Ultrafast solid–liquid–vapor phase change of a gold film induced by pico- to femtosecond lasers

Melting, vaporization and resolidification processes of thin gold film irradiated by a femtosecond pulse laser are studied numerically. The nonequilibrium heat transfer in electrons and lattice is described using a two-temperature model. The solid–liquid interfacial velocity, as well as elevated melting temperature and depressed solidification temperature, is obtained by considering the interfacial energy balance and nucleation dynamics. An iterative procedure based on energy balance and gas kinetics law to track the location of liquid–vapor interface is utilized to obtain the material removal by vaporization. The effect of surface heat loss by thermal radiation was discussed. The influences of laser fluence and duration on the evaporation process are studied. Results show that higher laser fluence and shorter laser pulse width lead to higher interfacial temperature, deeper melting and ablation depths.     

蒸气压对激光辐照靶材烧蚀速率的影响

研究了强激光辐照碳/碳复合材料靶材引起的烧蚀现象及蒸气压对烧蚀速率的影响。基于傅里叶定律,建立了强激光辐照靶材的热传导模型,模拟了忽略蒸气压影响时烧蚀速率随功率的变化;通过Mott-smith近似方法描述了Knudsen层间断区域,分析了间断两侧表面粒子状态参数;结合质量连续方程和蒸气压与温度关系方程,并由气体状态方程描述蒸气流状态,对蒸气压条件下激光烧蚀碳/碳复合材料靶材的速率随功率变化的关系进行了数值模拟。结果表明,在高能激光对靶材的烧蚀过程中,蒸气压力变化会导致靶材的饱和蒸气温度发生变化,进而影响烧蚀速率且使其随功率呈非线性变化,与忽略蒸气压作用时的线性变化规律相差较大,从理论上解释了忽略蒸气压导致的实验数据与理论结果的差异。     

Explosive vaporization of a water layer on a flat microheater

Explosion vaporization of a water layer of different thickness, induced by pulse heating in an inhomogeneous temperature field on the surface of a flat microheater coated with a submicron silicon-carbide layer, is experimentally studied. An optical method is used for recording the vaporization time history and dynamics of the steam blanket. Pulsed laser irradiation is applied for high-time-resolution photography of the vaporization process. The dynamics of filling the heater surface with the vapor phase and the lifetime of the main vapor bubble and the satellite bubble are estimated. Dependences of the vaporization temperature on the heater temperature growth are obtained. The initial temperature is 30°C; the temperature growth rate on the heater surfaces is about 180 MK/s.     

Thermal dynamics-based mechanism for intense laser-induced material surface vaporization

Laser material processing involving welding, ablation and cutting involves interaction of intense laser pulses of nanosecond duration with a condensed phase. Such interaction involving high brightness radiative flux causes multitude of non-linear events involving thermal phase transition at soild-liquid-gas interfaces. A theoretical perspective involving thermal dynamics of the vaporization process and consequent non-linear multiple thermal phase transitions under the action of laser plasma is the subject matter of the present work. The computational calculations were carried out where titanium (Ti) was treated as a condensed medium. The solution to the partial differential equations governing the thermal dynamics and the underlying phase transition event in the multiphase system is based on non-stationary Eulerian variables. The Mach number M depicts significant fluctuations due to thermal instabilities associated with the laser beam flux and intensity. A conclusive amalgamation has been established which relates material surface temperature profile to laser intensity, laser flux and the pressure in the plasma cloud.      

激光辐照下金属靶表面汽化现象研究

用高速纹影诊断技术研究激光辐照下金属表面发生汽化的特征及蒸气发展运动过程,结合汽化模型给出了凝聚相表面温度与激光入射功率密度的关系。由实验得到了不同入射激光功率密度下汽化表面(凝聚相)温度,蒸气羽阵面速度,前驱空气冲击波速度。     

Vapor Flows Along a Plane Condensed Phase with Weak Condensation in the Presence of a Noncondensable Gas

A steady flow of a vapor in a half space condensing at incidence onto a plane condensed phase is considered in the case where another gas that does not condense (the noncondensable gas) is present near the condensed phase. A systematic asymptotic analysis of the Boltzmann equation for hard-sphere molecules is performed in the case where condensation is weak, and the relation among the parameters of the vapor flow at infinity, those associated with the plane condensed phase, and the amount of the noncondensable gas is derived in an analytical form. The result supplements the numerical result for the relation for arbitrarily strong condensation obtained on the basis of a model Boltzmann equation and under the restriction that the vapor molecules are mechanically identical with the noncondensable-gas molecules [Taguchi et al., Phys. Fluids 15: 689 (2003)]. PACS numbers: 47.45.-n, 51.10.+y, 05.20.Dd, 05.60.-k     

