飞秒激光等离子体的光学诊断

使用飞秒激光探针法,对飞秒激光等离子体膨胀过程进行了光学诊断.实验得到了不同时刻的等离子体阴影图和干涉图.大尺度环形自生磁场对等离子体产生了挤压,磁场的横向约束导致等离子体的宏观非对称性和jet结构.等离子体在纵向的膨胀尺度呈现t^1/2变化规律.     

激光产生等离子体中不均匀性结构的研究

用可见光探针阴影成像方法研究了在激光产生等离子体中出现的不均匀性结构,特别是由于流体力学不稳定性而在薄膜靶后表面出现的小尺度喷流结构。     

计算激光等离子体参数的数学模型

在相关物理知识的基础上 ,给出了计算激光等离子体参数的数学模型 ,通过实验验证 ,用该模型所得结果与使用其他方法所得结果基本吻合 .     

强场中电子的经典行为

研究了圆偏振强激光在等离子体中产生的自生磁场及注入电子在由激光场和自生磁场构成的混合场中的经典行为.圆偏振强激光在等离子体中传播时,因逆Faraday效应,方向平行于传播方向的准静态磁场被激发.推出相对论情况下的自生磁场方程,并以此研究了自生磁场的特性.当电子或电子束注入激光脉冲,则电子将受到激光场和自生磁场的作用,满足共振条件的电子能得到加速.得出电子能量方程并计算了能量随时间的变化,就一种特殊情况计算了电子在混合场中的动力学行     

激光等离子体时、空、能三维分辨测试系统

介绍了激光等离子体时间、空间、能谱三维分辨测试系统的结构、原理和性能 ,并给出了利用该系统在实验中获得的典型结果 .     

槽状靶和平面靶产生的激光等离子体的实验研究 *

对两种几何形状靶产生激光等离子体的软X光和谐波发射的比较研究发现,相对常用的平面靶,槽状靶中的高阶离化态离子产生的X射线发射增强.槽状靶对激光更有效吸收是造成这一现象的主要原因.     

强激光诱导冲击波参数的计算研究

首先简要介绍了强激光诱导冲击波研究的重要性,然后讨论了强激光诱导冲击波的形成机理,在此基础上得到了利用材料的特性参数、激光参数和环境参数计算该冲击波参数的数学模型,并实际计算了激光参数为I_0=0～10~9w/cm~2,τ=15ns,λ=1.06μm,材料为铝时的结果,通过实验验证,其计算结果与其它方法所得结果基本吻合.     

粒子流特性对脉冲激光烧蚀淀积类金刚石薄膜的影响

采用两种不同脉宽 ( 30ns,5 0 0fs)的KrF准分子激光淀积类金刚石薄膜 ,通过等离子体发射光谱和离子探针的测量 ,研究了激光等离子体成分、平均离子能量和通量等随激光能量密度、淀积距离的变化 ,对两种脉宽的激光等离子体淀积的薄膜结构和性能进行了比较 ,并且研究了激光等离子体特性对淀积薄膜类金刚石性能的影响     

超强激光等离子体相互作用中产生的三次谐波

在不作稀薄等离子体和弱相对论近似的情况下,详细研究了超强激光(10~(18)W/cm~2)和等离子体相互作用过程中静电势及三次谐波发射对等离子体参数和激光参数的依赖关系,从而给出了三次相干谐波产生所需的最佳激光参数和等离子体参数.     

线聚焦激光产生等离子体中的自聚焦成丝及喷流结构

在线状聚焦激光辐照平面靶及柱形细丝靶的实验中,我们首次观察到线聚焦激光在等离子体中分裂成丝与等离子体的小尺度喷流结构,这些现象有可能构成当今以激光产生的等离子体为放大介质的X射线激光实验研究中的一类应设法克服的障碍.     

Ion soliton observation with laser induced fluorescence

Many theoretical and experimental studies of solitons in plasma have been performed [Phys. Fluids 16 (1973) 1668; Plasma Phys. 25 (1983) 943; IEEE Trans. Plasma Phys. PS 10 (1982) 180; Plasma Phys. 5 (1998) 4144] and most of the properties such as the relation between the amplitude, the velocity and the width, for soliton or soliton-dust interaction, have been obtained. The agreement between experiment and theoretical model is not always good [Phil. Mag. Ser. 39 (1895) 422; Phys. Rev. Lett. 17 (1966) 996; Phys. Rev. E 51 (1995) 4796]. The experimental observations typically involve Langmuir probes. However, the ion acoustic soliton propagation can be observed by laser induced fluorescence (LIF) in double plasma device. This direct observation of ion perturbation with LIF points out the importance of the optical pumping effect [Rev. Sci. Instrum. 72 (2001) 4372] in the measurement of fast velocity propagation of ion phenomena like solitons are. With the LIF we discovered that a train of soliton propagates easier in the device if a weak backward ion flux plasma, having a drift velocity in the range of 200 m/s is present; as faster the ion flux is, as close to the grid the solitons separation occurs; the precursors ions is in fact a collective phenomenon.     

Wave Front Tracking in Systems of Conservation Laws

This paper contains several recent results about nonlinear systems of hyperbolic conservation laws obtained through the technique of Wave Front Tracking.     

