The Chemical Origins of Plasma Contraction and Thermalization in CO2 Microwave Discharges

Thermalization of electron and gas temperature in CO2 microwave plasma is unveiled with the first Thomson scattering measurements. The results contradict the prevalent picture of an increasing electron temperature that causes discharge contraction. It is known that as pressure increases, the radial extension of the plasma reduces from similar to 7 mm diameter at 100 mbar to similar to 2 mm at 400 mbar. We find that, simultaneously, the initial nonequilibrium between similar to 2 eV electron and similar to 0.5 eV gas temperature reduces until thermalization occurs at 0.6 eV. 1D fluid modeling, with excellent agreement with measurements, demonstrates that associative ionization of radicals, a mechanism previously proposed for air plasma, causes the thermalization. In effect, heavy particle and heat transport and thermal chemistry govern electron dynamics, a conclusion that provides a basis for ab initio prediction of power concentration in plasma reactors.

Automated summary

Thermalization of electron and gas temperature in CO2 microwave plasma is revealed for the first time through Thomson scattering measurements. The results contradict the prevailing understanding of increasing electron temperature causing discharge contraction. The study demonstrates that associative ionization of radicals plays a significant role in the thermalization process of the plasma, providing a basis for ab initio prediction of power concentration in plasma reactors.