Changes in Structure of Subthreshold Discharge in Air Occurring with Decreasing Microwave Radiation Intensity

At microwave beam intensities in the range of 3.3–4.8 kW/cm 2 , changes in the characteristics were recorded of the subthreshold self-non-self-sustained microwave discharge (SNSS discharge) in air at atmospheric pressure. Qualitative changes were observed in the discharge glow structure when going over from higher to lower intensities: in the discharge head, the structure in the form of a set of contracted channels disappears and there appears the inhomogeneous diffuse glow along the entire discharge length. The changes in the discharge structure occurring as the beam intensity decreases from 4.8 to 3.3 kW/cm 2 are accompanied by a decrease in the propagation velocity of the discharge front from 1.7 × 10 3 to 2.8 × 10 2 cm/s, that is, the quadratic dependence of the discharge front velocity on the microwave intensity becomes violated. In this case, behind the discharge front, a decrease in the fluctuation level of the UV radiation intensity was also recorded.