Effect of shock heating on the stability of laser-driven targets

The shock heating of a laser-driven, direct-drive target can determine its stability by affecting Rayleigh-Taylor growth rates through target decompression and ablative stabilization. Measurements indicate that pulses that rise rapidly to 10(14) W/cm(2) produce shock-induced temperatures of approximately 25 eV, whereas more slowly rising pulses show less heating. Analysis of the observed target behavior produced by these two pulses demonstrates that shock heating improves hydrodynamic stability because ablative stabilization increases when the targets are preheated by shocks.     

Dynamic compaction of copper powder: 2D numerical simulation and experimental results

Abstract

A method for plate-impact dynamic compaction of copper powder has been developped. The optimization of the experimental set-up (impedance adjustments, tensile wave traps, relative thickness of impactor and target,…) is presented.

2D axisymetrical numerical simulations have been performed with a Lagrangian finite element code. Geometrical characteristics of the experimental set-up as well as the dynamic response of the powder (Reaugh equation of state) and of the material of the set-up have been taken into account. These simulations show that, due to the difference in shock velocities in the container and in the powder, the powder is submitted to 2D loading waves. As a matter of fact the powder may be loaded by a non-planar shock wave propagating in the as-expected direction, as well as by a sweeping wave initiated at the bottom of the powder container, and propagating obliquely from the bottom-up. This second wave loads the bottom of the powder first. The influence of the impactor thickness as well as its material on the shock front shape and on the shock density-pressure history of the material has been studied. 1D simulations are shown not to evaluate properly the stress history and the energy deposition in the powder sample.

Metallographic observations as well as X-ray tomography experiments have been performed on consolidated samples. A very good agreement has been found between results of 2D numerical simulations and the observed final shape and density maps of the samples. The shape of deformed powder particles are also in agreement with the expected shock history.     

冲击载荷下非晶态碳材料的动态响应模型研究

 在一维应变冲击加载条件下，采用两个石英压力传感器进行了双值应力历史测量，对非晶态碳材料的动态响应特性进行了分析研究。研究结果表明，非晶态碳材料的冲击响应是简单稳定的，在试验冲击应力范围内为非线性弹性响应，Hugoniot曲线呈上凸的，表明材料内部传播的不是一个冲击波而是一簇压缩波，因此可采用特征线方法来解该冲击波问题，并用Riemann积分法对冲击应力过程进行修正，得到材料更精确的Hugoniot方程。还采用COPS程序对该材料的冲击响应过程进行了数值模拟，数值模拟曲线与试验曲线是很吻合的。表明采用Riemann积分法处理是合理的，此方法可以在VISAR测量中得到应用。     

X射线烧蚀泡沫-固体靶增压机理研究

X射线烧蚀泡沫-固体靶时界面失配会使得泡沫与固体靶中压力增加, 这一现象可用于研究高压下物质的状态方程. 本文以CH泡沫-Al固体靶为例, 研究了X射线烧蚀双层靶时的增压现象. 提出仅当泡沫中有冲击波产生时, 在固体靶中才会有增压现象出现. 文中基于冲击波强波近似, 理论分析了冲击波过界面情况与冲击波、稀疏波作用过程, 得到增压倍数与材料密度, 多方气体指数的近似表达式. 同时表明固体靶中增压产生的冲击波为高压脉冲, 会被紧跟热波的稀疏波追赶并稀疏. MULTI一维计算结果验证了该结论.     

