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chi20181128上海光机所李儒新557454The non-linear propagation and filamentation of the intense femtosecond laser have attracted the attention of researchers since they were observed. Now, the femtosecond filamentation is known to be a dynamic balance between the nonlinear Kerr self-focusing effect and self-defocusing effect of plasmas generated through the multiphoton/tunneling ionization: a continuous, low-density plasma channel (1 ? 1016 cm-3 in air) with a lifetime of microsecond scale will be produced during the filamentation. The plasma channel has an excellent conductivity, and its position can be controlled remotely and precisely, which makes the filamentation being important in many potential applications, such as the atmospheric remote sensing, the terahertz generation and the artificial lightning and so on. At the same time, during the relaxation of the plasma, photo-oxidation reactions occur efficiently through collisions between ions and neutral molecules/atoms, which produce hygroscopic NH4NO3/HNO3 chemicals in the air. Simultaneously, laser energy is transferred into medium through thermal release which leads the thermal expansion of the filament region. The vigorous air flow motion will be induced by this thermal effect subsequently. When there is a temperature gradient in the air, the air with different temperature will be mixed and a supersaturated state can be formed inside. Condensation and even precipitation can occur efficiently in the supersaturated air with the help of hygroscopic chemicals and thermodynamic airflow motion. In recent years, femtosecond laser filamentation is considered as an alternative and creative technique for local weather modulation. This thesis is devoted to the study on the feasibility of intense femtosecond-laser-induced cloud formation and precipitation. A series of related experimental and theoretical studies of femtosecond laser-induced warm clouds, cold clouds and super-cooled clouds formation and precipitation were conducted. The main contents of this thesis include: 1. The background knowledge of femtosecond laser filamentation was reviewed, including the physical mechanisms of femtosecond laser filamentation in air and a series of basic nonlinear properties. Practical applications of femtosecond laser filamentation were also reviewed. At the same time, overviews of the cloud-micro-physical background knowledge and the technical development status of the weather modification were given. 2. By shooting a 7.5mJ/27fs @800nm femtosecond laser into a warm cloud at room temperature (T = 21 °C), we found that compared with laser parameters, the environmental relative humidity was the most critical factor which restricting the generation of laser-induced large size aerosols. And not only in the filament region but also beyond we observed a significant increase in the warm cloud optical thickness. The particle size distribution of the enhanced warm cloud depended tightly on the relative humidity and initial droplet density of the warm cloud. The hygroscopic chemical by-products of the filamentation (NH4NO3/HNO3) were proved to be the main cause for the warm cloud enhancement by the simulation results. While the enhancement observed beyond the filament zone was related to the tiny droplets condensation, which was generated when the pre-existing warm cloud droplets absorbed the near-infrared part of the laser energy and broken into fragments. 