Design of a large-sized microwave plasma jet device based on coaxial cable transmission

A large-sized, low-temperature plasma jets are crucial for industrial applications such as large-aperture optical processing, whole-machine surface cleaning, and solar cell surface texturing, where flexible and portable generators are necessary to meet high-efficiency manufacturing demands. This study developed a microwave plasma jet generator with a cylindrical resonant cavity, in which microwaves are transmitted via a coaxial cable and introduced into the cavity through a straight coil. Critical dimensions of the cavity were determined and optimized by analyzing the resonant frequency, electric field distribution, and microwave energy efficiency. Specifically, introducing a cylindrical inner conductor within the cavity could compress the microwaves, enhance the electric field, and reduce the cavity diameter. Modeling the plasma as an ideal microwave absorber enabled rapid coil optimization. A 2 mm diameter excitation coil directly connected to the cavity bottom achieved 99.8% microwave energy efficiency. Furthermore, experimental results confirmed the generation of an air plasma jet exceeding 9 mm in diameter at a microwave power of 200 W. The jet temperature displayed an approximately linear increase from 352 to 535 °C within the range of 70–200 W. When the microwave power is below 100 W, the microwave energy efficiency exceeds 90%.