Laboratory simulations of astrophysical blast waves with high energy lasers

We review recent efforts to simulate aspects of supernova remnants in laboratory experiments by creating energetic explosions with high energy lasers. High energy pulsed lasers are uniquely suited for these kinds of studies. By focusing a laser with pulse energy of a few joules to many hundreds of joules onto a solid target or into a dense gas target, explosive shock waves of very high Mach number can be created. With a well chosen set of laser and target parameters it has been shown by a number of groups that radiative blast waves can be produced. Such blast waves have dynamics dominated by radiation transport and exhibit unusual characteristics, the most important of which include hydrodynamic instabilities which may play an important role in the structure of the interstellar medium. As a result there are now prospects for gaining new insights into astronomical observations of supernova remnants by studying laboratory laser driven plasma systems.     

纳秒激光在铜靶材中诱导冲击波的实验研究

基于聚偏二氟乙烯压电传感器, 对铜靶材中纳秒激光脉冲诱导的冲击波传播过程进行了实验研究, 给出了铜靶材内冲击压强随激光脉冲能量和靶材厚度的变化规律. 实验结果表明: 500 mJ激光脉冲能量作用到2 mm厚的铜靶材产生的冲击压强达到2.1 MPa; 激光脉冲能量从200 mJ 增加到500 mJ, 在铜靶材厚度为2和4 mm条件下, 冲击压强分别增加了162%和231%; 而当铜靶材厚度从2 mm增加到6 mm时, 在400和500 mJ激光脉冲能量作用下, 铜靶材内冲击压强分别降低了32%和49%.     

Spatial and temporal study of shock waves generated by laser ablation for Ti target

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Self-similar analytical solutions for blast waves in inhomogeneous atmospheres with frozen-in-magnetic field

In the present paper, we have made an attempt to study the effects of the presence of a magnetic field on the cavity formation inside a blast wave propagating into a perfectly conducting gas with density varying as some power of distance from the plane or line of explosion. In order to obtain the closed form solutions for the flow variables inside the blast wave and to solve the problem of cavity formation analytically, a relation is taken between the ordinary pressure and the total pressure. It i s found that if the value of the inhomogeneity index is greater than a critical value (a function of M_A - Alfven Mach Number, - adiabatic index and i - wave geometry index), a contact discontinuity appears at some point inside the blast wave and the cavity formation occurs. The effect of the presence of magnetic field is found to increase the tendency of cavity formation.Received: 21 February 2003, Published online: 4 August 2003PACS: 47.40.-x Compressible flows; shock and detonation phenomena - 47.65.+a Magnetohydrodynamics and electrohydrodynamics     

Scattering of light waves by electron electrostatic waves in laser produced plasmas

The propagation of light waves in an underdense plasma is studied using one-dimensional Vlasov-Maxwell numerical simulation.It is found that the light waves can be scattered by electron plasma waves as well as other heavily and weakly damping electron wave modes,corresponding to stimulated Raman and Brilluoin-like scatterings.The stimulated electron acoustic wave scattering is also observed as a high scattering level.High frequency plasma wave scattering is also observed.These electron electrostatic wave modes are due to a non-thermal electron distribution produced by the wave-particle interactions.The collision effects on stimulated electron acoustic wave and the laser intensity effects on the scattering spectra are also investigated.     

大气环境下飞秒激光对铝靶烧蚀过程的研究

利用时间分辨的光阴影成像技术研究了在大气环境下飞秒激光烧蚀铝靶的动态过程. 在入射激光能量为4 mJ, 激光光斑超过1 mm时, 激光烧蚀区表面物质以近似平面冲击波形式向外喷射; 在同样激光能量下、激光光斑较小时(约0.6 mm), 激光烧蚀区以近似半球型冲击波形式向外喷射. 当激光能量比较大时(7 mJ), 发现空气的电离对于激光烧蚀靶材有着重要影响. 在光轴附近烧蚀产生的喷射物具有额外的柱状和半圆型的结构, 叠加在平面冲击波结构上.     

激光技术及强激光与物质相互作用实验

利用科研成果，开设激光技术及强激光与物质相互作用实验，让学生了解强激光与物质相互作用时的2个典型效应：二次非线性光学效应，高压冲击波（光力学）效应，实现了科研与实验教学的紧密结合。     

激光超声方法研究固-固界面波传播特性

对界面波的传播特性进行了理论及实验研究.首先探讨了界面波的求根问题，基于黎曼面分析，给出了求解界面波特征方程所有根的一般方法.理论上对三种常见的界面波——Stoneley波，Leaky Rayleigh及Leaky Interface波传播机理进行了分析，描述了三种界面波的波矢及位移势在两种介质中的状态.最后基于光弹效应原理, 利用全光学的激光超声手段对界面波进行了实验测量，实测结果与理论符合很好.     