背景气体对金属原子二维平面蒸发过程的影响

在原子蒸气法激光分离同位素中,金属原子蒸气宏观物理性质的空间分布会直接影响到分离过程的电离率和原料利用率.本文从分离过程的实际需求出发,建立了双组分气体的Bhatnagar-Gross-Krook模型方程组,并利用数值计算方法对方程进行求解,研究了背景气体对二维平面蒸发过程中原子蒸气宏观物理性质和蒸发速率的影响.研究结果表明:随着背景气体密度的增加,远离蒸发源位置处的金属原子蒸气密度增大,速度减小,温度升高,而近蒸发源位置处原子蒸气的性质则几乎不受影响,因而蒸发速率基本上不随背景气体密度发生变化.另外,随着尾料板温度的升高和对原子蒸气吸收率的增加,金属原子蒸气宏观物理性质受背景气体的影响逐渐下降.理论计算的结果对于分离装置的真空设计和光斑分布设计有较为重要的参考意义.     

Completely Integrable Equation for the Quantum Correlation Function of Nonlinear Schrödinger Equation

Correlation functions of exactly solvable models can be described by differential equations [1]. In this paper we show that for the non-free fermionic case, differential equations should be replaced by integro-differential equations. We derive an integro-differential equation, which describes a time and temperature dependent correlation function of the penetrable Bose gas. The integro-differential equation turns out to be the continuum generalization of the classical nonlinear Schr?dinger equation. Received: 15 January 1997 / Accepted: 21 March 1997     

Modeling of HMX combustion

A mechanism of HMX combustion was proposed and the corresponding model was developed under the assumption that the combustion wave consists of two zones, with consideration given to the reaction of decomposition and vaporization of the initial energetic material in the condensed phase and the subsequent decomposition of its vapor in the gas phase. An analysis of the results showed that, at low pressures, the burning rate is largely determined by the exothermic decomposition of the material in the condensed phase, but at pressure above ∼20 atm, the processes in the gas phase begin to play an increasingly important role, where the limiting process is the bimolecular activation reaction with the subsequent dissociation of HMX accompanied by the secondary reactions between the products. A comparison of the calculation results with experimental data showed that the model adequately describes a number of characteristics of the combustion wave and ballistic properties, such as the burning rate and its sensitivity to pressure and initial temperature.     

An alkali metal vapor laser amplifier

The operation of a transversely diode-pumped alkali metal vapor laser amplifier is theoretically studied. The amplifier operation is described by a rather intricate system of differential equations, which can be solved in the general case only numerically. In the case of intense incident radiation, an analytic solution is obtained which makes it possible to determine any energy characteristics of the laser amplifier and to find the optimal parameters of the active medium and pump radiation (temperature, buffer gas pressure, and intensity and width of the pump radiation spectrum).     

Influence of pressure-driven gas permeation on the quasi-steady burning of porous energetic materials

A theoretical two-phase-flow analysis is developed to describe the quasi-steady propagation, across a pressure jump, of a multi-phase deflagration in confined porous energetic materials. The difference, or overpressure, between the upstream (unburned) and downstream (burned) gas pressure leads to a more complex structure than that which is obtained for an unconfined deflagration in which the pressure across the multi-phase flame region is approximately constant. In particular, the structure of such a wave is shown by asymptotic methods to consist of a thin boundary layer characterized by gas permeation into the unburned solid, followed by a liquid-gas flame region, common to both types of problem, in which the melted material is preheated further and ultimately converted to gaseous products. The effect of gas flow relative to the condensed material is shown to be significant, both in the porous unburned solid as well as in the exothermic liquid-gas melt layer, and is, in turn, strongly affected by the overpressure. Indeed, all quantities of interest, including the burn temperature, gas velocity and the propagation speed, depend on this pressure difference, leading to a significant enhancement of the burning rate with increasing overpressure. In the limit that the overpressure becomes small, the pressure gradient is insufficient to drive gas produced in the reaction zone in the upstream direction, and all gas flow relative to the condensed material is directed in the downstream direction, as in the case of an unconfined deflagration. The present analysis is particularly applicable to those types of porous energetic solid, such as degraded nitramine propellants that can experience significant gas flow in the solid preheat region and which are characterized by the presence of exothermic reactions in a bubbling melt layer at their surfaces.     

