Plasma Characteristics of a Microwave Ion Engine.

For efficient use of solar electric propulsion, it is required to adjust operational parameters to the electrical power supply available on a spacecraft. For a microwave ion engine system, which is employed for asteroid sample return mission MUSES-C, such a throttling is conducted by varying the electrical power used for plasma generation via electron cyclotron resonance as well as by modifying the mass flow rate of xenon propellant. However, in some throttling conditions, microwave power deposition for plasma generation is prohibited, deteriorating propellant utilization efficiency. From probe measurement of a microwave ion source, the propellant utilization efficiency was strongly related to the plasma characteristics inside the discharge chamber. Very diffused plasma distributions were found in the case of low utilization efficiency, making contrast to localized plasma distributions for efficient propellant utilization.