飞秒脉冲烧蚀单晶硅的超快动力学

利用基于Pump-probe系统的超快时间分辨阴影图的方法,研究了空气中飞秒激光烧蚀单晶硅的动力学过程。实验采用脉宽为50 fs、平均能量密度约35 J/cm2的单脉冲激光烧蚀单晶硅,获取飞秒激光烧蚀单晶硅过程中等离子体和冲击波的形成和发展过程的时间分辨阴影图。实验结果表明：飞秒激光烧蚀单晶硅导致其表面物质喷发的过程是不连续的,分为明显的两次喷发过程。这表明飞秒激光与单晶硅作用的过程中,在不同的时间段可能由不同的机制主导,在前期可能是多光子电离为主,在后期可能是由多光子效应和雪崩效应共同作用。研究还发现,延迟时间较长时,冲击波形状发生畸变。     

飞秒激光在铝膜中驱动冲击波的特性

为探索研究飞秒激光在材料中驱动冲击波的相关特性,采用激光脉冲频域干涉测试技术对脉冲宽度35 fs、脉冲能量0.7 mJ、功率密度1014 W/cm2量级的飞秒激光脉冲在200 nm厚铝膜中驱动冲击波的过程进行了实验测量。通过测量冲击波在铝膜中的渡越时间,获得激光脉冲在铝材料中驱动的冲击波平均速度约为6 km/s;通过对不同时刻铝膜自由面频域干涉场测量结果的分析,获得铝材料自由表面速度达1 km/s,根据平面冲击波的关系,推算其冲击压强达到9 GPa。     

激光等离子体及点爆炸空气冲击波波前运动方程的研究

根据激光等离子体空气冲击波在自由空间中传播的约束条件导出的空气冲击波前运动方程与点爆炸波泰勒解进行比较,证明泰勒解是该冲击波波前传播方程在中场的近似形式.并将该冲击波运动方程与强爆炸空气冲击波的实验进行比较,计算结果与实验数据符合得非常好 关键词： 激光 核爆炸 等离子体冲击波     

激光间接驱动冲击波实验测量

在神光Ⅱ高功率激光装置上,利用光学条纹相机观测台阶靶背面的冲击波加热发光,对样品中冲击波的平面性和冲击波速度进行了测量。通过冲击波法测得的腔内辐射温度与软X光能谱仪的测量结果一致,验证了冲击波速度测量的可信度。实验结果表明：在8路能量各为260 J、波长为351 nm的激光注入腔靶时,激光间接驱动的冲击波阵面在288 mm范围内平面性较好,Al样品中的冲击波速度为3.15×106 cm/s,对应的冲击波压力为1.66×1012 Pa。     

Titanium micromachining by femtosecond laser

The ultimate goal of this research was to characterize the ablation depth with respect to pulse energy, translation speed, and consecutive passes in order to obtain the parameters to have smooth microchannel surfaces. A logarithmic dependence of the channel depth on the laser pulse energy was observed with two different distinct ablation regimes. Although the same fluence values were used with two different lens sizes, the slopes of these ablation regimes were quite different. 100 mm lens has a small optical penetration length with steeper ablation slope in the first regime, whereas the 15 mm lens has the opposite. In the second part of the ablation regime, the slope was lower for 100 mm lens as compared to 15 mm lens. Furthermore, spike formation has been seen in 100 mm lens study at 0.308, 0.370, 0.431, and 0.493 J/cm² fluence values yet no spike formations have been seen in 15 mm lens study.     

神光-III激光装置时标激光和任意反射面速度干涉仪探针光源产生技术

在惯性约束聚变研究中, 时标激光是对物理诊断数据进行分析的重要时间标尺, 而任意反射面速度干涉仪(VISAR)光源则是冲击波精密诊断必不可少的探针光源. 通过对物理需求的分析, 提出对时标激光与VISAR光源共用脉冲产生单元, 采用时分复用技术实现二者在同一台幅度调制器上的精密整形, 经1×2分束后再通过声光开关进行选择输出, 从而降低了系统造价, 便于集中控制. 采用了脉冲稳偏、高稳定空间放大、高精度温控谐波转换技术及可快速插拔精密复位的光纤耦合和传能技术, 实现了时标和VISAR光源脉冲的高稳定输出. 研制的时标激光系统可产生与主激光高精度同步的12路二倍频、4路三倍频时标信号, 为神光-III激光装置物理实验提供了重要的时间基准. 产生的VISAR光源脉冲在经过光纤系统和Nd: YAG棒状放大器后, 通过温控LBO晶体倍频, 然后经1 mm芯径的多模传能光纤传输至成像型VISAR系统, 为物理实验提供了单纵模、高亮度、可精密整形的脉冲激光. 系统已用于VISAR诊断物理实验, 获得了完整的冲击加载、减速的图像, 从而为冲击波调速及相关高压物理实验提供了可靠的技术手段.     

