Space and time analysis of the nanosecond scale discharges in atmospheric pressure air: I. Gas temperature and vibrational distribution function of N2 and O2

Reliable experimental data on nanosecond discharge plasmas in air become more and more crucial considering their interest in a wide field of applications. However, the investigations on such nonequilibrium plasmas are made difficult by the spatial non-homogeneities, in particular under atmospheric pressure, the wide range of time scales, and the complexity of multi-physics processes involved therein. In this study, we report spatiotemporal experimental analysis on the gas temperature and the vibrational excitation of N-2 and O-2 in their ground electronic state during the post-discharge of an overvoltage nanosecond-pulsed discharge generated in a pin-to-plane gap of air at atmospheric pressure. The gas temperature during the pulsed discharge is measured by optical emission spectroscopy related to the rotational bands of the 0-0 vibrational transition N-2(C-3 Pi(u), nu = 0) -> N-2 (B-3 Pi(g), nu = 0) of nitrogen. The results show a rapid gas heating up to 700 K in tens of nanoseconds after the current rise. This fast gas heating leads to a high gas temperature up to 1000 K measured at 150 ns in the first stages of the post-discharge using spontaneous Raman scattering (SRS). The spatiotemporal measurements of the gas temperature and the vibrational distribution function of N-2 and O-2, also obtained by SRS, over the post-discharge show the spatial expansion of the high vibrational excitation of N-2, and the gas heating during the post-discharge. The present measurements, focused on thermal and energetic aspect of the discharge, provide a base for spatiotemporal analysis of gas number densities of N-2, O-2 and O atoms and hydrodynamic effects achieved during the post-discharge in part II of this investigation. All these results provide space and time database for the validation of plasma chemical models for nanosecond-pulsed discharges at atmospheric pressure air.