Equilibrium and non-equilibrium gas discharges sustained by powerful radiation of the terahertz frequency range in noble gases

The results of a study of the spatial dynamics of a discharge in pure noble gases and their mixtures (helium with the addition of argon) in a wide range of gas pressures (0.2–1.5 bar) are presented. The studies were carried out for several frequencies of heating radiation (250, 263 GHz and 1 THz) in a wide range of power densities in the focal spot (from fractions of a kW/cm$^2$ to several MW/cm$^2$). At relatively low power densities, the discharge propagation had an equilibrium character, while at high power densities it was nonequilibrium. Various mechanisms of propagation of nonequilibrium and equilibrium discharges are discussed.