Numerical modeling of ultrashort-pulse laser ablation of silicon

Silicon ablation by a single ultrashort laser pulse is simulated through a computer model. The agreement between results obtained through the model and experimental data found in the literature supports the hypothesis made by the authors in considering thermal evaporation as the dominant ablation mechanism in silicon. Two distinctive thresholds are defined for the ablation procedure leading to a better interpretation of experimental data. The dependence of ablation fluence thresholds on both wavelength and pulse width is discussed. An approximate analytical model describing the crater formation process is proposed and indicative results are presented.     

Sub-micron ablation of metallic thin film by femtosecond pulse laser

The ability to machine very small features in a material has a wide range of applications in industry. We ablated holes into thin film of 100 nm thickness made from various metals by femtosecond pulsed laser ablation. Using a Ti:Sapphire laser which supplies a laser pulse of 150 fs duration at central spectrum wavelength of 400 nm, we have produced a series sub-micron holes, whose diameters are less than 200 nm with a focused laser spot of 1.7 μm. We found that the material damage threshold has a great influence on the quality of the produced features. Experimental results shows that the heat-affected zone and the degree of being affected reduce with the increase of threshold value.     

激光空泡溃灭及其产生的声波

强激光脉冲在水中的自聚焦传输、成丝、击穿及击穿后产生的声波,近年来成为强激光脉冲传输应用研究领域的一个热点。强激光脉冲电离产生的空泡在溃灭过程中,对周围水体产生压缩,从而在水下产生声波。在现有理论基础上,考虑空泡含气量、水的粘滞系数和水的表面张力系数对空泡运动及其产生声波的影响。给出双空泡运动方程,分析空泡间距对空泡溃灭和其产生声压关系。根据实际情况,计算不同温度情况下单空泡和双空泡运动过程及其产生的声压,并进行相互比较。计算结果表明： 水温越高,空泡运动时达到的最小半径越小,空泡振荡周期和溃灭时间延长,产生声压越高。随双空泡间距减小,空泡溃灭时达到的最小半径线性减小,而其产生的最大声压则单调增大。     

Numerical simulation of the ultrasonic waves generated by ring-shaped laser illumination patterns

The generation of ultrasound in aluminum plate subjected to ring-shaped laser beam illumination has been studied quantitatively by using the finite element method. The superposition effects of surface acoustic waves on the top surface and the bulk ultrasonic waves on the rear surface of specimen have been obtained in a single simulation. The typical displacement profiles of the bulk ultrasonic wave at various depths along the central axis of the ring are obtained for three different radii, and the effect of the ring radius on the focal depths of the compression and shear mode are determined. The numerical results confirm that the focal depth of a bulk acoustic mode is determined by the directivity patterns of the acoustic mode generated by point-like laser sources via a thermoelastic mechanism, which depends on the physical constants of elastic medium.     

Motion and acceleration of electrons in high-intensity laser standing waves

The motion and the energy of electrons driven by the ponderomotive force in linearly polarized high-intensity laser standing wave fields are considered. The results show that there exists a threshold laser intensity, above which the motion of electrons incident parallel to the electric field of the laser standing waves undergoes a transition from regulation to chaos. We propose that the huge energy exchange between the electrons and the strong laser standing waves is triggered by inelastic scattering, which is related to the chaos patterns. It is shown that an electron's energy gain of tens of MeV can be realized for a laser intensity of 10²⁰ W/cm².     

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利用时空分辨的测量技术,测定了XeCl紫外激光烧蚀金属Cu消融粒子的发射光谱随时间与空间的强度分布。通过在不同的氢气压强下拍摄到的XeCl308nm紫外激光烧蚀金属Cu消融粒子的发光羽照片,发现激光消融粒子发光羽的颜色在不同区域有不同的颜色,不同区域的发光羽颜色随环境气压的改变而变化。随环境气压的增大,发光羽不仅逐渐变小,而且逐渐变淡。对激光消融粒子发光羽的机理进行了探讨,激光消融粒子发光羽的发光动力学模式在不同区域有不同的发光模式,且随环境气压的变化而改变。定性地解释了所观察的实验现象。