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 Omega 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.