Fully ionized nanosecond discharges in air: the thermal spark

The formation and decay of the thermal spark generated by a single nanosecond high-voltage pulse between pin electrodes are characterized in this study. The influence of air pressure in the range 50-1000 mbar is investigated at 300 K. By performing short-gate imaging and Optical Emission Spectroscopy (OES), we find that the thermal sparks exhibit an intense emission from excited electronic states of N+, in contrast with non-thermal sparks for which the emission is dominated by electronic transitions of N2. Spark thermalization consists of the following steps: (i) partial ionization of the plasma channel accompanied by N2 emission, (ii) creation of a fully ionized filament at the cathode characterized by N+ emission, (iii) formation of a fully ionized filament at the anode, (iv) propagation of these filaments toward the middle of the interelectrode gap, and (v) merging of the filaments. The formation of the filaments, steps (ii) and (iii), occurs at sub-nanosecond timescales. The propagation speed of the filaments is on the order of 104 m/s during step (iv). For the 1-bar condition, the electron number densities are measured from the Stark broadening of N+ and Hα lines, with spatial and temporal resolution. The electron temperature is also determined, from the relative emission intensity of N+ excited states, attaining a peak value of 48,000 K. In the post-discharge, the electron number density decays from 4x1019 to 2x1018 cm-3 in 100 ns due to an isentropic expansion. During this decay phase, the plasma is found to be in chemical equilibrium. Comparisons are given with previous experiments from the literature. Expressions for the Van der Waals and resonant broadenings of H, Hβ, and several lines of O, O+, N and, N+ are derived in the appendix.