Characteristics of an atmospheric-pressure line plasma excited by 2.45 GHz microwave travelling wave

An atmospheric-pressure line plasma was produced by microwave discharge using a slot antenna with travelling microwave power. Two different types of plasma mode, i.e., "pseudo" and "real" line plasma were investigated using a high-speed camera under different discharge conditions, such as slot gap width and power. Using wide slot gaps (0.5 mm) and low powers (<1.0 kW), the pseudo line plasma mode, i.e., the time-averaged line plasma mode with the fast movement of small plasmas along the slot, was observed. By reducing the slot gap width to 0.1 mm and by increasing the peak microwave power, the plasma mode changed from the pseudo to real line plasma mode, i.e., the spatiotemporally uniform plasma mode along the slot. A gas temperature was obtained from N2 second positive band spectra as low as 400 K. The movement of the plasma in the pseudo line plasma mode was well explained by a one-dimensional diffusion model including the spatial distribution of the ionization rate in a moving plasma.