Optical Characterization of a Non-Thermal Microwave Argon Plasma Column at Intermediate Pressure in the Presence of S8.

Physicochemical mechanisms taking place in a non-thermal microwave argon-sulfur plasma column are studied by analyzing the effect of solid sulfur ( S8 ) on argon plasma properties at various injected power and pressure conditions. Direct optical imaging reveals an enhancement of the plasma radial contraction upon the addition of sulfur at low power, which contrasts with a transition from contacted to diffusive regime at high power. This phenomenon is further examined by optical emission spectroscopy to obtain the population of atomic sulfur (from actinometry on S atoms using Xe as the actinometer gas) and the neutral gas temperature (from the broadening of the 826, 840 and 842 nm argon emission lines). Depending on the operating conditions, the number density of atomic sulfur varies between 10 ¹⁹ and 10 ²¹ m ⁻³ . As for the neutral gas temperature, it shows a significant rise upon the addition of sulfur. By correlating the population of S atoms with the corresponding thermal vaporization fluxes at the measured neutral gas temperature, it can be concluded that vaporization must involve other energy inputs linked to, for example, ion, electron and photon irradiations of S8. Further analyses of line intensity ratios along the plasma column reveal a decrease of the electron temperature associated with decreasing densities of atomic and dimer sulfur. Based on the obtained results, several mechanisms are discussed to explain the enhancement and annihilation of the plasma radial contraction in these conditions.