空间碎片典型材料激光烧蚀反喷羽流实验研究

建立了观测和记录不同激光入射角度烧蚀6061铝合金靶材等离子体反喷羽流特性的实验装置,对实验结果图像进行了处理,并对处理结果进行了数值拟合。拟合结果表明,激光辐照靶材后100ns内,等离子体反喷羽流大致分布区域为靶面外5mm×5mm。激光以不同角度入射时,等离子体反喷速度相对于靶面法线方向大致呈轴对称分布。当激光相对靶面法线方向小角度范围内入射时,激光烧蚀引起的冲量主要沿靶面法线方向,反喷羽流沿靶面法方向的速度为20~40km·s-1。激光斜入射时,反喷羽流沿靶面法线方向的速度要大于激光垂直入射的情况。高斯函数可以很好地描述等离子体反喷羽流速度分布。     

Experimental investigation of plume expansion dynamics of nanosecond laser ablated Al with small incident angle

Experimental study of expansion dynamics of pulsed-laser ablation plasmas from Al is presented. A systematic investigation of plasma plume expansion is done. The laser beam is focused on the target with an incident angle between 0° and 20°. The results show that the plume growth is almost normal to the target surface, irrespective of the incident angle of the laser. Besides, the time evolution of the plasma plume geometry ratio at different incident angles shows that the incident angle of laser beam influences very slightly its shape at later delay time. The results imply that when the incident angle is small (ranging from 0° to 20°), the influence of the incident angle on the plume expansion is rather trivial.     

Modeling of laser induced breakdown spectroscopy for very low-pressure conditions

Laser Induced Breakdown Spectroscopy (LIBS) can be considered as a prominent technology for compositional analysis of materials in low-pressure space applications. In space applications, usually LIBS is conducted in a low-pressure environment and proper understanding of the plasma parameters is significant for any improvement in the system. A model is developed to describe the heating and subsequent melting, vaporization and ionization of a target material during LIBS process. A numerical model based on one-dimensional thermal conductivity equation is being used to simulate the target evaporation and a hydrodynamic model is used to simulate plume expansion. Further, an experimental approach of measuring spectral emission from the ablation plume using emission spectroscopy and estimating the plasma state, such as the ionization species, and average plasma temperature, is investigated. An important result of this work is that for different ambient conditions, laser ablation plume dynamics can be estimated.     

Influence of ambient gas ionization on the deposition of clusters formed in an ablation plume

The effect of inert gas ionization on the dynamics of a laser ablation plume expanding through a background inert gas is studied. Charge transfer reactions between ablated ions and neutral background gas atoms yield to the formation of a charged layer on the plume expansion front. The energy lost by ablated ions when the plume is slowed down is calculated. The observed microstructure differences between carbon films prepared by pulsed laser deposition in helium, where the ionization mechanism is absent and respectively in argon, where it is present, are well correlated to model predictions.     

Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale molecular dynamics simulations. The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution. At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gas-phase molecules, and small clusters. The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement. Spatially resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume. A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed. Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume. Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters. The cluster-size distributions can be relatively well described by a power law Y(N)∼N^-τ with exponents different for small, up to ∼15 molecules, and large clusters. The decay is much slower in the high-mass region of the distribution. Received: 13 October 2001 / Accepted: 18 July 2002 / Published online: 25 October 2002 RID="*" ID="*"Corresponding author. Fax: +1-434/982-5660, E-mail: lz2n@virginia.edu     

Numerical study of ultra-short laser ablation of metals and of laser plume dynamics

Numerical modeling is used to investigate the physical mechanisms of the interaction of ultra-short (sub-picosecond) laser pulses with metallic targets. The laser–target interaction is modeled by using a one-dimensional hydrodynamic code that includes the absorption of laser radiation, the electronic heat conduction, the electron-phonon or electron–ion energy exchange, as well as a realistic equation of state. Laser fluences typical for micromachining are considered. The results of the 1D modeling are then used as the initial conditions for a 2D plasma expansion model. The dynamics of laser plume expansion in femtosecond regime is investigated. Calculations show that the plasma plume is strongly forward directed. In addition, a two-peaked axial density profile is obtained for 400 nm laser wavelength. The calculation results agree with the experimental observations. PACS 52.38.Mf; 02.60.Cb     

Short-laser-pulse-driven emission of energetic ions into a solid target from a surface layer spalled by a laser prepulse

An efficient emission of picosecond bunches of energetic protons and carbon ions from a thin layer spalled from a organic solid by a laser prepulse is demonstrated numerically. We combine the molecular dynamics technique and multi-component collisional particle-in-cell method with plasma ionization to simulate the laser spallation and ejection of a thin (∼20–30 nm) solid layer from an organic target and its further interaction with an intense femtosecond laser pulse. In spite of its small thickness, a layer produced by laser spallation efficiently absorbs ultrashort laser pulses with the generation of hot electrons that convert their energy to ion energy. The efficiency of the conversion of the laser energy to ions can be as high as 20%, and 10% to MeV ions. A transient electrostatic field created between the layer and surface of the target is up to 10 GV/cm. Received: 13 March 2001 / Accepted: 20 March 2001 / Published online: 20 June 2001     

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.     

