A Novel Three-Layer Coaxial Atmospheric Pressure Microwave Plasma Jet

An atmospheric pressure microwave plasma jet (APMPJ) based on three-layer coaxial structure and rectangular waveguide power divider is proposed in this article. The inner and middle conductors of the three-layer coaxial structure form the inner coaxial, and the middle and outer conductors form the outer coaxial. The ratio of microwave power fed into the inner coaxial ( $P_{1}$ ) and outer coaxial ( $P_{2}$ ) can be adjusted by the short plane of the end of the rectangular waveguide. Clear differences in discharge characteristics, such as shape, electron density, electron temperature, gas temperature, and so on, are experimentally obtained under different microwave power and mass flow rate ratios of the inner coaxial ( $Q_{1}$ ) and the outer coaxial 2 ( $Q_{2}$ ). Maintaining a total incident power of 20 W and a total gas flow rate of 12 SLM, by adjusting the ratio of $Q_{1}$ and $Q_{2}$ , the diameter of the plasma jet excited by the three-layer coaxial structure can be increased by up to two times compared to the two-layer coaxial structure. Keeping the total gas flow rate constant, the diameter of the plasma jet excited by the three-layer coaxial structure is at least three times higher compared to the two-layer coaxial structure by adjusting $P_{1}$ and $P_{2}$ . This result shows that the plasma excited by the three-layer coaxial structure has a larger action area. In addition, the degradation efficiency of RhB solution by the plasma jet excited when the inner and outer coaxials were operated simultaneously was compared with that of the plasma jet excited when the inner coaxial was operated alone, and the three-layer coaxial structure showed an increase in the degradation efficiency of RhB of about 50%. The results show that this study is expected to be helpful in increasing the plasma jet area and regulating the discharge characteristics of the plasma jet, and the APMPJ has been experimentally verified to be conducive to improve treatment efficiency and energy utilization. In addition, the gas temperature at the tail of the plasma jet was tested using an infrared thermal imager and was approximately 299.5 K when $P_{1}$ and $P_{2}$ were 20 and 0 W, respectively, which suggests that it is suitable for use in biomedical applications and so on.