激光驱动冲击波传播稳定性的计算分析

 采用简化材料状态方程,用1维特征线方法计算了激光驱动冲击波在多层材料中稳定传播的距离。计算结果与用1维三温激光-靶耦合JB程序的数值模拟结果进行了比较,二者符合得比较好。通过计算发现：冲击波在Au中的稳定传播距离受基底厚度的影响很大,稳定传播距离随基底厚度的增加先增大后减小。     

激光驱动冲击波传播稳定性的计算分析

采用简化材料状态方程,用1维特征线方法计算了激光驱动冲击波在多层材料中稳定传播的距离。计算结果与用1维三温激光-靶耦合JB程序的数值模拟结果进行了比较,二者符合得比较好。通过计算发现：冲击波在Au中的稳定传播距离受基底厚度的影响很大,稳定传播距离随基底厚度的增加先增大后减小。     

Nanodomain shock wave in near-field laser–material interaction

In this work, molecular dynamics simulation is conducted to explore the shock wave phenomena in a nanodomain in near-field laser–material interaction. A large system consisting of over 800,000 atoms is studied. The work focuses on the kinetic and physical properties of the disturbed gas compression driven by the high speed movement of the molten particulates ejected from the solid target in a nanodomain. The quick interaction between solid and gas atoms compresses the gas and forms a steep shock wave front, which moves at a supersonic speed. The fast compression of gas also induces a steep interface of density, temperature and pressure distribution, which is viewed as typical characteristics of nanoscale shock waves. Evolutions of shock wave front position, velocity and Mach number are also explored and show quick decay during wave propagation.     

斜入射激光驱动的冲击波在样品中传播特性的实验研究

在“神光-Ⅱ”高功率激光装置上，利用弯折靶、双台阶靶实验及通过对等离子体喷射状态的实验测量，研究了激光斜入射情况驱动的冲击波传播特性.结果表明，即使在大角度(约45°)斜入射的激光驱动下，靶材料中的冲击波依然是沿着靶面法线方向传播的，并能形成很好的一维正击波. 关键词： 斜入射 冲击波 弯折靶 双台阶靶 激光状态方程     

Microstructure study of direct laser fabricated Ti alloys using powder and wire

A compositionally graded material has been fabricated using direct laser fabrication (DFL). Two types of feedstock were fed simultaneously into the laser focal point, a burn resistant (BurTi) alloy Ti-25V-15Cr-2Al-0.2C powder and a Ti-6Al-4V wire. The local composition of the alloy was changed by altering the ratio of powder to wire by varying the feed rate of the powder whilst maintaining a fixed feed rate of wire-feed. For the range of compositions between about 20% and 100% BurTi only the beta phase was observed and the composition and lattice parameter varied monotonically. The grain size was found to be much finer in these functionally graded samples than in laser fabricated Ti64. Some samples were made using the wire-feed alone, where it was found that the microstructure is different from that found when using powder feed alone. The results are discussed in terms of the power requirements for laser fabrication of powder and wire samples.     

强激光与稠密等离子体作用引起的冲击波加速离子的研究

用二维PIC（Particle-in-Cell）程序模拟研究了强激光与稠密等离子体靶作用产生的无碰撞静电冲击波的结构和这种冲击波对离子的加速过程,研究发现由于冲击波前沿附近的双极电场的作用,具有一定初速度的离子能被该双极场俘获并获得加速,最终能够被加速到两倍冲击波速度.冲击波加速可以得到准单能的离子能谱,叠加在通过鞘层加速机理产生的宽度离子能谱上.还对不同激光强度和不同等离子体密度情况下形成的冲击波进行了比较.研究表明,强度相对较低的激光在高密度等离子体中可以产生以一定速度传播的静电孤波结构,后者只能加速 关键词： 强激光 稠密等离子体 无碰撞静电冲击波 离子加速     

约束层对激光驱动冲击波压力影响机理的理论研究

针对刚性约束层、柔性约束层以及液体约束层，从激光诱导冲击波阵面状态、汽化物(包括气体和等离子体)扩散以及冲击波的反射进行分析，发现对于脉宽小于冲击波通过汽化物层的时间间隔的短脉冲激光，约束层并不能直接提高冲击波的冲量，而对于脉宽大于冲击波通过汽化层时间间隔的激光，其增强冲击效果是通过约束汽化物的扩散，提高压力幅值和由于冲击波在约束层与工件表面的多次反射而延长对工件的作用时间来实现的.刚性约束层能最大地增加冲击冲量，而柔性约束层和液体约束层的主要优点是其形状可与非平面形工件表面符合. 关键词： 激光 约束层 扩散 反射波     

激光引致激波演化数值研究

在高重复频率激光推进的研究中,激波的合并是发生在激波演化后期的,同时由于脉冲间隔短,脉冲宽度对流场演化的影响也需要详细研究。考虑了激光辐照过程对流场演化的影响,通过数值计算对激波演化特性进行了研究。结果表明,初期波阵面的椭球形状逐渐转化为一个球形,球心与击穿点的距离随时间逐渐减小并最终趋于稳定。基于激波合并的应用,当激波马赫数在1~2之间时,给出了激波波阵面半径随时间的变化规律,以及激波高压区长度和波峰压强随激波波阵面半径变化规律的经验公式。     

A suppressor to prevent direct wave-induced cavitation in shock wave therapy devices

Cavitation plays a varied but important role in lithotripsy. Cavitation facilitates stone comminution, but can also form an acoustic barrier that may shield stones from subsequent shock waves. In addition, cavitation damages tissue. Spark-gap lithotripters generate cavitation with both a direct and a focused wave. The direct wave propagates as a spherically diverging wave, arriving at the focus ahead of the focused shock wave. It can be modeled with the same waveform (but lower amplitude) as the focused wave. We show with both simulations and experiments that bubbles are forced to grow in response to the direct wave, and that these bubbles can still be large when the focused shock wave arrives. A baffle or "suppressor" that blocks the propagation of the direct wave is shown to significantly reduce the direct wave pressure amplitude, as well as direct wave-induced bubble growth. These results are applicable to spark-gap lithotripters and extracorporeal shock wave therapy devices, where cavitation from the direct wave may interfere with treatment. A simple direct-wave suppressor might therefore be used to improve the therapeutic efficacy of these devices.     

