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首页> 外文期刊>Plasma Sources Science & Technology >Fully ionized nanosecond discharges in air: the thermal spark
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Fully ionized nanosecond discharges in air: the thermal spark

机译：空气中完全电离纳秒放电：热火花

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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 N_2. Spark thermalization consists of the following steps: (i) partial ionization of the plasma channel accompanied by N_2 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 10~4 m s~(-1) during step (iv). For the 1 bar condition, the electron number densities are measured from the Stark broadening of N~+ and H_α 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 derWaals and resonant broadenings of H_α, H_β, and several lines of O, O~+, N and, N~+ are derived in the appendix.
机译：本研究描述了pin电极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毫巴范围内气压的影响。通过进行短栅极成像和光学发射光谱（OES），我们发现，与非热火花相比，热火花表现出强烈的N~+激发电子态发射，而非热火花的发射主要由N_2的电子跃迁控制。火花热化包括以下步骤：（i）伴随着N_2发射的等离子体通道的部分电离，（ii）在阴极产生以N~+发射为特征的完全电离的灯丝，（iii）在阳极形成完全电离的灯丝，（iv）这些灯丝向极间间隙的中间传播，以及（v）灯丝的合并。细丝的形成，即步骤（ii）和（iii），发生在亚纳秒的时间尺度上。在步骤（iv）中，长丝的传播速度约为10~4ms-1。在1bar条件下，电子数密度是通过N~+和H_α线的斯塔克加宽测量的，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，达到峰值48000k。在后放电中，电子数密度在100ns内从4×10~（19）衰减到2×10~（18）cm-3。我们表明，这种衰减曲线可以解释为等离子体在化学平衡中的等熵膨胀。与文献中先前实验的比较支持这一结论。附录中导出了H_α、H_β以及O、O~+、N和N~+的几行的范德瓦尔斯和共振展宽表达式。

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