  This paper studies a continuous atmospheric-pressure discharge maintained by
focused subterahertz radiation of a 263 GHz gyrotron. A plasma torch
propagating towards 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
hydrogen lines exceeds a critical value for the frequency of the heating field.
