Investigation of the initial phases of nanosecond discharges in liquid water
Investigation of the initial phases of nanosecond discharges in liquid water
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DOI：
10.1088/1361-6595/ab87b7
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发表时间：
2020-04
影响因子：
3.8
通讯作者：
M. Šimek;P. Hoffer;Ján Tungli;V. Prukner;J. Schmidt;P. Bílek;Z. Bonaventura
中科院分区：
文献类型：
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作者：
M. Šimek;P. Hoffer;Ján Tungli;V. Prukner;J. Schmidt;P. Bílek;Z. Bonaventura
在这项工作中，我们研究了纳秒持续时间的高压(HV)脉冲在去离子水中产生的微放电的初始阶段。我们应用具有极高的时间(低至30ps)和空间(低至1μm)分辨率的光电诊断。冻结的干涉图和阴影图显示了三个截然不同的事件。第一个是亚临界(无放电)事件，其特征是折射率的周期性扰动，这些折射率离开阳极表面，并以声速被拉离，形成由阳极尖端形状定义的膨胀包络。在模拟实际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模拟揭示了静压和电致伸缩压力之间的动态平衡引起的扰动的基本特征，与实验观测一致。第二种是暗的或不发光的放电事件，其特征是从阳极尖端开始生长出几个孤立的非常微小的树状结构。根据HV幅度的不同，初始结构在高压脉冲开始后延迟2-3 ns出现，随后以平均速度∼1×10~5-2×10~5m S−1扩展，在几纳秒内产生由细毛状细丝组成的非常致密的灌木状结构。第三，发光放电事件，(几乎同时)跟随暗放电事件，并揭示由非发光灌木状结构的延伸确定的简单得多的树状形态。观测事件的特征尺寸从大约1μm(非发光细丝的典型直径)到数十微米(发光细丝的特征直径)不等。此外，我们讨论了由于周期性的高压脉冲在阳极区发展的微泡的可能作用，并验证了当用脱气取代未脱气的去离子水时，发光相的UV-VIS-NIR光谱特征发生了显著变化。
In this work, we examine initial phases of micro-discharges produced in deionised water by high-voltage (HV) pulses of nanosecond duration. We apply opto-electrical diagnostics with extremely high temporal (down to 30 ps) as well as spatial (down to 1 μm) resolution. Frozen interferometric and shadowgraph images show three distinct events. The first, the subcritical (no-discharge) event, is characterised by periodic perturbations of the index of refraction which depart from the anode surface and are pulled away at the speed of sound as an expanding envelope defined by the shape of the anode tip. One-dimensional hydrodynamic modelling of the 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, the dark or non-luminous discharge event, is characterised by the onset of a few isolated very tiny tree-like structures growing from the anode tip. Depending on the HV amplitude, the initial structures occur with a delay of ∼2–3 ns after onset of the HV pulse and subsequently expand with average velocity of ∼1 × 105–2 × 105 m s−1, creating very dense bush-like structures made of thin hair-like filaments in a few nanoseconds. The third, the luminous discharge event, 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 μm (typical diameter of non-luminous filaments) to tens of micrometres (characteristic diameters of luminous filaments). Furthermore, we address a possible role of microbubbles developing in the anode region due to the periodic HV pulses and verify that the UV-vis-NIR spectrometric signatures of the luminous phase notably change when replacing non-degassed deionised water with degassed.