Analyzing the dispersion relation of azimuthal oscillations in a miniature microwave discharge ion thruster

Electric propulsion systems for small satellites have been widely developed in recent years. Liquid and solid propellants offer advantages in reducing tank volume and mass, making these propellants particularly suitable for CubeSats. Water is one of the candidates due to its low cost and the potential for local refueling through in-situ resource utilization (ISRU) on asteroids and the Moon. In the water-propelled electric propulsion systems, a microwave discharge ion thruster is approaching on-orbit demonstration. However, when water is used as a propellant instead of xenon, which is a conventional propellant for electric propulsions, performance degradation remains a major challenge. Numerical simulations have indicated that azimuthal oscillations within the microwave discharge neutralizer are associated with thruster performance. These oscillations have recently been confirmed experimentally, and the mode of these oscillations may be linked to thruster performance. In that experiment, a positively biased collector was placed in front of the microwave discharge neutralizer to capture electrons. The collector was azimuthally segmented to measure azimuthal oscillations. In this study, in addition to this diagnostic approach, the inside wall of the thruster is also segmented azimuthally to measure current fluctuations. The resulting data are used to determine the dispersion relation with the aim of investigating the instability mechanisms responsible for these oscillations.