Measurements of Rotational Temperature and Density of Molecular Nitrogen in Spark-Plug Assisted Atmospheric-Pressure Microwave Discharges by Rotational Raman Scattering

Rotational Raman scattering was adopted for measuring the spatiotemporal evolutions of the rotational temperature (Tr) and the density (nN2) of molecular nitrogen at the electronic ground state in spark-plug assisted atmospheric-pressure microwave discharge. The rotational temperature had a spatial distribution with the peak at the plasma center. The temporal evolution of Tr suggests a long confinement time of heat in this plasma source. Because of the long confinement time of heat, the plasma was in a nonequilibrium state for at least ~1 ms after the initiation of the discharge. We observed the depletion of nN2 as well as the decrease in the partial pressure of N2 (PN2). They were more significant at a longer distance from the plasma center. The depletion of nN2 at the plasma center was roughly explained by the increase in Tr. On the other hand, we have speculated that the significant decreases in nN2 and PN2 in the peripheral region of the plasma is owing to dissociation of N2 followed by nitriding of solid surfaces.