Features of a Supersonic Ionization Wave in Argon at Atmospheric Pressure in a Sub-Threshold Microwave Field

In the subthreshold microwave discharge (the wavelength of 4 mm) in argon at the pressure of 750 Torr at intensities of the wave beam from 3 to 7.8 kW/cm2, the velocity of the ionization front, measured from the phase change of the reflected wave, increases approximately as the intensity on the wave beam axis in the power of 3/2 from 0.5 × 105 to 2.9 × 105 cm/s. This velocity of the discharge front in argon is 20–30 times higher than the velocity of the discharge front in air at the same intensities in the microwave beam. The glow structure of the discharge at intensities higher than 3 kW/cm2 is similar to the glow structure in molecular gases. An even finer structure of the discharge is observed at glow intensities on the wave beam axis below 3 kW/cm2. The gas temperature in the discharge at radiation intensity of 6 kW/cm2 is 6 kK. It is concluded that the volume of high-temperature regions in the discharge is 0.01 of the volume of the discharge region. The problem of the mechanism of the argon ionization and the possibility of the development of ionization-overheating instability in a non-self-sustained microwave discharge in the UV halo of the discharge front are discussed.