Effect of the Split-Ring Resonator Width on the Microwave Microplasma Properties

In this paper, a microstrip split-ring resonator microwave-induced plasma source was developed and the effect of the resonator strip width on the microplasma properties was studied. The end-goal of this paper is to implement the resonator into an in-space micropropulsion system. We thus need to understand the behavior of the microplasma to optimize the thruster performance. A range of resonators with widths from 1.5 to 6 mm were designed to generate a microplasma in argon. Simulations of the electric fields were performed to determine the peak gap electric field. Single Langmuir probe measurements were taken at a pressure of 1 torr to obtain the plasma density and electron temperature. The microwave power was varied from 4.7 to 11.34 W (37-41 dBm). The simulations showed that with an increase in the microstrip width, the maximum electric field strength is obtained for the 3-mm-wide resonator. The electron temperature is also the highest for the 3-mm-wide resonator sitting at ~5.6 eV with 11.34 W of power. However, the maximum plasma density was obtained for the 1.5-mm resonator, ~1.6 × 1017/m3. There is an indirect relationship between the resonator electric field and the resulting microplasma properties. A theoretical analysis of a resonator-based miniature ion engine showed similar performance to the existing miniature ion engines.