【作者】 唐建；

【导师】 程祖海； 左都罗；

【作者基本信息】 华中科技大学 ， 电子科学与技术， 2013， 博士

【摘要】 将高功率激光进行聚焦后,会引起被照射物质击穿产生高温高压等离子体,并伴随有强烈的冲击波效应和闪光。于是,研究者们开始将激光诱导等离子体的这些特性应用到航空航天、核能开发、工业监测、环境污染和生物工程等领域。同时,随着对激光诱导等离子体相关机理和特性研究的不断推进,激光诱导等离子体的相关应用也必将得到更为深入的发展。本论文利用脉冲CO2激光器在空气中聚焦诱导产生等离子体,从理论上和实验上对激光诱导等离子体的击穿、膨胀、辐射过程进行了比较系统的研究,并分析了激光与等离子体相互作用时的能量沉积过程。本论文首先对脉冲CO2激光诱导空气等离子体的击穿和演化过程进行了理论分析和实验观测。由于C02激光的单光子能量非常小,多光子吸收电离过程对等离子体中的电子增长几乎没有贡献,自然环境中存在的自由电子碰撞雪崩电离是空气击穿的主要机制。在实验上,利用ICCD相机窄门宽序列采样的方法,探测得到了轴向和径向空气等离子体在击穿过后的膨胀过程。同时,结合激光支持的球面爆轰波模型,研究了在不同焦距和入射激光能量下空气等离子体的膨胀过程。实验结果表明,在较长的焦距和较高的入射激光能量下,等离子体具有更大的体积和更快的膨胀速度；激光支持爆轰波的结束时间与等离子体吸收面的激光功率密度密切相关,激光功率密度越高,等离子体爆轰波结束的时间就越晚。另外,结合测量得到的等离子体波前面积,计算得到了在等离子体波前位置获得的推力。结果显示,采用长焦距击穿产生的等离子体在波前位置获得的推力要比短焦距的要大。随后,对激光诱导击穿空气等离子体的发射光谱进行了细致的实验研究和理论分析。在理论上,基于局域热力学平衡态下的等离子体发射光谱理论,并结合空气等离子体Saha电离平衡方程,计算了空气等离子体中的平衡组分和连续光谱。结果表明,随着温度升高,高电离态粒子的浓度越来越高,空气等离子体的电离度也随之升高；并且单原子结构的粒子占据了等离子体中的绝对比例,而分子结构的粒子几乎可以忽略不计；同时,随着电子温度和电子密度升高,等离子体的连续光谱会逐渐增强,但是总的看来,轫致辐射占据了空气等离子体中连续光谱的主导位置。另外,基于局域热力学平衡态下的等离子体发射光谱理论,并在光学薄的假设下,建立了空气等离子体的线状光谱模型和连续光谱模型。利用该模型对实验测得的空气等离子体发射光谱进行了拟合,并计算得到了电子温度和电子密度。在实验上,为了获得等离子体在激光脉冲作用时间内的演化过程,搭建了一套具有时间和空间分辨的激光诱导空气等离子体发射光谱诊断实验平台,实现了轴向和径向的时空分辨等离子体发射光谱诊断。利用时间分辨发射光谱诊断装置,测量得到了不同激光脉冲能量条件下激光诱导空气等离子体的发射光谱特性。结果表明,在高能量激光诱导产生的空气等离子体中,离子线状光谱能够维持的时间远比低能量激光诱导产生的等离子体短,并且高能量激光产生的等离子体会表现出更为强烈的连续光谱强度。轴向和径向的时空分辨空气等离子体发射光谱的诊断由成像光谱仪的狭缝耦合方式实现。实验中发现,空气等离子体在轴向上的光强逆着激光方向逐渐增大,而径向等离子体的光谱分布则出现了一个向外围“分离”的现象。通过利用本论文建立的线状光谱模型计算了电子温度和电子密度随着时间和空间的变化规律。结果表明,在抛物聚焦面中产生的空气等离子体会形成一层对CO2激光不透明的反射面,使激光能量不能穿透这层临界面而到达等离子体中心,因此会使激光发生反射而对这层临界面外围的等离子体进行二次加热,而最终形成等离子体辐射光强在径向上分离。更多还原

