Discharge in the atmosphere in a Gaussian beam of subthreshold millimeter waves

The propagation velocities of a subthreshold discharge excited in air at atmospheric pressure by a pulsed microwave beam with a Gaussian field distribution, a wavelength of $4$ mm, and an intensity up to $30$ kW/cm$^2$ have been measured by means of optical and microwave diagnostics. It has been shown that the motion of a discharge front along the path of the beam toward the region of an increasing microwave field is accompanied by an increase in the velocity from subsonic (${\sim}\,10^4\,$ cm/s) to supersonic (${\sim}\,(6{-}8)\times 10^4\,$ cm/s). At the same time, motion toward the decreasing field region is accompanied by a decrease in the velocity from supersonic to subsonic. It has been found that the maximum temperature of the gas in the discharge at velocities of $\sim 10^4$ cm/s reaches $\sim 5.3$ kK.