油墨红外光谱相似度与计算机配色精度的关系研究

为了获得高精度的配色样品,提出了一种配色方法-成分分析配色法。该方法将化学分析法与计算机配色相结合,其核心是选取与目标色成分最接近的油墨进行配色,从而实现高精度的色彩匹配,为计算机配色发展提供新的思路。与目标色成分相近的油墨配色效果验证：使用某种油墨印制目标色,并用相同的油墨进行配色,以实现目标色成分与配色色样成分的一致性。使用三个不同品牌的油墨对目标色进行配色,比较配色精度及效率。使用泗联牌三种颜色油墨以任意比例通过印刷适性仪IGT-CI(荷兰)印制目标色,这些目标色包括间色和复色,各3个色样;使用配色软件X-Rite color master(美国)建立泗联、东洋、牡丹三个品牌油墨的配色基础数据库,并对不同目标色进行配色。结果表明使用与目标色相同的泗联油墨的配色精度远高于东洋、牡丹两个品牌的油墨,配色色差整体都很小,校正1～2次就能得到小于1.0的色差,最小达到0.36,几乎实现了目标色的同色同谱匹配。实验验证了成分分析配色法的核心“选取与目标色成分最接近的油墨进行配色,可以实现高精度色彩匹配”的可行性。判别目标色与配色油墨在成分上区别的化学分析工具探讨：为了判别目标色色料与配色油墨在成分上有区别,尝试使用“红外光谱相似度”作为判别的分析工具。使用红外光谱仪Thermo Nicolet 6700(美国)测出泗联、东洋、牡丹三个品牌的三种颜色油墨的红外光谱图,使用OMNIC软件中的相关性算法得到它们与目标色油墨的红外光谱相似度,并计算出平均相似度;将各品牌油墨的红外光谱相似度与其配色实验的精度进行对比分析,评价红外光谱相似度作为化学分析判别工具的有效性。结果表明泗联牌油墨与目标色的平均红外光谱相似度为100%,东洋的为86.53%,牡丹的为64.63%。当校正次数相同时,泗联油墨配色色差最小,配色精度最高;东洋次之,是泗联油墨配色色差的2倍左右;牡丹最差,是泗联油墨配色色差的3倍以上。配色结果与其红外光谱相似度的规律是一致的,目标色油墨与配色油墨之间的红外光谱相似度越高,越容易得到高精度的配色样品。实验证明了用成分分析配色法获得高精度的色彩匹配是可行的,使用红外光谱相似度作为目标色与配色油墨在成分上的分析工具对判别配色精度是有效的。今后的工作将探讨红外光谱相似度与配色精度间的相关性数值关系,以及进一步寻求更为有效的化学分析方法来判断目标色色料与配色油墨之间的成分关系。     

Numerical Simulation of Shock Wave Generation for Ignition of Precompressed Laser Fusion Target

In this work, we investigate the formation of a converging shock wave in a homogeneous spherical target, whose outer layer was heated by a flux of monoenergetic fast electrons of a given particle energy. Ablation pressure generating the wave forms at spherical expansion of a layer of a heated substance, whose areal density remains constant throughout the entire heating process and equal to the product of the initial heating depth and density of the target. The studies are carried out based on numerical calculations using a one-dimensional hydrodynamic code as applied to ignition of a precompressed target by a shock wave (shock ignition), one of the most promising techniques of laser fusion ignition.     

0.35 μm激光驱动冲击波的实验测量

 采用波长为0.35 μm的高功率激光脉冲聚焦辐照楔形铝靶,用光学条纹相机观测靶背面冲击波加热发光的特性。并对冲击波压力的定标关系和楔形样品冲击波速度的计算方法进行了介绍,对实验结果进行了简要的分析。结果表明：实验测量的冲击压力与定标公式的结果较为接近。     

Fabrication of graded density impactor via underwater shock wave and quasi-isentropic compression testing at two-stage gas gun facility

We show direct evidence that underwater shock wave enables us to bond multithin plates with flat, parallel, and high-strength interfaces, which are key requirements for functionally graded material (also called graded density impactor). This phenomenon is ascribed to the super short duration of the high-speed underwater shock wave, reducing the surface tension, diffusion, evaporation, deposition, and viscous flow of matter. Thin magnesium, aluminum, titanium, copper, and molybdenum foils were welded together and designed with the increase in density. Experimental evidence and numerical simulation show that well bonding between the multilayer structures. Microstructure examinations reveal that the dominant interfacial form shifts from waviness to linearity. Graded density impactor with multilayer structure is proved that can produce quasi-isentropic compression in two-stage gas gun experiment with a designed pressure loading profile, which suggests a feasible method to simulate the conditions we want to study that were previously inaccessible in a precisely controlled laboratory environment.     

