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.

中文翻译：

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空气中完全电离的纳秒放电：热火花

本研究表征了针电极之间单个纳秒高压脉冲产生的热火花的形成和衰减。在 300 K 下研究了 50-1000 mbar 范围内气压的影响。通过执行短门成像和光学发射光谱 (OES)，我们发现热火花表现出来自 N+ 激发电子态的强烈发射，在与非热火花相比，非热火花的发射主要是 N2 的电子跃迁。火花热化包括以下步骤：(i) 伴随 N2 发射的等离子体通道的部分电离，(ii) 在以 N+ 发射为特征的阴极处形成完全电离的灯丝，(iii) 在阴极形成完全电离的灯丝阳极，(iv) 这些细丝向电极间间隙的中间传播，以及 (v) 细丝的合并。细丝的形成、步骤 (ii) 和 (iii) 发生在亚纳秒的时间尺度上。在步骤(iv)期间，长丝的传播速度约为104 m/s。对于 1-bar 条件，电子数密度是从 N+ 和 Hα 线的斯塔克展宽测量的，具有空间和时间分辨率。电子温度也由 N+ 激发态的相对发射强度确定，峰值为 48,000 K。在放电后，由于等熵，电子数密度在 100 ns 内从 4x1019 衰减到 2x1018 cm-3扩张。在这个衰变阶段，发现等离子体处于化学平衡状态。与之前文献中的实验进行了比较。