Plasma dynamics from laser ablated solid lithium

Emission plasma plume generated by pulsed laser ablation of a lithium solid target by a ruby laser (694 nm, 20 ns, 3 J) was subjected to optical emission spectroscopy: time and space resolved optical emission was characterised as a function of distance from the target surface. Propagation of the plume was studied through ambient background of argon gas. Spectroscopic observations can, in general, be used to analyse plume structure with respect to an appropriate theoretical plasma model. The plume expansion dynamics in this case could be explained through a shock wave propagation model wherein, the experimental observations made were seen to fit well with the theoretical predictions. Spectral information derived from measurement of peak intensity and line width determined the parameters, electron temperature (T _e) and electron number density (n _e), typically used to characterise laser produced plasma plume emission. These measurements were also used to validate the assumptions underlying the local thermodynamic equilibrium (LTE) model, invoked for the high density laser plasma under study. Some interesting results pertaining to the analysis of plume structure and spatio-temporal behaviour of T _e and n _e along the plume length will be presented and discussed.     

Computational and experimental study of the cluster size distribution in MAPLE

A combined experimental and computational study is performed to investigate the origin and characteristics of the surface features observed in SEM images of thin polymer films deposited in matrix-assisted pulsed laser evaporation (MAPLE). Analysis of high-resolution SEM images of surface morphologies of the films deposited at different fluences reveals that the mass distributions of the surface features can be well described by a power-law, Y(N) ∝ N^−t, with exponent −t ≈ −1.6. Molecular dynamic simulations of the MAPLE process predict a similar size distribution for large clusters observed in the ablation plume. A weak dependence of the cluster size distributions on fluence and target composition suggests that the power-law cluster size distribution may be a general characteristic of the ablation plume generated as a result of an explosive decomposition of a target region overheated above the limit of its thermodynamic stability. Based on the simulation results, we suggest that the ejection of large matrix-polymer clusters, followed by evaporation of the volatile matrix, is responsible for the formation of the surface features observed in the polymer films deposited in MAPLE experiments.     

发射光谱法研究纳秒激光烧蚀硅等离子体特性

利用调Q Nd3+∶YAG激光器三倍频355 nm激光脉冲烧蚀空气环境的硅样品,观测不同脉冲激光能量下产生的等离子体在380～420 nm范围内的时间-空间分辨等离子体发射光谱,观测到在等离子体羽膨胀初期存在N+发射光谱。在局域热力学平衡近似条件下,根据时间-空间分辨等离子体发射光谱计算得到等离子羽体电子温度和电子密度随时间延时存在二次指数衰减变化,等离子体羽体电子温度和电子密度的空间分布近似呈Lorentz分布,发现在确定激光脉冲能量下电子密度空间分布最大值偏离光谱强度最大空间位置并对产生原因进行分析,探讨了等离子体羽参数与激光脉冲能量的关系。     

Effect of ZnO plasma plume dynamics on laser ablation

The dynamic behaviors and optical properties of a ZnO plasma plume produced by pulsed laser ablation using a Nd:YAG laser (wavelength: 532 nm, pulse width: 3 ns) were studied by fast photography using a commercial gated charge coupled device (CCD) camera linked with a delay circuit and by optical emission spectroscopy at various ambient oxygen pressures. Fast photography was conducted with a resolving power of 0.25 μs and the expansion behaviors of the laser ablation plume were observed. Plasma plume expansion velocity decreased with oxygen partial pressure. The flow of the plasma plume in the early stage of expansion of up to 3 ms agreed well with the drag model.     

Effect of ionization on laser-induced plume self-similar expansion

The dynamics of a laser ablation plume during the first stage of its expansion, just after the termination of the laser pulse is modelled. The one-dimensional expansion of the evaporated material, considered as an ideal fluid, is governed by one-fluid Euler equations. For high energetic ions, the charge separation can be neglected and the hydrodynamics equations solved using self-similar formulation. Numerical solution is obtained, first when the laser fluence range is low enough to deal with a neutral vapor, and in a second stage, when ionization effects on the expansion are taken into account, for different material targets. As a main result, we found that the presence of ions in the evaporated gas enhances the self-similar expansion.     

