In this work, we examine initial phases of micro-discharges produced in deionized water by high-voltage pulses of nanosecond duration. We summarize results achieved by applying opto-electrical diagnostics with extremely high temporal (down to 30 ps) as well as spatial (down to m) resolutions. We have obtained frozen interferometric and shadowgraph images evidencing three distinct events. The first one, subcritical (no-discharge) event, is characterised by periodic perturbations of the index of refraction which depart from the anode surface and are pulled away with the speed of sound as an expanding envelope defined by the shape of the anode tip. One-dimensional hydrodynamic modelling of subcritical phase under conditions mimicking curvatures of real anode tips reveals basic characteristics of perturbations caused by dynamic balance between the hydrostatic and electrostrictive pressures consistent with experimental observations. The second one, dark or non-luminous discharge event, is characterised with the onset of few isolated very tiny tree-like structures growing from anode tip. Depending on the HV amplitude, the initial structures occur with delay of 2-3) ns after onset of HV pulse and subsequently expand with average velocity of (1-2)E7 cm/s creating very dense bush-like structures made of thin hair-like filaments in few nanoseconds. The third one, luminous discharge, follows (nearly simultaneously) the dark discharge event and unveils much simpler tree-like morphology determined by the extension of non-luminous bush-like structures. Characteristic dimensions of observed events range from about 1 micrometre (typical diameter of non-luminous filaments) to tens of micrometres (characteristic diameters of luminous filaments). Furthermore, we have addressed a possible role of microbubbles developing in the anode region due to the periodic HV pulses and verified that the UV-vis-NIR spectrometric signatures of the luminous phase notably change when replacing non-degassed DI water with the degassed one.

中文翻译：

---

液态水中纳秒放电初始阶段的研究

在这项工作中，我们检查了由纳秒持续时间的高压脉冲在去离子水中产生的微放电的初始阶段。我们总结了通过应用具有极高时间（低至 30 ps）和空间（低至 m）分辨率的光电诊断所获得的结果。我们已经获得了证明三个不同事件的冻结干涉和阴影图图像。第一个，亚临界（无放电）事件，其特征在于折射率的周期性扰动，其离开阳极表面并随着声速被拉开，作为由阳极尖端的形状定义的扩展包络。在模拟真实阳极尖端曲率的条件下对亚临界相进行的一维流体动力学建模揭示了由与实验观察一致的静水压力和电致伸缩压力之间的动态平衡引起的扰动的基本特征。第二个，黑暗或不发光的放电事件，其特征是从阳极尖端生长出少数孤立的非常微小的树状结构。根据 HV 振幅，初始结构在 HV 脉冲开始后延迟 2-3) ns 出现，随后以 (1-2)E7 cm/s 的平均速度扩展，形成由细毛制成的非常密集的灌木状结构- 几纳秒内的细丝。第三个，发光放电，跟随（几乎同时）暗放电事件并揭示由不发光的灌木状结构的延伸决定的更简单的树状形态。观察到的事件的特征尺寸范围从大约 1 微米（不发光灯丝的典型直径）到几十微米（发光灯丝的特征直径）。此外，我们已经解决了由于周期性 HV 脉冲而在阳极区域形成微气泡的可能作用，并验证了当用脱气去气水代替未脱气去离子水时，发光相的 UV-vis-NIR 光谱特征显着变化。观察到的事件的特征尺寸范围从大约 1 微米（不发光灯丝的典型直径）到几十微米（发光灯丝的特征直径）。此外，我们已经解决了由于周期性 HV 脉冲而在阳极区域形成微气泡的可能作用，并验证了当用脱气去气水代替未脱气去离子水时，发光相的 UV-vis-NIR 光谱特征显着变化。观察到的事件的特征尺寸范围从大约 1 微米（不发光灯丝的典型直径）到几十微米（发光灯丝的特征直径）。此外，我们已经解决了由于周期性 HV 脉冲而在阳极区域形成微气泡的可能作用，并验证了当用脱气去气水代替未脱气去离子水时，发光相的 UV-vis-NIR 光谱特征显着变化。