Dynamic damage and failure in viscoplastic materials

In this paper, a dynamic damage model in ductile solids under the application of a dynamic mean tensile stress is developed. The proposed model considers void nucleation and growth as parts of the damage process under intense dynamic loading (strain rates epsilon greater than or equal to 10(3) s(-1)). The evolution equation of the ductile void has the closed form, in which work-hardening behavior, rate-dependent contribution and inertial effects are taken into account. Meanwhile, a plate impact test is performed for simulating the dynamic fracture process in LY12 aluminum alloy. The damage model is incorporated in a hydrodynamic computer code, to simulate the first few stress reverberations in the target as it spalls and postimpact porosity in the specimen. Fair agreement between computed and experimental results is obtained. Numerical analysis shows that the influence of inertial resistance on the initial void growth in the case of high loading rate can not be neglected. It is also indicated that the dynamic growth of voids is highly sensitive to the strain rates.

Spall damage in aluminum-alloy

A void growth relations for ductile porous materials under intense dynamic general loading condition is presented. The mathematical model includes the influence of inertial effects, material rate sensitivity, as well as the contribution of void surface energy and material work-hardening. Numerical analysis shows that inertia appears to resist the growth of voids. The inertial effects increase quickly with the loading rates. The theoretical analysis suggests that the inertial effects cannot be neglected at high loading rates. Plate-impact tests of aluminum alloy are performed with light gas gun. The processes of dynamic damage in aluminum alloy are successfully simulated with a finite-difference dynamic code in which the theoretical model presented in this paper is incorporated.

Dynamic failure in ductile porous materials

In this paper, a mathematical model of dynamic fracture in porous ductile materials under intense dynamic general loading is developed. The mathematical model includes the influence of inertial effects and material rate sensitivity, as well as the contribution of surface energy of a void and material work-hardening. In addition, the condition of the void compaction is considered as well. The threshold stresses for the void growth and compaction are obtained. A simple criterion for ductile fracture which is associated with material distention and plastic deformation is adopted. As an application of the theoretical model, the processes of two-dimensional spallation in LY12 aluminum alloy are successfully simulated by means of two-dimensional finite-difference Lagrangian code.

Statistical-theory of multiphoton excitation of polyatomic-molecules based on the wigner function

This paper is aimed at establishing a statistical theory of rotational and vibrational excitation of polyatomic molecules by an intense IR laser. Starting from the Wigner function of quantum statistical mechanics, we treat the rotational motion in the classical approximation; the vibrational modes are classified into active ones which are coupled directly with the laser and the background modes which are not coupled with the laser. The reduced Wigner function, i.e., the Wigner function integrated over all background coordinates should satisfy an integro-differential equation. We introduce the idea of ``viscous damping'' to handle the interaction between the active modes and the background. The damping coefficient can be calculated with the aid of the well-known Schwartz–Slawsky–Herzfeld theory. The resulting equation is solved by the method of moment equations. There is only one adjustable parameter in our scheme; it is introduced due to the lack of precise knowledge about the molecular potential. The theory developed in this paper explains satisfactorily the recent absorption experiments of SF6 irradiated by a short pulse CO2 laser, which are in sharp contradiction with the prevailing quasi-continuum theory. We also refined the density of energy levels which is responsible for the muliphoton excitation of polyatomic molecules.

Laser action from 1,3,5,7-tetramethyl-2,6-diethyl-8-n-propyl pyrromethene-BF2

We measured spectroscopic and laser action properties of a novel 8-position substituted pyrromethene-BF2, namely 1,3,5,7-tetramethyl-2,6-diethyl-8-n-propyl pyrromethene-BF2 complex. The laser action was performed with the corresponding dye solution in ethanol, which was placed in a Littman-type laser cavity pumped by the second harmonic of a Q-switched Nd:YAG laser. The spectroscopic measurements clearly indicated that the corresponding dye solution in ethanol exhibited intense absorption in the visible spectral region with large fluorescence quantum yield. It possesses rather low triplet-triplet absorption in the spectral region 460-550 nm and almost negligible triplet-triplet absorption in the lasing spectral region. As a consequence, it lases nearly as efficiently as commercially available benchmark laser dyes such as Rhodamine-6G and outperformed them in wavelength tunability in our laser cavity and pump geometry. (C) 2002 Optical Society of America.

7 fs超快强激光驱动Ar原子产生支持单个阿秒脉冲的高次谐波连续谱

采用脉冲宽度为7 fs，脉冲能量为0．4mJ的超快强激光脉冲与气体盒子中Ar原子作用获得了高次谐波截止区连续谱，并发现当驱动激光稳定在不同的载波包络相位时，高次谐波的谱结构、谱调制深度和连续谱的带宽都有很大区别。在某些载波包络相位时获得了平滑的连续谱，调制深度小于17％，连续带宽达10eV，从而支持时域上获得变换极限500as的单个阿秒脉冲。

Laser-confined fusion

An approach for producing a large quantity of neutrons is proposed. It involves compression of a fuel foil and confinement of the resulting plasma between two intense laser pulses. It is shown that two circularly polarized laser pulses of amplitude a=7 illuminating a deuterium-tritium foil of areal density 3.3 X 10(18) cm(-2) can produce about 4.2 X 10(6) neutrons per joule of the input laser energy.

