飞秒激光成丝在云室中诱导降雪物理机制的实验和理论研究

Femtosecond laser filamentation-induced water condensation is a new research topic in the application fields of femtosecond laser filamentation in the recent years. This would provide a new and potential way for the local weather control in the atmospheric environment based on the technique of femtosecond intense laser considering of its ability of being able to propagate in air for a long distance of several even tens of kilometers as a self-guided filament, and denotes important scientific significance and potential applications. This project would investigate the mechanisms of femtosecond laser filament-induced snowfall in a cloud chamber by combining experimental study with numerical simulation, focusing on the later process of condensation nuclei growing to fall which has been hardly investigated so far. The research contents include: studying on the process of air motion with different patterns induced by femtosecond laser filament in a cloud chamber and revealing their key influence factors; investigating the microscopic growing process of condensation nuclei and showing the role of air motion with the different patterns in this process; considering of the ionization effect induced by femtosecond laser filamentation, and developing the present numerical model to explain the mechanisms of snowfall. The goal is to increase the efficiency of femtosecond laser filamentation-induced water condensation, consequently accelerate the development in the related application fields.

飞秒激光成丝诱导水凝结是近年来在飞秒强激光成丝应用领域中出现的一个新的研究课题。鉴于飞秒强激光在大气中能够实现几公里甚至几十公里的长距离自导引传输，这方面的研究将为基于飞秒强激光技术的大气环境局部天气的控制提供极具潜力的新途径，具有重要的科学意义和应用前景。本项目围绕目前研究甚少的凝结核长大直到可以沉降的后期过程，将实验研究和数值模拟紧密结合，开展飞秒激光成丝在云室中诱导降雪的物理机制研究。具体内容包括：研究飞秒激光成丝诱导降雪过程中气流的运动过程，揭示形成不同形态气流的关键影响因素；探测凝结核长大的微观物理过程，确定不同形态气流运动在凝结核长大过程中所起的作用；进一步考虑飞秒激光成丝带来的电离效应，发展和完善现有数值模型，理论方面探究降雪形成的物理机制。目标是为进一步提高飞秒激光成丝诱导水凝结的效率提供参考，从而推动相关应用领域的发展。

飞秒激光成丝诱导水凝结是近年来在成丝领域出现的一个新的研究课题。鉴于飞秒强激光在大气中能够实现几公里甚至几十公里的自导引传输，这方面的研究将为大气环境局部天气的控制提供极具潜力的新途径，具有重要的科学意义和应用前景。本项目从开题到2018年底，主要开展了如下研究：1) 通过改变透镜焦距、飞秒激光的脉冲能量、宽度、啁啾等参数，实验研究了飞秒激光成丝在诱导水凝结过程中形成不同形态的气流、降雪量、硝酸浓度等的影响因素，结果发现激光丝的长度、强度和强度分布的均匀性是影响气流形态和降雪量的关键因素。2) 通过改变不同的气体氛围、采用具有时间分辨的探测手段、固体靶辅助、加热丝及吹风等实验手段进行人工降雪实验，详细研究了飞秒激光成丝诱导水凝结及降雪过程的微观物理机制。结果表明：吸湿性的硝酸并不是飞秒激光成丝诱导水凝结过程中必不可少的凝结核，激光丝产生的高浓度等离子体和光丝下方的涡流运动在水凝结和降雪过程中起着最为重要的作用。3）理论模拟了不同形态涡流运动的形成过程，结合实验结果，解释了不同形态涡流运动产生的机理。结果发现，强而长的激光丝可以释放出更多的热量，有助于形成丝上方的涡流运动。4）实验探究了提高飞秒激光诱导人工降雪效率的可行性方法。结果表明，低强度的皮秒激光可以先与空气中预存的小水滴或者冰晶相互作用，继而发生强烈的碰撞电离和爆炸效应，致使总的水凝结量增加。飞秒激光成丝辅助AgI烟剂可将雪水中的硝酸浓度增加10倍以上，从而提高了降雪效率。高压放电产生的离子风在水凝结的过程中产生了非常重要的作用，电晕放电可以将水凝结的效率提升4个数量级。这些结果为激光在人工影响天气领域中的实际应用提供了参考，推动了相关领域的发展。

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