Experimental investigation of ionization front propagating in a 28 GHz gyrotron beam: Observation of plasma structure and spectroscopic measurement of gas temperature

Abstract

Atmospheric millimeter-wave discharge was investigated experimentally using a 28 GHz gyrotron. The propagation velocity of an ionization front, plasma structure, and vibrational and rotational temperatures of nitrogen molecules were measured at a beam intensity lower than 1.0 GW/m2, which is below the breakdown threshold. Results show that the propagation velocity of an ionization front increased monotonically with beam intensity and decreased with ambient pressure. In addition, four typical plasma structures having different space occupancies were observed. Furthermore, at any beam intensity below 0.5 GW/m2, the vibrational temperature was found to be saturated at about 6000 K. The corresponding electron number density is almost equal to the cut-off density. Finally, it was suggested that the propagation velocity depends on the plasma space occupancy.