Sterilization characteristics by a coaxial microwave plasma flow at atmospheric pressure

A new type of atmospheric low temperature plasma source was developed utilizing a coaxial cable for microwave transmission rather than conventionally used waveguides. The plasma source has advantages such as portability, simple configuration, and ability to operate with wide range of gases and its mixtures. It mainly consists of a cavity, a quartz discharge tube, a coaxial cable, a microwave power source and a gas supply system. Using this plasma source, we tried to clarify sterilization characteristics of Bacillus subtilis. The log reduction numbers of Bacillus subtilis was obtained at least 5 (10^-5) under the following operation conditions: gas flow rates of Ar (14 SLM) with mixing oxygen of 0.5 SLM and input power is 400 W. Distributions of temperature and gas velocity were also measured two-dimensionally by using E-type thermocouple and Pitot tube, respectively. The temperature distributions and the peak value of temperature were influenced by the gas flow rate and the kinds of mixing gas. The effective sterilization is obtained under the condition of the substrate surface temperature of 353 K.