A Microwave Plasma Tube Based on a Modified Evanescent-Mode Cavity Resonator

This paper presents a new method for generating a tubular plasma jet using a modified evanescent-mode (EVA) cavity resonator. Unlike conventional EVA structures, the proposed design incorporates a curved outer cavity and a contoured central post to enhance gas flow and create a circular capacitive region that also serves as the gas outlet. Electromagnetic simulations reveal that the micro-gap at the top of the post concentrates electric fields on the order of $1\times10^{6}$~V/m, enabling efficient gas breakdown at low input power. A prototype operating at 2.25~GHz was fabricated and experimentally characterized with input powers of 100~mW to 20~W and flow rates of 1 to 100~slpm. The device achieves stable plasma ignition at powers as low as 250~mW and produces a plasma tube up to 5~mm in length at a helium flow rate of 40~slpm. Because of its low-power operation, extended jet geometry, and robust plasma stability.