【作者】 夏雄平；

【导师】 易林；

【作者基本信息】 华中科技大学 ， 光学， 2012， 博士

【摘要】 随着强激光技术的不断发展,激光等离子体相互作用由于在惯性约束核聚变(ICF)和电子加速等方面的重要应用而成为当前研究的一个热点。本论文研究了激光等离子体相互作用中产生的温密等离子体中非线性特性,主要包括自聚焦、光束分裂和激光驱动电子加速等非线性特性问题。主要研究内容和取得的成果如下：1.研究了激光和欠稠密碰撞性等离子体中电子温度对介电函数和激光传播的影响。得出了依赖于电子温度变化的介电函数和激光传播的波动方程。结果发现,当激光在欠稠密碰撞性等离子体中传播时,由于电子之间的碰撞非线性效应导致电子密度发生再分布,从而引起介电函数发生改变；电子温度的变化导致的电子之间的碰撞非线性效应呈现出一种弱—强—弱变化趋势,导致激光在传播过程产生三类不同的现象：稳定的分离,震荡的分离和稳定的自聚焦。2.研究了稠密等离子体与强激光的相互作用过程中的光束分裂现象。基于在稠密等离子体中描绘激光传播的波动方程,利用非傍轴理论第一次预言了一种新的非线性参量不稳定性：三分裂的光强分布。同时分析了产生分裂光强分布的可能原因,并调查发现更高的四分裂和其它的分裂现象不能产生。这一发现丰富了激光等离子体作用中的非线性研究。3.研究了单一型和混合型指数衰变非均匀性等离子体中的电磁波的相对论自聚焦。研究发现,由于明显的相对论影响和存在的等离子体的非均匀性,两者联合影响着非均匀等离子体中的介电函数和电磁波的传播。特别是,通过对比两种具有相反性质的指数衰变型非均匀等离子体与强电磁波的相互作用,发现非均匀性等离子体内在的数学性质(如单调性和极值等)对电磁波的传播性质存在直接的影响,这种发现对于在实验中发现新的类型的等离子体具有一定的指导意义。同时一个重要发现：尽管等离子体的非均匀性在ICF实验中会引入不稳定性,然而联合相对论效应和等离子体的非均匀性能产生超强的短脉冲电磁波,这种发现可用来设计产生强超短脉冲电磁波和新的粒子加速器。4.研究了脉冲激光与碰撞性等离子体相互作用中的非线性碰撞热。通过使用麦克斯方程,流体方程和电子欧姆热的传输方程,在考虑有质驱动和非线性欧姆热的情况下,得到了等离子体中不同激光强度下的电磁场、电子密度和电子温度的分布情况。结果发现随着激光强度的增加,电磁场的震荡波长会减少,而其幅度会增加,同时发现随着激光强度的增加,电子密度、电子温度和有效介电函数的震荡峰值会变得非常的尖锐和震荡波长会逐渐降低。5.研究了磁性等离子体中脉冲激光对电子的有质驱动加速问题。研究结果表面,在激光与等离子体相互作用过程中,等离子体的频率和电子的旋转频率对脉冲激光的群速度有明显的影响。在脉冲激光的传播过程中,产生的有质驱动力会驱动电子发生强烈的震荡,随着传播距离的增大,这种震荡会逐渐减弱,最终趋向稳定,使得电子获得高的能量增益。而在此过程中,由于脉冲激光电场的影响,会产生自生磁场,自生磁场会导致电子以一定的频率发生旋转,此时在脉冲激光频率和电子的旋转频率满足ω=ω。会产生共振,进一步增强有质驱动力,从而使电子的震荡得到共振增强,进一步提高电子能量的增益。更多还原

【Abstract】 With the development of intense laser technology, the nonlinear interaction of lasers with warm dense plasma has been a subject of active research for decades due to its relevance to inertial confinement fusion (ICF) and particle acceleration. Beam self-focusiong, splitted beam intensity and electron acceleration in laser-plasma interaction are investigated. The research contents and the obtained results are as follows:1. Dielectric constant and laser beam propagation in an underdense collisional plasma are investigated, using the wave and dielectric function equations, for their dependence on the electron temperature. Simulation results show that, due to collision nonlinearity results in the distribution of electron density, and lead to modification of dielectric constant. The collision nonlinearity presents a weak-strong-weak variation trend by the variation of electron temperature, and lead to three kind various beam propagation phenomenon: steady defocusing, oscillatory defocusing and self-focusing.2. The phenomenon of splitted beam is researched in intense laser and overdense plasma interaction. Based on the modified nonlinear wave equation describing the interaction of intense laser and overdense plasmas in nonparaxial region, and first find a new parameter instability:three-splitted beam intensity profile. In the work, we analyze the reason of splitted intensity profile, and show that four-splitted and other intensity profile cannot be find in the system. The new parameter instability would enrich the nonlinear research of laser-plasma interaction.3. Based on Wentzel-Krammers -Brillouin (WKB) approximation and higher order paraxial ray theory, we investigate electromagnetic beam self-focusing in interaction between Gaussian electromagnetic beam and two axially exponential decay underdense inhomogeneous (include single and mixed type) plasma with completely opposite characters, respectively. The simulation results show that, the combination influence of relativistic nonlinearity and plasma inhomogeneity determine the variation of dielectric constant and propagation of electromagnetic beam in plasma. Especially, an interesting and important finding which the intrinsic mathematic (such as:monotonicity and extremum) qualities of plasma inhomogeneity may has many internal relations with electromagnetic beam propagation in plasma. On one hand, increasing plasma inhomogeneity and beam relativistic self-focusing and filamentation increasing, as well as bring to filamenation instability in ICF. On the other hand, if we can combine two influences of relativistic nonlinearity and plasma inhomogeneity, it is able to generate particularly intense and short pulses. The founding may be designed particularly intense and short pulses and new particle acceleration.4. The nonlinear interaction of a laser pulse with a homogenous unmagnetized underdense plasma, taking ohmic heating and the effects of ponderomotive force into account, is theoretically studied. Snce the ponderomotive force modifies the electrons density and temperature distribution, the nonlinear dielectric permittivity of plasma is obtained in non-relativistic regime. Furthermore, electric and magnetic fields, electron density, temperature distribution, and the effective permittivity variations are obtained in terms of plasma length by making use the steady state solutions of the Maxwell and hydrodynamic equations. It is shown that the oscillations wave length of electric and magnetic fields decreases when the laser intensity increases. At the same time, in this case, electron density oscillations become highly peaked. Also, the amplitude of the electron temperature oscillations increase and their wavelength decreases.5. In the paper, we examine electron acceleration by laser pulse in magnetized plasma. The laser pulse group velocity is less than the speed of light and hence electrons can resonantly interact with the pulse. The basic mechanism involves acceleration of electrons by the axial gradient in the ponderomotive potential of the laser. Research results show that, in the laser-plasma interaction, electron plasma frequency and cyclotron frequency can obvious effect the group of laser pulse. The electron acceleration depends on the ratio of laser frequency to electron cyclotron frequency, amplitude of the laser pulse and plasma density. Due to the influence of ponderomotive nonlinearity, lead to electron transverse oscillation, and the formation of intense plasma wave, as last results in electron acceleration and high energy gain. While in magnetic plasma, due to the influence of electron field of laser beam, result form the self-generated magnetic field, which it cause electron cyclotron at a frequency. When the frequency of laser beam and electron cyclotron frequency meetω=ωc and form resonance, self-generated magnetic field would strengthen the ponderomotive nonlinearity and increase electron transverse oscillation, finally further increase electron energy gain.更多还原