The formation and decay of the thermal spark generated by a single nanosecond highvoltage 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 subnanosecond timescales. The propagation speed of the filaments is on the order of 10 4 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 4×10 19 to 2×10 18 cm -3 in 100 ns. We show that this decay curve can be interpreted as the isentropic expansion of a plasma in chemical equilibrium. Comparisons with previous experiments from the literature support this conclusion. 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.

We report on Raman scattering and optical emission spectroscopy (OES) measurements in recombining atmospheric pressure plasmas of air and nitrogen. An inductively coupled plasma torch is used to create an equilibrium plasma, which is then forced to rapidly recombine by flowing through a water-cooled tube. For all conditions, temperature measurements are performed using OES and Raman scattering at the exit of tubes of varying lengths. The density of atomic nitrogen is also determined. Evidence of strong chemical nonequilibrium is found in a number of cases. For these cases, we observe that the rotational temperatures measured with OES differ from those measured with Raman scattering, and that the atomic nitrogen density is elevated with respect to equilibrium. A power balance analysis confirms that a large fraction of gas enthalpy is stored in the non-recombined nitrogen atoms. For cases where the plasma remains in equilibrium, we perform numerical simulations using the Eilmer3 computational fluid dynamics (CFD) code. Eilmer3 does not predict the observed drop in gas temperature measured using Raman scattering and OES. Prior efforts by the CFD community have also failed to correctly predict this temperature drop. The results presented in this paper are therefore intended as validation test cases for CFD simulations.