Spatiotemporal evolutions of gas density and gas temperature in spark-plug assisted atmospheric-pressure microwave discharges

The gas temperature ($T_{\rm g}$) and gas density ($n$) are important parameters in plasma-assisted combustion. The gas heating results in the concomitant reduction of the gas density. The reduction of the gas density affects the reduced electric field ($E/n$) in the plasma, and correspondingly all the plasma parameters which depend on $E/n$. In this work, a rotational Raman scattering technique was used for measuring spatiotemporal evolutions of $T_{\rm g}$ and the density ($n_{\rm N2}$) of molecular nitrogen in spark-plug assisted atmospheric-pressure microwave discharges generated in N$_2$-He mixture (${\rm N}_2/{\rm He} = 700/50$ Torr) with pulsed microwave power of 360 W. Measured values of $T_{\rm g}$ in the discharge phase suggests that the discharge is in a nonequilibrium state. Unexpected, significant depletion of $n_{\rm N2}$ (up to 95\%) was observed in the intermediate stage of the discharge phase and in the early afterglow at a distance of 4.5 mm from the discharge center. Although the most probable mechanism for the significant depletion of N$_2$ is dissociation, further investigation is necessary to confirm the huge degree of dissociation.