Few-cycle spatiotemporal soliton wave excited by filamentation of a femtosecond laser pulse in materials with anomalous dispersion

The nonlinear dynamics of 1.6-mu m fs laser pulses propagating in fused silica is investigated by employing a full-order dispersion model. Different from the x-wave generation in normally dispersive media, a few-cycle spatiotemporally compressed soliton wave is generated with the contrary contributions of anomalous group velocity dispersion (GVD) and self-phase-modulation. However, at the tailing edge of the pulse forms a shock wave which generates separate and strong supercontinuum peaked at 670 nm. It is also the origin of conical emission formed both in time and frequency domain with the contribution of normal GVD at visible light.

Nonlinear propagation of fs laser pulses in liquids and evolution of supercontinuum generation

Nonlinear propagation of fs laser pulses in liquids and the dynamic processes of filamentation such as self-focusing, intensity clamping, and evolution of white light production have been analyzed by using one- and two-photon fluorescence. The energy losses of laser pulses caused by multiphoton absorption and conical emission have been measured respectively by z-scan technique. Numerical simulations of fs laser propagation in water have been made to explain the evolution of white light production as well as the small-scale filaments in liquids we have observed by a nonlinear fluorescence technique. (c) 2005 Optical Society of America.

High-power extracavity pulse compression in solid materials

A novel technique for high-power extracavity pulse compression with a nonlinear solid material is demonstrated. Before spectral broadening by self-phase modulation in the solid material, a short filament generated in argon is used as a spatial filter, which works for a uniform spectrum broadening over the spatial profile. Compensated by chirped mirrors, a 15-fs pulse is generated from a 32-fs input laser pulse. A total transmission larger than 80% after the solid material is achieved.

Phase dependence of electron acceleration in a tightly focused laser beam

Electron acceleration using a tightly focused ultraintensity laser beam is investigated numerically and strong phase dependence is found. The acceleration is periodic to the variety of the initial laser field phase, and the accelerated electrons are emitted in pulses of which the full width is the half period of the laser field. When a 10 PW intense laser beam is used, the electron with energy less than 1 Mev can be accelerated up to energies about 1.4 GeV. The optimal initial condition for electron acceleration is found. (C) 2005 American Institute of Physics.

Long lifetime plasma channel in air generated by multiple femtosecond laser pulses and an external electrical field

The lifetime of a plasma channel produced by self-guiding intense femtosecond laser pulses in air is largely prolonged by adding a high voltage electrical field in the plasma and by introducing a series of femtosecond laser pulses. An optimal lifetime value is realized through adjusting the delay among these laser pulses. The lifetime of a plasma channel is greatly enhanced to 350 ns by using four sequential intense 100fs( FWHM) laser pulses with an external electrical field of about 350kV/m, which proves the feasibility of prolonging the lifetime of plasma by adding an external electrical field and employing multiple laser pulses. (c) 2006 Optical Society of America.

紧聚焦的超短超强激光脉冲在真空中加速斜入射的相对论电子

研究了紧聚焦的线偏振飞秒超强高斯激光脉冲俘获并剧烈加速斜入射低能相对论电子的效应，发现被俘获的电子在激光脉冲纵向有质动力的强大加速作用下，可以获得GeV量级的能量，并详细研究了入射电子的初能量、斜入射角、电子与激光脉冲的相对延迟时间和激光脉冲宽度等条件对电子能量增益的影响，发现当激光脉宽超过10λ时，脉宽对电子能量增益影响不大．

激光同步辐射作为阿秒X射线辐射源的特性研究

研究了逆流相对论电子与激光脉冲相互作用获得激光同步辐射的频率上移、微分散射截面等特性.发现逆流相对论电子与短脉冲激光相互作用,可以获得阿秒X射线辐射脉冲.短脉冲激光条件下得到的后向散射光的频率上移与长脉冲激光条件下得到的后向散射光的频率上移是完全一致的,同时发现随着入射电子初始能量的增加,散射光的准直性越来越好,后向散射光脉冲的脉宽越来越短.

飞秒强激光场中大尺寸氩团簇爆炸机理的实验研究

By using time-of-flight spectroscopy, the ionization and explosion of large argon clusters ( (n) over bar = 3 x 10(3) - 3 x 10(6)) in the intense femtosecond pulsed laser field (60 fs,2 x 10(16) W/cm(2)) has been studied, and the dependence of average energy of ions emitted from argon clusters on the gas backing pressure has been measured. By comparing the average ion energies obtained with two different supersonic conical nozzles and considering the Hagena's scaling law of clusters, we have found that the average ion energy is determined by the cluster size when the laser parameters are kept unchanged. The experimental results indicated that when the cluster size is less than 3 x 10(5) atoms per cluster, the Coulomb repulsion force is the dominating factor in the expansion mechanism. Beyond this size, for 3 x 10(5) < (n) over barn < 3 x 10(6), the expansion is the result of the combined effect of both the Coulomb repulsion force and the hydrodynamic force, and the latter will play the dominating role for increasing cluster size.