Development and Investigation of a Two-Layer Metal—Ceramic Material for Protective Barriers in Conditions of High-Speed Impact

The possibility of improving the physico-mechanical characteristics of composite materials used in protective structures against a high-speed impact is considered. By means of self-propagating high-temperature synthesis, a two-layer cermet is obtained: the front layer is a cermet based on titanium diboride with a titanium nickelide bond, and the back layer is a titanium alloy. The study by the computational-experimental method of the impact resistance of this composite in comparison with a homogeneous titanium plate showed a qualitative advantage expressed in the absence of a shock crater in a cermet plate after a collision with a steel spherical impactor and stronger deformation and prefracture of the impactor. The two-layer cermet has a high resistance to the penetration of the steel impactor.     

Formation of Dusty Plasma Clouds at Meteoroid Impact on the Surface of the Moon

The nature of two dusty plasma clouds appearing because of the impact of a meteoroid on the surface of the Moon has been discussed. It has been shown that one of the clouds is formed by particles (or fragments) of regolith ejected into free space by a shock wave induced by the meteoroid impact on the surface of the Moon and the second cloud is formed by solidified melt droplets. The main characteristics of these clouds, including the cloud expansion rates, the characteristic sizes of particles in both clouds, and the concentrations and charges of particles, have been calculated. The calculated cloud expansion rates are in qualitative agreement with the observational data.     

Multiple tone generation by transonic compressors

An axial flow compressor operating at relative tip speeds in excess of the speed of sound generates a multiple tone acoustic field. A theory is presented which explains this tone field in terms of the production and propagation of the shock waves associated with the supersonic elements of the blading. It is shown how the strengths and positions of these shock waves are sensitive to the small blade-to-blade differences found in practical builds. This causes the complete shock pattern to be unique to each compressor. The propagation of the shock waves along the inlet ducting is analysed. This indicates that whilst the overall strength of the shock system decays, the relative differences between the shock waves are magnified in the propagation process. Ultimately, all similarity of the shock system to its original form is lost, and the steady rotation of the new shock system results in the multiple tone field. The analysis shows how features of the compressor design, as well as the degree of non-uniformity in the blade geometries, can influence the level and character of the radiated field.     

Instanton-like self-dual solution to lattice Euclidean Gravity: Difference from Eguchi–Hanson solution to continuous gravity

An analytical solution has been obtained for the spherical isothermal expansion of the outer layer of a ball whose mass increases at a constant areal density of the heated layer, which is equal to the product of the initial values of the depth of heating and the density of the layer for the entire time of expansion into vacuum. This solution differs from the known solution for the isothermal spherical expansion of a given mass of a material in a slower decrease in the density and, as a result, in the pressure of the expanding material with the time. In particular, it describes the expansion of the boundary layer of the ball heated by a flow of fast electrons in application to the problem of the ignition of an inertial confinement fusion target by a shock wave induced because of the heating of the target by the flow of laser-accelerated fast electrons (shock ignition).     

Self-phase modulation via similariton solutions of the perturbed NLSE Modulation instability and induced self-steepening

The perturbed nonlinear Schrodinger equation (PNLSE) describes the pulse propagation in optical fibers, which results from the interaction of the higher-order dispersion effect, self-steepening (SS) and self-phase modulation (SPM). The challenge between these aforementioned phenomena may lead to a dominant one among them. It is worth noticing that the study of modulation instability (MI) leads to the inspection of dominant phenomena (DPh). Indeed, the MI triggers when the coefficient of DPh exceeds a critical value and it may occur that the interaction leads to wave compression. The PNLSE is currently studied in the literature, mainly on finding traveling wave solutions. Here, we are concerned with analyzing the similarity solutions of the PNLSE. The exact solutions are obtained via introducing similarity transformations and by using the extended unified method. The solutions are evaluated numerically and they are shown graphically. It is observed that the intensity of the pulses exhibits self steepening which progresses to shock soliton in ultra-short time (or near t = 0). Also, it is found that the real part of the solution exhibits self-phase modulation in time. The study of (MI) determines the critical value for the coefficients of SS, SPM, or high dispersivity to occur.     

