By employing multi-frame laser interferometry, shadow, and schlieren imaging, we trace the formation of a nanosecond spark discharge in millimeter-sized air gaps formed by a point cathode and flat anode or vice-versa. We discover that the electrical breakdown of the discharge gap is associated with extremely fast (<<1 ns) explosive formation of micron-sized cathode and anode spots. We find that the characteristic delay between the instants of the anode and cathode spot initiation can be much shorter than 1 ns. The spots appear as highly ionized near-electrode plasmas with an electron density ne ~ 1019–1020 cm-3. The spots then give rise to highly ionized spark channels with pronounced filamentary structures. Our findings indicate that the extremely fast formation of anode spots is associated with an ultrafast gap breakdown promoted by an ultrafast ionization wave. The role of the ultrafast ionization wave governed by the rapidly evolving cathode spot is discussed as a fundamental mechanism of the breakdown.

中文翻译：

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大气放电中阳极点的极快形成指向一个基本的超快击穿机制

通过采用多帧激光干涉测量、阴影和纹影成像，我们追踪了在由点阴极和平面阳极形成的毫米级气隙中纳秒火花放电的形成，反之亦然。我们发现放电间隙的电击穿与微米级阴极和阳极斑点的极快（<<1 ns）爆炸性形成有关。我们发现阳极和阴极点启动瞬间之间的特征延迟可以远短于 1 ns。这些斑点表现为高度电离的近电极等离子体，电子密度为 ne ~ 1019–1020 cm-3。然后这些斑点产生具有明显丝状结构的高度电离的火花通道。我们的研究结果表明，阳极斑点的极快形成与超快电离波促进的超快间隙击穿有关。由快速演化的阴极点控制的超快电离波的作用被讨论为击穿的基本机制。