3. By using a 22 TW laser, experimental study on the microphysics of laser-induced water condensation and precipitation in cold clouds (-5 oC) was conducted. In sub-millisecond timescale after each laser shot being arrived, the shock wave would compress the surrounding wet air and caused the formation of supersaturation state in the air, and it would also shatter the pre-existing cloud droplets into fragments. This would induce the rapid formation of large size droplets (300–400 μm). In 20 ms after each laser shot being arrived, ice crystals with a diameter of 400-500 μm were observed in a cold area (T < ?30 °C) near the cold plate of the cloud chamber. The calculation results showed that these ice crystals were formed by the direct phase change from liquid and they would follow the downdraft air flow motion below the filament and precipitate to the bottom of the cloud chamber eventually. 4. In order to study the interaction of femtosecond laser and ice-rich cold cirrus clouds, a cloud chamber was designed and set up to simulate the temperature and relative humidity conditions of the cirrus clouds. The temperature inside the cloud chamber can be precisely controlled within the range of ?60 °C to ?25 °C. With the help of a humidifier, it can accurately simulate the temperature and relative humidity conditions of the cirrus clouds and other cold clouds, which is convenient to carry out a series of studies on the interaction of femtosecond laser and super-cold clouds.2019atalunwen21912111212466Femtosecond laser;Filamentation;Water condensation;PrecipitationStudy on the feasibility of intense femtosecond-laser-induced cloud formation and precipitation飞秒强激光诱导成云致雨的可行性研究飞秒强激光在空气中非线性传输以及成丝现象,自从被发现以来就备受广大科研工作者的关注。随着相关研究的不断深入,飞秒激光成丝被认为是光克尔自聚焦效应和等离子体自散焦效应之间形成的动态平衡过程。这一平衡过程,形成了连续的低密度等离子体通道(空气中1 ? 1016 cm-3),寿命持续微秒量级。产生的等离子体通道,因具有良好的导电性、位置远程精确可控、定位准确灵活等特点,在大气遥感探测,太赫兹产生,人工引雷等领域有重要的潜在应用。同时,在等离子体的复合过程中,电子和中性原子/分子相互碰撞,会有效地激发一系列的光氧化反应。通过这些光氧化反应,飞秒光丝在空气中传播时,会产生吸湿性化学物质,如HNO3和NH4NO3等。同时,等离子体产生时吸收的激光能量会通过热释放转移到介质中,在光丝区域产生热效应,诱导周围气流发生剧烈热力学运动,在环境存在温差时,促进不同温度气体混合,形成过饱和态,辅助吸湿性化学物质发生水汽凝结,甚至沉降过程。近几年来,飞秒激光技术被认为是可以应用于人工影响天气领域的创新技术之一。 本论文针对飞秒强激光诱导成云致雨的可行性开展相关研究,研究内容围绕飞秒激光与暖云、冷云及过冷云相互作用过程中,诱导水汽凝结及沉降的可行性,展开一系列相关的实验及理论研究,主要内容包括: 1.综述飞秒激光成丝背景知识,包括飞秒激光在空气中成丝的丰富的物理机理以及衍生的一系列基本的非线性特性,及飞秒激光成丝的应用等内容。同时,综述传统人工影响天气作业技术相关的云雾微物理背景知识,及技术发展现状。 2.开展飞秒激光与室温条件下(21 °C)的暖云相互作用的研究。研究了暖云条件下,影响飞秒激光诱导的0.3-2.0 μm气溶胶生成的主要参数情况,实验发现,相比于激光参数,环境相对湿度是制约飞秒激光诱导较大尺寸气溶胶生成的最关键因素。此外,发现在飞秒激光照射下,光丝位置以及光丝前后均有暖云光学厚度显著的增强。新生暖云的粒径分布紧密依赖于环境相对湿度以及暖云的初始云滴浓度。模拟结果证明,光丝产生吸湿性化学副产物是诱导暖云增益的主要原因,但光丝区域之外的暖云增益,可能仅与飞秒激光近红外部分产生的热效应,击碎预先存在的暖云液滴,诱导水凝结,形成细小液滴相关。 3.开展百太瓦飞秒激光(22 TW)与冷云(-5 oC)相互作用诱导水凝结/沉降的微物理过程研究。发现在激光脉冲到达后的亚毫秒时间尺度内,冲击波通过挤压周围湿冷气体形成过饱和态及击碎预先存在云滴等,可以促进大尺寸凝结液滴(300–400 μm)快速形成。在激光脉冲到达20 ms后,在靠近云室底板的过冷区域发现(T < -30°C)直径为400-500 μm的冰相颗粒,计算分析证明该部分冰粒是由液滴直接相变而来,并随后会随着光丝下方的下沉气流沉降到云室底板上。 4.为了研究飞秒激光与富含冰晶的冷云-卷云的相互作用,我们设计并搭建了一个模拟卷云环境的实验云室。该云室内的温度可以精确控制在?60 °C至?25 °C的范围内。配合加湿器与温湿度传感器,可以精确模拟卷云以及其他冷云的温湿度条件,方便开展飞秒激光与过冷云相互作用的系列研究。飞秒激光;成丝;水凝结;沉降中国科学院上海光学精密机械研究所胡兴凯光学工程硕士
中文题目: 飞秒强激光诱导成云致雨的可行性研究
外文题目: Study on the feasibility of intense femtosecond-laser-induced cloud formation and precipitation
作者: 胡兴凯
导师姓名: 李儒新
学位授予机构: 中国科学院上海光学精密机械研究所
答辩时间: 20181128
中文关键词:
飞秒激光;成丝;水凝结;沉降
英文关键词:
Femtosecond laser;Filamentation;Water condensation;Precipitation
中文摘要:
英文摘要:
文献类型:学位论文
学位级别: 硕士
正文语种: chi
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