Influence of the Microwave Frequency and Discharge Chamber Height on the Ion Extraction Performance of a Water Ion Thruster

Abstract

A water-propellant ion thruster was proposed and studied using microwave discharge for a propulsion system of CubeSats aiming for a high delta-V. In this paper, we measured the ion production cost and the microwave reflectivity of the water-propellant ion thruster with a changing discharge chamber height and microwave frequency. As a result, we found lower and upper limitations of the frequencies to sustain the plasma. The lower limitation depended on the discharge chamber height, and the upper one showed no dependency. Microwaves from 4.4 to 5.4GHz were found to sustain the plasma stably for all the heights. In this range, the ion production cost became low as the height was set low. On the other hand, the characteristics of the reflectivity largely differed by the height. Furthermore, the characteristics changed in the experiments using xenon propellant, which implied that the microwave frequency should be optimized depending on the chamber shape and the propellant type.