Determination of temperature and concentration of a vapor–gas mixture in a wake of water droplets moving through combustion products

Characteristic temperatures and concentrations of a vapor–gas mixture in a wake of water droplets moving through combustion products (initial temperature 1170 K) were determined using the Ansys Fluent mathematical modeling package. We investigated two variants of motion: motion of two droplets (with sizes from 1 mm to 3 mm), consecutive and parallel, and motion of five staggered droplets. The influence of the relative position of droplets and also of distances between them (varied from 0.01 mm to 5 mm) on temperatures and concentrations of water vapor was established. The distances determine the relation between the evaporation areas and the total volume occupied by a droplet aggregate in the gas medium. The results of modeling for conditions that take into account vaporization on the droplet surface at average constant values of evaporation rate and also with consideration of the change in the latter, depending on the droplet temperature field, are compared. We determined conditions under which the modeling results are comparable for the assumption of a constant vaporization rate and with regard to the dependence of the latter on temperature. The earlier hypothesis on formation of a buffer vapor layer (“thermal protection”) around a droplet, which decreases the thermal flow from the external gas medium, was validated.     

Dynamics of a vapor bubble in a nonuniformly superheated fluid at high superheat values

Results of numerical simulation of the growth of a vapor bubble in a nonuniformly superheated liquid are presented. The effect of the nonuniformity of the temperature on the growth dynamics of the vapor bubble is studied. The simulation conditions corresponded to saturation and underheating of the liquid in the volume to the saturation temperature. The nonuniformity of the temperature results in a significant decrease of the bubble growth rate at the thickness of the superheating layer, which is comparable with the radius of the separation bubble. Numerical results are compared with the experimental data for the growth of a vapor bubble near a cylindrical heater. The numerical results for strong superheating agree well with the experimental data at the initial stage of the vapor bubble growth. The measured values of the bubble radius exceed those calculated in the presence of vaporization fronts. This excess can be explained by the presence of an additional supply of vapor to the central bubble from the vaporization front.     

激光辐照下复合材料树脂基热分解3维温度场模型

采用控制体积法,从质量和能量守恒角度,推导了激光辐照下复合材料树脂基热分解时温度变化的基本方程。用多步模型描述树脂的热分解反应。提出了热分解气体的1维运动假设,这使得可以在3维情况下考虑热分解气体对流传输的影响,且在不引入力学量的前提下实现3维温度场模型的封闭。考虑到树脂基复合材料的各向异性,对部分分解材料的导热率计算公式进行了推导,并重新推导了部分分解材料对激光的吸收系数计算公式。在一定的简化下,推导的能量守恒方程与文献结果一致。     

Oscillatory regimes of vapor condensation

The problem of the condensation of supersaturated vapor in an open system at a constant rate of production of a monomer and a continuous flow of a carrier gas that removes the products of condensation from the system is considered. It is shown both analytically and by numerical experiment that with a decreasing rate of the carrier gas to below a critical magnitude, the condensation regime becomes oscillatory; namely, time oscillations of the cluster-size distribution in the vapor to be condensed are set in. The cause of the phenomenon is in the suppression of the rate of nucleation and the presence of large clusters.     

Free Massive Fermions¶Inside the Quantum Discrete sine-Gordon Model

We extend the notion of space shifts introduced in [FV3] for certain quantum light cone lattice equations of sine-Gordon type at root of unity (e.g. [FV1,FV2,BKP,BBR]). As a result, we obtain a compatibility equation for the roots of central elements within the algebra of observables (also called current algebra). The equation, which is obtained by exponentiating these roots, is exactly the evolution equation for the?“classical background” as described in [BBR]. As an application for the introduced constructions, we derive a one to one correspondence between a special case of the quantum light cone lattice equations of sine-Gordon type and free massive fermions on a lattice, as a special case of the lattice Thirring model constructed in [DV]. Received: 2 December 1996 / Accepted: 19 January 1999     

