Measurement of Microwave Absorbance by Plasma of a Miniature Microwave Discharge Ion Thruster

Abstract

In recent years, small spacecraft missions are increasingly utilized in wide variety of applications. They promote technology demonstration and space development by the short-term and low-cost development. Further progress of small spacecraft needs small thrusters—propulsion system with light weight, low operating power, and high efficiency. A miniature and low-power microwave discharge ion thruster is one of the candidates. The miniature ion thruster can operate with 1 W microwave power and it has specific impulse higher than 1000 s. However, microwave behaviors in the transmission line of complex geometry and discharge chamber with plasma absorption make it difficult to modify or improve the thruster. It is important to understand overall behavior of the microwave from the input connector to the discharge chamber. In this study, 3D microwave analysis was conducted in the transmission line and discharge chamber and it was revealed that impedance mismatching, even if it is short-distance, has an important effect, making resonance. Secondly, that resonance was formulated using a signal flow graph, and reflectivity of the plasma was experimentally measured using that formulation. A set of the techniques makes it possible to estimate the net plasma absorption of the device with different transmission lines or to optimize the transmission line for a newly designed plasma source.