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

Renewable electricity-driven electrodeless microwave (MW) plasma systems have attracted significant attention for achieving zero carbon emissions in environmental and energy applications, such as hydrocarbon reforming and nitrogen fixation. While extensive studies have focused on the energy efficiency and plasma state analysis for these applications, the conditions required to sustain plasma have been relatively underexplored. In this study, we experimentally characterized electrodeless MW plasma under atmospheric pressure by measuring the temperature and electron density. The necessary conditions for sustaining the plasma were then determined through chemical equilibrium calculations and microwave skin depth analysis. The temperatures of both CO₂ and N₂ plasmas were approximately 6000 K, and the electron densities closely matched those predicted for thermal equilibrium at the corresponding temperatures, indicating that the plasmas were in a state of local thermodynamic equilibrium. From the skin depth analysis, the minimum gas temperatures required to sustain the plasmas—based on the criterion that the skin depth of microwave-sustained plasma must be smaller than the effective plasma radius—were calculated to be above 5300 K for the observed power coupling, supporting our experimental results.