Non-equilibrium Atmospheric-Pressure Plasma Torch Sustained in a Quasi-optical Beam of Subterahertz Radiation

This paper studies a continuous atmospheric-pressure discharge maintained by focused subterahertz radiation of a 263-GHz gyrotron. A plasma torch propagating toward microwave radiation was sustained on an argon flow exiting a metal gas tube in the surrounding atmosphere of air. Using a high-speed camera, the spatial structure of the torch was studied, and the temporal dynamics on timescales of the order of 20 ns was examined. The emission spectra of the optical wavelength range were used to estimate the excitation temperature of argon atoms and electron density. It was shown that a discharge of this type is substantially non-equilibrium, and the electron temperature is many times higher than the vibrational and gas temperatures, being 1.5–1.7 eV in order of magnitude. The electron density measured by the Stark broadening of the Нα and Нβ hydrogen lines exceeds a critical value for the frequency of the heating field and is at a level of 1.5 1015 cm−3.