Shock-wave compression of hydrogen isotopes condensed under megabar pressures

Experiments on shock wave compression of solid hydrogen (protium) up to 66 GPa and deuterium (in the initially liquid or solid phase) up to 123 GPa using spherical devices where a steel ball (impactor) is explosion-accelerated to 23 km/s are described. The experimental data are compared with those obtained by American researchers for liquid deuterium and protium, who used conventional explosives and light-gas guns as energy sources in the experiments carried out on the Z machine at Sandia National Labs, where an impactor was accelerated by an ultrahigh magnetic field, or on the NOVA laser at the Livermore National laboratory. Our data agree well with those derived by Sandia Labs.     

激光驱动高压冲击波的实验观察

本文介绍能量约500J,脉宽约1.2ns的高功率激光脉冲聚焦辐照铝箔靶或铝台阶靶,用可见光超快速扫描相机观察靶背面冲击加热发光的情况。实验表明,采用特别的组合透镜聚焦,使激光冲击波的平面性得到明显的改善。实验测得铝台阶中的冲击波速度为17.6μm/ns,并推算出相应的冲击波压力达4.4Mbar。 关键词：     

Behavior of aluminum near an ultimate theoretical strength in experiments with femtosecond laser pulses

The dynamics of the motion of the free surface of micron and submicron films under the action of a compression pulse excited in the process of femtosecond laser heating of the surface layer of a target has been investigated by femtosecond interferometric microscopy. The relation between the velocity of the shock wave and the particle velocity behind its front indicates the shock compression to 9–13 GPa is elastic in this duration range. This is also confirmed by the small (≤1 ps) time of an increase in the parameters in the shock wave. Shear stresses reached in this process are close to their estimated ultimate values for aluminum. The spall strength determined at a strain rate of 10⁹ s⁻¹ and a spall thickness of 250–300 nm is larger than half the ultimate strength of aluminum.     

Flow characteristics and micro-scale metallic particle formation in the laser supersonic heating technique

The characteristics of the supersonic flow of the laser heating technique for producing micro-scale metallic particles were investigated in this study. A numerical model was established to predict the flow fields and particle trajectories leaving a spray nozzle with shock wave effects. The compressible flow of the shock waves and the trajectories of particles in diameters of 1–20 μm were simulated and compared with the flow visualization. In the experiment, a pulsed Nd-YAG laser was used as heat source on a carbon steel target within the nozzle, and the carbon steel particles were ejected by high-pressure air. The result shows that the shock wave structures were generated at various entrance pressures, and there is a significant increase in the amount of carbon steel particles and the spraying angles by increasing the entrance air pressure.     

强激光产生的强磁场及其对弓激波的影响

强激光照射金属线圈后,会在打靶点附近的背景等离子体中诱发冷电子的回流,在金属丝内形成强电流源,从而产生强磁场.本文利用神光II高功率激光器产生的强激光照射金属丝靶,产生了围绕金属丝的环形强磁场.利用B-dot对局域磁感应强度进行了测量,根据测量结果,结合三维模拟程序,反演得到磁场的空间分布.再利用强激光与CH平面靶相互作用产生的超音速等离子体撞击该金属丝,产生了弓激波.通过光学成像手段研究了磁场对冲击波的影响,发现磁场使得弓激波的轮廓变得不明显并且张角变大.同时,通过实验室天体物理定标率,将金属丝表面等离子参数变换到相应的天体参数中,结果证明利用该实验方法可以在实验室中产生类似太阳风的磁化等离子体.     

Numerical models of pressure pulse generation by imploding metalliners

The authors describe numerical calculations of pressure pulse generation using imploding liners. Liners are metal cylinders that are magnetically compressed by an intense axial current flow from a high-power pulse generator. The simulations cover the acceleration of the liner, collision with an internal diagnostic target, followed by compression and shock wave heating of the target. With the projected current waveform of the Atlas capacitor bank (in development at Los Alamos National Laboratory), initial results suggest that it may be possible to achieve pressures exceeding 3000 Gpa (30 Mbar) in a 4 mm diameter sample over an interval of 100-200 ns. The simulations were carried out with Crunch, a new one-dimensional hydrodynamics package for advanced personal computers. The program uses finite-element techniques to solve the coupled problems of hydrodynamics and magnetic diffusion. Crunch fully supports loading and interpolating Sesame equation-of-state tables. The program exhibits excellent stability, even for collisions between material shells and shock convergence on axis