Mechanisms of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation

For the last decade, a variant of pulsed laser ablation, Resonant-Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE), has been studied as a deposition technique for organic and polymeric materials. RIR-MAPLE minimizes photochemical damage from direct interaction with the intense laser beam by encapsulating the polymer in a high infrared-absorption solvent matrix. This review critically examines the thermally-induced ablation mechanisms resulting from irradiation of cryogenic solvent matrices by a tunable free electron laser (FEL). A semi-empirical model is used to calculate temperatures as a function of time in the focal volume and determine heating rates for different resonant modes in two model solvents, based on the thermodynamics and kinetics of the phase transitions induced in the solvent matrices. Three principal ablation mechanisms are discussed, namely normal vaporization at the surface, normal boiling, and phase explosion. Normal vaporization is a highly inefficient polymer deposition mechanism as it relies on collective collisions with evaporating solvent molecules. Diffusion length calculations for heterogeneously nucleated vapor bubbles show that normal boiling is kinetically limited. During high-power pulsed-FEL irradiation, phase explosion is shown to be the most significant contribution to polymer deposition in RIR-MAPLE. Phase explosion occurs when the target is rapidly heated (10⁸ to 10¹⁰ K/s) and the solvent matrix approaches its critical temperature. Spontaneous density stratification (spinodal decay) within the condensed metastable phase leads to rapid homogeneous nucleation of vapor bubbles. As these vapor bubbles interconnect, large pressures build up within the condensed phase, leading to target explosions and recoil-induced ejections of polymer to a near substrate. Phase explosion is a temperature (fluence) threshold-limited process, while surface evaporation can occur even at very low fluences.     

A model of laser ablation with temperature-dependent material properties,vaporization, phase explosion and plasma shielding

Laser ablation of metals using nanosecond pulses occurs mainly due to vaporization. However, at high fluences, when the target is heated close to its critical temperature, phase explosion also occurs due to homogeneous nucleation. Due to a wide variation in target temperature, the material properties also show a considerable variation. In this paper, a model of laser ablation is presented that considers vaporization and phase explosion as mechanisms of material removal and also accounts for the variation in material properties up to critical temperature using some general and empirical theories. In addition, plasma shielding due to inverse bremsstrahlung and photo-ionization is considered. The model predicts accurately (within 5 %) the phase explosion threshold fluence of Al. The predictions of ablation depth by the model are in reasonable agreement with experimental measurements at low fluences. Whereas, the degree of error marginally increases at high laser fluences.     

Excimer laser deposition of thin amorphous carbon films

A new carbon film deposition technique, based upon excimer laser vaporization of graphite in a flowing gas system has been developed. The low temperature vapor (LTV) technique alleviates high temperatures occurring in most other deposition methods. In this technique the UV laser ablation occurs in an inert flowing gas atmosphere. Atoms and molecules evaporated from graphite are cooled by gas entrainment before condensing on a substrate. The resulting films of amorphous carbon or hydrogenated amorphous carbon are free from strain. Measurement of the optical band gap of these films shows that E_g can be controlled by the hydrogen content of the carrier gas.     

Modeling of multicomponent-fuel drop-laden mixing layers having a multitude of species

A formulation representing multicomponent-fuel (MC-fuel) composition as a probability distribution function (PDF) depending on the molar mass is used to construct a model of a large number of MC-fuel drops evaporating in a gas flow, so as to assess the extent of fuel specificity on the vapor composition. The PDF is a combination of two Gamma PDFs, which was previously shown to duplicate the behavior of a fuel composed of many species during single drop evaporation. The conservation equations are Eulerian for the flow and Lagrangian for the physical drops, all of which are individually followed. The gas conservation equations for mass, momentum, species, and energy are complemented by differential conservation equations for the first four moments of the gas-composition PDF; all coupled to the perfect gas equation of state. Source terms in all conservation equations couple the gas phase to the drops. The drop conservation equations for mass, position, momentum, and energy are complemented by differential equations for four moments of the liquid-composition PDF. The simulations are performed for a three-dimensional mixing layer whose lower stream is initially laden with drops. Initial perturbations excite the layer to promote the double pairing of its four initial spanwise vortices to an ultimate vortex. The drop temperature is initially lower than that of the surrounding gas, initiating drop heating and evaporation. The results focus on both evolution and the state of the drops and gas when layers reach a momentum-thickness maximum past the double vortex pairing; particular emphasis is on the gas composition. Comparisons between simulations with n-decane, diesel, and three kerosenes show that at same initial Reynolds number and Stokes number distribution, a single-component fuel cannot represent MC fuels. Substantial differences among the MC-fuel vapor composition indicate that fuel specificity must be captured for the prediction of combustion.