Influence of initial micro-porosity of target on material ejection under nanosecond laser pulses

Heating and explosive destruction of the near surface volume of metal with initial micro-porosity under nanosecond laser pulse is simulated. It is shown that presence of initial micro-porosity may influence and modify the ejection mechanisms from melt and from solid phase. Thermal and gas dynamic processes in a gas bubble growing in laser irradiated melt are analysed. Experiments show rather different surface morphology defined by micro-explosions and melt ejection for different metals. Possible explosion of micro-pores and solid particles release under nanosecond laser irradiation is analysed for metals with high melting point. It is shown that the stress attains the values that are close to the damage threshold.     

Laser ablation of silicon and copper targets. Experimental and finite elements studies

The ablation process induced by excimer lasers is a collective phenomenon that basically involves two phenomena: the laser radiation–matter interaction and the dynamic of the ablation plume. The laser parameters, the thermal and optical properties of the material, and the surface morphology are critical factors in the ablation mechanisms affecting the direction of the ablation plume expansion. In this study, the role of the surface roughness and the evolution of its morphology under the laser irradiation were investigated. Assuming a thermal ablation model, a theoretical study of the initial steps of the laser ablation process by a finite element method using ANSYS (6.1) was performed. Different ablation experiments were carried out on silicon and copper targets using a XeCl laser. The target surface morphology changes were observed by SEM and the plume deflection was recorded by a digital camera. An acceptable agreement between the experimental and simulated results was found. This study contributes to a better understanding of the physical processes involved in the laser ablation and the relations between the plume deflection angle and the surface roughness. PACS 79.20.Ds; 81.40.Gh; 44.05.+e     

Molecular imaging by Mid-IR laser ablation mass spectrometry

Mid-IR laser ablation at atmospheric pressure (AP) produces a mixture of ions, neutrals, clusters, and particles with a size distribution extending into the nanoparticle range. Using external electric fields the ions can be extracted and sampled by a mass spectrometer. In AP infrared (IR) matrix-assisted laser desorption ionization (MALDI) experiments, the plume was shown to contain an appreciable proportion of ionic components that reflected the composition of the ablated target and enabled mass spectrometric analysis. The detected ion intensities rapidly declined with increasing distance of sampling from the ablated surface to ∼4 mm. This was rationalized in terms of ion recombination and the stopping of the plume expansion by the background gas. In laser ablation electrospray ionization (LAESI) experiments, the ablation plume was intercepted by an electrospray. The neutral particles in the plume were ionized by the charged droplets in the spray and enabled the detection of large molecules (up to 66 kDa). Maximum ion production in LAESI was observed at large (∼15 mm) spray axis to ablated surface distance indicating a radically different ion formation mechanism compared to AP IR-MALDI. The feasibility of molecular imaging by both AP IR-MALDI and LAESI was demonstrated on targets with mock patterns. Presented at the 9-th International Conference on Laser Ablation, 2007 Tenerife, Canary Islands, Spain     

Detailed Characteristics of Expansion Velocity of Si from Laser Ablated SiC

Optical emission of plasma is used to investigate the characteristics of dynamics distribution in the plume gen- erated by ablation of a SiC sample using Nd：YAG laser. The plume expansion dynamics is characterized by time-of-flight measurement. We find that the profiles of Si （I） （390.55 nm） split into two components and the Si （1I） （634. 71 nm） spectra show two distinct expansion dynamics regions. The time-of-flight measurement of Si（ll） （634. 71 nm） under different laser irradianee conditions, from 0.236 G W/cm^2 to 1.667 G W/cm^2, are presented and discussed.     

脉冲激光烧蚀材料等离子体反冲压力物理模型研究与应用

分析了脉冲激光烧蚀材料等离子体等温膨胀阶段的物理特性,建立了脉冲激光烧蚀材料等离子体压力三维方程与动力学模型.应用所建模型,数值分析了单脉冲激光烧蚀青铜金刚石砂轮等离子体相关特性,得到等离子体的反冲压力最大值870 Pa出现在约25 ns后,距离砂轮表面距离约0.05 mm处.相关条件下开展脉冲激光烧蚀青铜金刚石砂轮试验,采用高速相机观测烧蚀砂轮过程中的飞溅现象;采用光栅光谱仪测量等离子体空间发射光谱,计算了等离子体电子温度、电子密度以及反冲压力.实验表明脉冲激光烧蚀青铜金刚石砂轮等离子体反冲压力可以不计,同时也验证了气体方程与动力学模型的正确性和可行性,对脉冲光纤激光烧蚀工艺优化具有启示意义.