On the necessary condition for the generation of atmospheric-pressure electrodeless microwave plasma

This study investigates the necessary conditions for sustaining electrodeless microwave (MW) plasma at atmospheric pressure with efficient power coupling. The analysis focused on two different plasma gases of CO₂ and N₂, commonly used for hydrocarbon reforming and for hazardous gas decomposition and nitrogen fixation, respectively. First, the plasma temperature and electron number density were measured experimentally to characterize the states of the produced plasmas. Then, chemical equilibrium calculations and MW skin depth calculations in the produced plasmas were performed to determine the necessary conditions for sustaining the plasma. The experiments showed that, regardless of changes in specific energy input, the temperatures of the plasmas in CO₂ and N₂ were 6300 K and 6500 K, respectively. Additionally, the electron number densities in CO₂ and N₂ were at nearly the same levels as those for thermodynamic equilibrium, indicating that the produced plasmas are in local thermodynamic equilibrium (LTE). Lastly, from the skin depth analysis, it was found that the temperature of the LTE plasma should be greater than 5300 K for CO₂ and N₂ to achieve a skin depth that enables 95% power coupling efficiency, a finding that was consistent with the experimental observation.