Thermalization Between Electrons and Heavy Species in CO₂ Microwave Plasmas Revealed by Thomson Scattering

Thomson scattering has been implemented for the first time in CO₂ microwave plasma to study electron properties. Resolving the Thomson spectra has been enabled by (i) attenuation of Rayleigh scattered light using a volume Bragg grating and (ii) a disentangling of the Raman and Thomson signatures, made possible by the high gas temperatures (4000-7000 K) and large degree of dissociation. The plasma contracts with pressure, which is accompanied by an increasing gas temperature and decreasing electron temperature. At low pressures (<100 mbar) electron temperatures are found between 1 and 2 eV with gas temperatures <0.5 eV. At pressures above approximately 150 mbar, electron and gas temperatures equilibrate at ~0.6 eV (or ~7000 K). Electron density and ionization fraction increase as electron temperature decreases. Radial profiles of electron density and atomic oxygen emission (3s⁵S⁰ <- 3p⁵P) overlap for contracted conditions, but not in the transient regime between diffuse and contracted plasma; here electron density has a wider profile than oxygen emission, a result of optical contraction. Initial analysis, using 1D fluid modeling, indicates C+O associative ionization is the main mechanism providing ionization at the high gas temperatures, albeit with C fractions <1%. This mechanism explains the simultaneous increase in ionization fraction and decrease in electron temperature. These results are at odds with the current picture of non-equilibrium in moderate pressure CO₂ microwave plasma.