Ion Pitch Angle Effects in Microwave ECR Ion Thruster
Ming-Hsueh, Shen1*, Hui-Kuan, Fang2, Sunny W. Y. Tam2, Yei-Chin, Chao1, Yueh-Heng, Li1
1Department of Aeronautics and Astronautics, National Cheng Kung University, Tainan, Taiwan
2Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan
* presenting author:Ming-Hsueh Shen,
Ion thruster is a form of electric propulsion by generating thrust from ion acceleration. Unlike higher thrust chemical propulsions, electric propulsion, especially ion thruster, has strength of higher specific impulse (ISP), which means higher propulsion efficiency. Accordingly, it can meet the requirement of deep space exploration mission. Ion thrusters have been used for many space missions, such as Deep Space 1 in 1998 from NASA, Artemis in 2001 from ESA, Hayabusa 2 in 2014 from JAXA etc.

In the near future, small, lightweight, low power satellites with correspondingly low power thrusters, such like CubeSat, will be implemented for deep space explorations. We have designed a micro ion thruster with 2.2 cm in inner diameter and 20 cm in length. Electron Cyclotron Resonance (ECR) Plasma is generated under 875 G background magnetic fields supplied by magnetic coils. The ions are initially accelerated from plasma generation region to the pre-acceleration region, and further accelerated to high speed in main acceleration region. Ions are accelerated by parallel electric fields supplied by metal grids. In the thruster, ion gyro-radius is 0.13 cm and its temperature is close to room temperature.

To enhance thruster efficiency, ion transparency should be much higher than neutral particle (fuel) transparency in the pre-acceleration region. Ions should also be accelerated to higher initial speed before entering main acceleration region. To investigate the combination effects of electric and magnetic fields on ion motion in this region, single particle motion simulation is performed by SIMION software. 3D simulation results show that the ion transparency is higher than 90% at all pitch angles when the position offset of incident ion is less than 3mm from thruster's axial line. For offset higher than 5mm, ion transparency is zero for all positive pitch angles, and less than 30% for negative pitch angles. Ion lensing effect, which reduces ion pitch angle effect and increases ion transparency, can be raised by increasing thruster boundary potential. Then the optimum pre-acceleration length can be determined to be the same as ion lensing focal length.

Keywords: Electric Propulsion, Ion Thruster, Ion Pitch Angle, ECR Plasma, SIMION