Characterization of the CO2 microwave plasma based on the phenomenon of skin-depth-limited contraction

The subatmospheric CO2 microwave plasma is known to contract to a narrow filament with rising pressure as result of a mode transition. This changing state of contraction is investigated in relation to its dielectric properties, in order to directly relate the discharge parameters to the discharge radius. The electron density and gas temperature are measured, respectively, by 168 GHz microwave interferometry and Doppler broadening of the 777 nm oxygen emission lines. The plasma is operated in steady state with 1400 W at 2.45 GHz, between 100 mbar and 400 mbar. Electron density values in the central region range from 1018 to 1020 m−3 between the discharge modes, while the gas temperature increases from 3000 K to 6500 K, in good agreement with previously reported values. Based on the dielectric properties of the discharge in relation to the plasma radius, it is found that the discharge column constitutes a radius of a single skin depth. Implications of these insights on the conditions of previously reported CO2 dissociation experiments are discussed.