Vortice formation induced by laser filamentation in gas
Experimental setup for the femtosecond laser-induced airflow formation. A femtosecond laser beam is generated by a chirped-pulse amplification laser system. A green laser beam is used as the probe beam and co-propagates with the femtosecond laser beam into the chamber.
Intense femtosecond lasers can achieve self-guided transmission over a long distance in the Earth's atmosphere, which provides a more environmentally friendly alternative to current technologies for local weather control and rainmaking/snowmaking. Researchers from the State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics (SIOM), Chinese Academy of Sciences (CAS) conducted a series of studies about water condensation induced by femtosecond laser filamentation. Recently, this team proved that vortices formed both below and above the filaments in different ambient gases. The details of the study are reported in Chinese Optics Letters, Vol. 14, No. 3, 2016 (Vortices formation induced by femtosecond laser filamentation in a cloud chamber filled with air and helium, corresponding author: Prof. Jiansheng Liu).
The laser-induced water condensation has been confirmed both in the atmosphere and in a cloud chamber. In this process, laser-induced airflow was identified to play a key role in water condensation and snow formation. However, the conditions and the formation mechanism for vortices have not been fully understood yet.
In order to understand the mechanisms of the formation of the laser-induced airflow, airflow generation in different ambient gases was observed. The experimental and theoretical results show that vortices always form both below and above the filaments. In the experiments, femtosecond filaments were generated when intense ultrashort laser pulses were fired into a cloud chamber filled with air or helium. Vortex motion induced by laser filaments in a cloud chamber filled with air was observed only below the filaments, while vortex motion in a cloud chamber filled with helium was observed both below and above the filaments. The airflow in the cloud chamber was simulated by numerically solving the continuity energy and momentum equations of gas while using laser filaments as heat sources. The simulation results show that vortices also form above the filaments in air. The airflow above the filament was not observed in the experiments with air because of the formation of smaller particles followed by a weak Mie scattering signal. The results are essential for understanding the process of the formation of laser-induced airflow and the mechanisms of water condensation.
The following work will be focused on the physical mechanisms of vortex formation using different experimental methods and theoretical simulation in order to lay foundations for achieving weather control by lasers.
激光丝在气体中诱导涡流的形成
图片说明：飞秒激光诱导气流形成实验装置。实验中，啁啾脉冲放大激光系统输出超短飞秒激光脉冲，绿光激光被用作探测光，飞秒光和探测光同轴射入云雾室。
飞秒强激光在大气中能够实现长距离（千米、甚至几十千米量级）自导传输，为局部天气控制并实现人工降雨、降雪提供了极具潜力且更加环保的新途径。中国科学院上海光学精密机械研究所强场激光物理国家重点实验室的研究团队在该方面做了一系列的工作并取得一定成果。近期，该团队通过实验观察了不同气体氛围中形成的涡流，证实了光丝周围都存在涡流运动。相关研究成果发表在Chinese Optics Letters 2016年第3期上（Vortices formation induced by femtosecond laser filamentation in a cloud chamber filled with air and helium, 通讯作者：刘建胜研究员）。
目前，激光诱导水凝结在云雾室和现实大气中皆被证实。激光诱导产生的涡流在水凝结过程中起着十分重要的作用。然而，人们对涡流形成的条件和机理还没有完全了解。
为了理解激光诱导形成的气流，科研人员首先将超短激光脉冲入射到空气或氦气中，形成一串等离子体通道，俗称激光丝。他们在空气中只观察到了光丝下方明显的涡流运动，而在氦气中光丝下方和上方都观察到了明显的涡流运动。理论模拟中，科研人员将光丝设定为一个热源，然后根据气体的连续性方程、能量以及动量方程，运用二维模型数值模拟了云雾室内气流的运动，从理论上证明在空气中光丝上方也存在涡流运动。空气氛围实验中观察不到光丝上方的气流运动是由光丝上方形成的凝结颗粒较小导致的。该研究结果对理解光丝诱导气流运动的形成过程及相关机理有重要意义。
后续工作主要是利用不同的实验方法及数值模拟对激光诱导水凝结的物理机制进行更为深入的研究，从而为实现激光控制天气奠定基础。