disc, star, and cross antennas. The ion beam current of the star antenna was the largest among the three configurations and that of the disc antenna was the smallest. The effect of good coupling with plasma exceeded the effect of large losses on the antenna surface under low mass flow rate with low plasma density. The effect of large losses on the antenna surface also exceeded the effect of good coupling with plasma under high mass flow rate. The results showed that the optimum antenna shape depends on the desired mass flow level due to the tradeoff between microwave plasma coupling efficiency and surface-area recombination. It was also found that the disc antenna was the best at low mass flow rate and the star antenna was the best at high mass flow rate.
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