Visualization of particle trajectories in the laser shock cleaning process

The laser shock cleaning (LSC) method has recently attracted substantial attention since it can remove micro/nano-scale contaminant particles from a solid surface without direct exposure of the surface to laser irradiation. However, despite the importance of the particle detachment and redeposition mechanisms in the LSC process, the behavior of the particles during the cleaning process has never been analyzed experimentally. In this work, the motion of the micrometer-scale particles detached by a laser-induced plasma/shock wave is visualized by a photoluminescence imaging technique. The technique yields time-resolved particle trajectories under typical conditions of the LSC process, with and without a gas jet blowing. Discussions are made on the behavior of the detached particles and redeposition mechanisms.     

Real time NDE of laser shock processing with time-of-flight of laser induced plasma shock wave in air by acoustic emission sensor

Acoustic emission sensor is used to research the time-of-flight of the shock wave induced by laser-plasma in air for real time nondestructive evaluation (NDE) of laser shock processing. The time-of-flight of the shock wave propagating from the source to the sensor declines nonlinearly and similarly at the different distances for different laser energies. The velocity of the shock wave at the distance of 30 mm increases faster than that of the distance of 35 mm. The relationship between the laser energy and the distance is almost linearly when the signal with distortion is measured by acoustic emission sensor. Finally, Taylor solution is used to analyze the experimental results, and the empirical formula between the energy of the shock wave and the laser energy is established, which will provide a theoretical basis for real time NDE of laser shock processing.     

Influence of air pressure on mechanical effect of laser plasma shock wave

The influence of air pressure on mechanical effect of laser plasma shock wave in a vacuum chamber produced by a Nd:YAG laser has been studied. The laser pulses with pulse width of 10ns and pulse energy of about 320mJ at 1.06$\mu $m wavelength is focused on the aluminium target mounted on a ballistic pendulum, and the air pressure in the chamber changes from $2.8\times 10^{3}$ to 1.01$\times $10$^{5 }$Pa. The experimental results show that the impulse coupling coefficient changes as the air pressure and the distance of the target from focus change. The mechanical effects of the plasma shock wave on the target are analysed at different distances from focus and the air pressure.     

辐射驱动实验中预热对冲击波速度诊断的影响

使用主动式任意反射面速度干涉仪进行了平面靶预热膨胀量的观测,并对间接驱动下不同能量和靶型的预热膨胀量进行了计算。实验发现,在5 kJ单端驱动的激光能量作用下,平面样品后界面的膨胀量可达2.1 m左右。优化靶设计后,在冲击波速度测量实验中使用双端驱动方式,通过测量膨胀量修正台阶厚度,可以获得更加精确的动态台阶厚度值。结合精确获得的冲击波传输时间,在匀速传输的条件下可以获得高精度的冲击波传输速度值。该方法在辐射驱动超高压条件下具有很好的适用性,可以为状态方程实验提供高精度的冲击波速度数据。     

辐射驱动实验中预热对冲击波速度诊断的影响

使用主动式任意反射面速度干涉仪进行了平面靶预热膨胀量的观测,并对间接驱动下不同能量和靶型的预热膨胀量进行了计算。实验发现,在5kJ单端驱动的激光能量作用下,平面样品后界面的膨胀量可达2.1μm左右。优化靶设计后,在冲击波速度测量实验中使用双端驱动方式,通过测量膨胀量修正台阶厚度,可以获得更加精确的动态台阶厚度值。结合精确获得的冲击波传输时间,在匀速传输的条件下可以获得高精度的冲击波传输速度值。该方法在辐射驱动超高压条件下具有很好的适用性,可以为状态方程实验提供高精度的冲击波速度数据。     

Multi-scale method study of nano-void under the shock wave

The dynamic behavior of nano-void in metal Al under shock loading is simulated by using the multi-scale numerical simulation method. This method couples the molecular dynamics method used in local defect domain and the finite element method overall domain. The macroscopic failure of material is analyzed based on the microscopic dynamic behavior of nano-void. Our results show that the collapse of nano-void depends on the shock strength. Both a high and a low stress region are formed in the region of void after void collapse, whereas only a low stress region is formed under no collapse. The local hot spot is displayed in the temperature field, and a detailed analysis reveals that the hot point initiation is caused by recompression of atoms in void.     

Effects of process variables on laser direct formation of thin wall

In this paper, effects of process variables on wall thickness, powder primary efficiency and speed of forming a thin metallic wall in single-pass coaxial laser cladding are investigated, and some resolution models are established and testified experimentally. With some assumptions, each of wall thickness, powder primary efficiency and formation speed can be defined as a function of the process variables. Wall thickness is equal to width of the molten pool created in single-pass laser cladding and determined by laser absorptivity, laser power, initial temperature, scanning speed and thermo-physical properties of clad material. Powder primary efficiency and formation speed are both dependent on an exponential function involving the ratio of melt pool width, which is decided by the process variables, to powder flow diameter. In addition, formation speed is influenced by powder feed rate. In present experiment, a 500 W continual-wave (CW) CO₂ laser is used to produce thin-wall samples by single-pass coaxial laser cladding. The experimental results agree well with the calculation values despite some errors.