Increase of Power Efficiency in Coaxial Transmission Line Resonator by Using a Spacer to Compensate for Plasma Impedance

This article presents a method to improve the power efficiency of microwave plasma generators which have a resonator structure after plasma ignition. A coaxial transmission line resonator, one of the microwave plasma generators, was used to design to have an input impedance of 50 Ω in the absence of plasma. However, after plasma ignition, the plasma impedance affects the input impedance of the resonator, and the device efficiency decreases. The proposed resonator includes a spacer at the shorted end; the spacer has the necessary impedance to compensate for the plasma impedance. The spacer increased the device efficiency by up to 27.2% and increased the length of the plasma jet by 2.5 mm compared to the conventional one. Optical emission spectrum analysis confirmed that the intensities of Ar spectrum and reactive oxygen/nitrogen species spectra were also increased by 50% and 30% by using the spacer, respectively.