Plasma hysteresis caused by high-voltage breakdown in gridded microwave discharge ion thruster μ10

Abstract

A cluster of gridded microwave discharge ion thrusters μ10 contributed to the success of the deep space exploration missions of the Japanese asteroid explorers Hayabusa and Hayabusa2. To meet the demands of the next mission DESTINY+, the maximum ion beam current was increased to 200 mA from the 170 mA used in Hayabusa2 by redesigning the magnetic field and grid configuration. However, this thruster exhibited the plasma hysteresis caused by a high-voltage breakdown. Specifically, the ion beam current (i.e. thrust) has two modes at a given propellant flow rate, namely high- and low-current modes. In nominal operation, the thruster operates in high-current mode. However, once a high-voltage breakdown occurs, the ion beam current transitions to and remains in low-current mode, which significantly decreases thrust efficiency. In this paper, based on an investigation of the physical mechanism that causes the hysteresis, we proposed a recovery method to return the thruster operation to high-current mode. In the investigation, we focused on the plasma mode transition, where plasma is generated inside the waveguide. We experimentally investigated the hysteresis by varying the grid configuration and the waveguide geometries while maintaining the magnetic field. The results show that the neutral density and plasma conditions inside the waveguide are strongly related to the hysteresis, which indicates that the main cause of the hysteresis is plasma generation inside the waveguide.