【作者】 张宏超；

【导师】 陆建；

【作者基本信息】 南京理工大学 ， 仪器科学与技术， 2009， 博士

【摘要】 本文主要从实验和理论上研究了纳秒激光诱导空气击穿机制,激光支持爆轰波(LSDW)形成机制与传播特性,激光等离子体的电子密度和冲击波波后气体分子的时空演化规律,以及空气等离子体的碰撞。首先,建立了具有数据自动采集功能的马赫-曾德尔数码化干涉测量系统,获得了高质量的等离子体干涉条纹图：提出了针对激光等离子体干涉图的处理方法,得到了高分辨的激光诱导空气等离子体折射率三维空间分布图。提出了空气击穿时间的测量方法,并得到了空气击穿时间和作用激光功率密度间的定量关系。实验发现,激光的透射率与其入射能量近似成反比。当入射激光能量较小时,激光的击穿时间与击穿功率密度之积为定值；随着入射能量的增加,并超过某阈值时,击穿时间将达到饱和。进而利用雪崩电离的机制对实验现象给予了解释,并提出了等效脉冲的概念。提出了一种基于阴影法和点扫描法的LSDW传播速度测量方法。通过观察等离子体二维屏蔽特征和对等离子体二维屏蔽分布图的处理,获得了不同作用激光能量导致的LSDW传播速度随时间的变化关系。提出了利用激光诱导电子气冲击波的Taylor模型来解释LSDW的形成机制,理论与实验结果吻合得较好。通过实验研究了纳秒激光诱导空气等离子体的时空演化过程,观察到等离子体通道的形成,发现激光等离子体动力学膨胀是形成等离子体通道的主要机制,且等离子体膨胀速度迅速衰减是等离子体通道塌陷的主要原因。通过对激光诱导冲击波波后气体分子密度三维分布随时间变化过程的研究,发现了冲击波后的分子密度先增长后衰减的现象,以及分子密度分布从非均匀向均匀分布的演化过程,并认为等离子体膨胀初期内部的各向异性是其主要因素。实验研究了激光诱导空气等离子体间的碰撞特性,观察到了碰撞区域冲击波后分子密度和等离子体电子密度的增强现象,提出冲击波碰撞形成的分子迟滞层是导致冲击波后分子密度的增强的主要原因,而电子密度的增强是由于自由电子受到了被迟滞层阻碍的正离子吸引并聚集的结果。本文的研究结果对激光诱导空气击穿的理论研究、激光的应用及相关检测技术的发展具有一定的促进作用。更多还原

【Abstract】 In this dissertation, the laser induced gas breakdown, laser supported detonation wave (LSDW) and propagation mechanisms were systemically investigated in experiment and theory. The evolution of the electron density and the gas density behind the shock wave, as well as the gas plasma collision were investigated.First of all, an interferometry system based on Mach-Zehnder interferometer for laser plasma diagnoses was established. It has ability to automatically acquire the experimental data. The hingh quality interferogram was aquired. The method for the processing of the laser interferogram was proposed and high resolution imagings of the 3D distribution of the plasma refractive index were achieved.A method for determining the breakdown time of the air was proposed, and further to determine the quantitative relation of the breakdown time and the breakdown intensity. It was found that the transmissivity is inversely proportional to the incident pulse energy approximately. The arithmetic product of the breakdown time and the breakdown intensity is fixed with small pulse energy. However, with increasing the pulse energy, when a threshold was achieved, the breakdown time became saturation. This phenomenon was explained with the avalanche ionization mechanism and the concept of the equivalent pulse was proposed.Based on the shadowgraph and point scan technology, a method of measurement of the LSDW velocity was proposed. The characteristics of the plasma two-dimensional shielding were investigated. And then the LSDW velocity as a function of the pulse energy and time was obtained by the plasma two-dimensional shielding image processing. The mechanism of the formation of the LSDW was explained with the Taylor model, and the theory agrees with the experiments results well.The evolution of the plasma electron density 3D distribution was investigated in experiment. The results show that the plasma channel has been formed in the early stage of laser induced air plasma, and the plasma expanding velocity rapid attenuation accelerates the plasma channel collapses. The evolution of the gas density 3D distribution behind shock wave front was also investigated. The result shows that, at the initial stage, the gas density was increasing at first and then decreasing, and the evolutionary process of the gas density from non-uniform density distribution to the uniform distribution duo to the anisotropic expansion of the plasmaLaser induced gas plasma collision was investigated in experiment. From the results, the enhancement gas density behind the shock wave front and the electron density were observed at the collision zone. We think that the enhancement of the gas density behind the shock wave was induced by the formation of the gas stagnation layer, while the enhancement of the electron density was induced by the attraction of the positive ions which were stagnated by the gas molecules.The results of this dissertation will facilitate the theoretical study of laser induced gas breakdown, laser applications and the related development of detection technology.更多还原