【Abstract】 When a high power laser is focused on any material, an optical breakdown of the material will be occurred, and produces the high temperature and high pressure plasma, accompanying with a strong shock wave and spark. This laser induced plasma also attracts researchers’attention, and is applied widely in the aerospace, nuclear energy, industrial monitoring, environment pollution, biological engineering and other fields. With a deeper knowledge of the related mechanism and characteristics of laser induced plasma, these applications of laser induced plasma will get more and more in-depth development.In this dissertation, the laser induced air plasma was generated by the pulsed CO2laser focused in air. The theories and the experiments of laser induced air plasma breakdown, expansion, spectral radiation process had been investigated, and the energy deposition interaction between the pulsed laser and plasma was analyzed at the same time.Firstly, we investigated the optical breakdown and expansion of air plasma generated by pulsed CO2laser in both theory and experiment. Because of the small single photon energy of the CO2laser, the initial free electron produced from the multiphoton absorption ionization could be ignored, while the initial free electron was all produced by the avalanche ionization in laser field. The axial and radial laser induced air plasma expansions were detected by the sequential acquiring method of ICCD camera. Combined with the spherical laser supported detonation wave model, air plasma expansion processes with different focal length and different incident laser energy were investigated, the experimental results showed that air plasma had bigger volume and higher expansion rate when using the longer focal length and higher incident laser energy, and the termination of laser supported detonation wave was closely related to the laser power density in focusing area, the higher laser power density, the later termination of laser supported detonation wave. In addition, based on the measurement of the area of the plasma wave front, we calculated the thrust at the position in front of the plasma wave, which showed that the thrust generated by long focal length of plasma was bigger than the short focal length.Subsequently, we studied the laser induced air plasma by the spectroscopic diagnosis in both theory and experiment. Based on the assumption that the plasma in the local thermodynamic equilibrium state, and combining the Saha ionization of air plasma equilibrium equation, we calculated the equilibrium composition and continuous spectrum of air plasma radiation. With the increase of temperature, ionization state of air particle became more and more high, corresponding to the degree of ionization of air plasma also became more and more high, and the single atomic structure of the particle occupied absolute proportion of plasma, while the molecular structure of the particle were almost negligible. In addition, with the increase of electron temperature and electron density, the intensity of continuous spectrum would strengthen gradually; however, bremsstrahlung dominated the continuous spectrum in the air plasma.Furthermore, based on the emission spectra theory of the local thermodynamic equilibrium plasma, assuming the air plasma in optical thin, we established line spectra model and continuous spectra model of air plasma, and carried on the fitting of the emission spectra of air plasma to obtain the electron temperature and electron density in plasma.On the experiment, in order to obtain the evolution of the laser induced plasma in the stage of laser pulse duration, we built a time and space resolution of laser induced air plasma emission spectra diagnosis experiment platform, and realized the axial and radial directions respectively by the plasma emission diagnosis. By using time-resolved emission diagnosis device, different incident energy of laser induced air plasma emissions were measured, the results showed that the ion line spectrum radiation generated by higher energy laser could maintain longer time, and plasma produced by higher energy laser showed more intense continuum radiation.In addition, the emission spectra of the axial and radial air plasma were diagnosed by using the imaging spectrometer slit coupling method. In this space-resolved emission spectra diagnosis experiment, we found that the intensity of air plasma increased gradually against the direction of incident laser, and the spectral distribution of the radial plasma showed a "separation" phenomenon to the periphery. Then, the spatio-temporal evolution of the electron temperature and electron density in air plasma were calculated by the line spectra model we established, the results showed that air plasma would generate a layer which was opaque for CO2laser, and the laser energy could not penetrate this critical layer to arrive the plasma center, therefore, the reflected laser would heat the plasma outside this critical layer again, leading to the separation of the plasma radiation intensity on the radial direction.更多还原