Strichartz Estimates for Wave Equations with Coefficients of Sobolev Regularity

Wave packet techniques provide an effective method for proving Strichartz estimates on solutions to wave equations whose coefficients are not smooth. We use such methods to show that the existing results for C ^{1, 1} and C ^{1, α} coefficients can be improved when the coefficients of the wave operator lie in a Sobolev space of sufficiently high order.     

一类双曲型一维波动方程反问题解的整体唯一性

本文给出了一类双曲型一维波动方程反问题的解的整体唯一性定理。     

A Phase Space Transform Adapted to the Wave Equation

Wave packets emerged in recent years as a very useful tool in the study of nonlinear wave equations. In this article we introduce a phase space transform adapted to the geometry of wave packets, and use it to characterize and study the associated classes of pseudodifferential and Fourier integral operators.     

STABILITY FOR IMPOSING ABSORBING BOUNDARY CONDITIONS IN THE FINITE ELEMENT SIMULATION OF ACOUSTIC WAVE PROPAGATION＊

It is well-known that artificial boundary conditions are crucial for the efficient and accurate computations of wavefields on unbounded domains. In this paper, we investigate stability analysis for the wave equation coupled with the first and the second order absorbing boundary conditions. The computational scheme is also developed. The approach allows the absorbing boundary conditions to be naturally imposed, which makes it easier for us to construct high order schemes for the absorbing boundary conditions. A thirdorder Lagrange finite element method with mass lumping is applied to obtain the spatial discretization of the wave equation. The resulting scheme is stable and is very efficient since no matrix inversion is needed at each time step. Moreover, we have shown both abstract and explicit conditional stability results for the fully-discrete schemes. The results are helpful for designing computational parameters in computations. Numerical computations are illustrated to show the efficiency and accuracy of our method. In particular, essentially no boundary reflection is seen at the artificial boundaries.     

Mathematical analysis of the waves propagation through a rectangular material structure in space-time

We consider propagation of waves through a spatio-temporal doubly periodic material structure with rectangular microgeometry in one spatial dimension and time. Both spatial and temporal periods in this dynamic material are assumed to be the same order of magnitude. Mathematically the problem is governed by a standard wave equation t(ρut)−z(kuz)=0 with variable coefficients. We consider a checkerboard microgeometry where variables cannot be separated. The rectangles in a space-time checkerboard are assumed filled with materials differing in the values of phase velocities but having equal wave impedance . The difference between dynamic materials and classical static composites is that in the former case the design variables will also be time dependent. Within certain parameter ranges, the formation of distinct and stable limiting characteristic paths, i.e., limit cycles, was observed in [K.A. Lurie, S.L. Weekes, Wave propagation and energy exchange in a spatio-temporal material composite with rectangular microstructure, J. Math. Anal. Appl. 314 (2006) 286-310]; such paths attract neighboring characteristics after a few time periods. The average speed of propagation along the limit cycles remains the same throughout certain ranges of structural parameters, and this was called in [K.A. Lurie, S.L. Weekes, Wave propagation and energy exchange in a spatio-temporal material composite with rectangular microstructure, J. Math. Anal. Appl. 314 (2006) 286-310] a plateau effect. Based on numerical evidence, it was conjectured in [K.A. Lurie, S.L. Weekes, Wave propagation and energy exchange in a spatio-temporal material composite with rectangular microstructure, J. Math. Anal. Appl. 314 (2006) 286-310] that a checkerboard structure is on a plateau if and only if it yields stable limit cycles and that there may be energy concentrations over certain time intervals depending on material parameters. In the present work we give a more detailed analytic characterization of these phenomena and provide a set of sufficient conditions for the energy concentration that was predicted numerically in [K.A. Lurie, S.L. Weekes, Wave propagation and energy exchange in a spatio-temporal material composite with rectangular microstructure, J. Math. Anal. Appl. 314 (2006) 286-310].     

WAVE COMPUTATION ON THE HYPERBOLIC DOUBLE DOUGHNUT

We compute the waves propagating on the compact surface of constant negative curvature and genus 2 that is a toy model in quantum chaos theory and cosmic topology. We adopt a variational approach using finite elements. We have to implement the action of the fuchsian group by suitable boundary conditions of periodic type. Despite the ergodicity of the dynamics that is quantum weak mixing, the computation is very accurate. A spectral analysis of the transient waves allows to compute the spectrum and the eigenfunctions of the Laplace-Beltrami operator. We test the exponential decay due to a localized dumping satisfying the assumption of geometric control.     

Wave propagation analysis in buried pipe conveying fluid

A study of wave propagation in buried pipe conveying fluid is presented in the paper. The Flüggle shell model is adopted for pipe and surrounding solid is modeled as elastic matrix by using Winkle model. Wave dispersion curves of a buried vacant pipe and a pipe conveying fluid are obtained numerically by considering coupling conditions. Results show that wave velocity exhibits sharp drop points in dispersion curves, and remains to an identical values before and after the points for both of vacant pipe and pipe conveying fluid. Effects of wall thickness, elastic matrix properties and fluid velocity are also discussed.