Igor Smurov

Thermal model of nanosecond pulsed laser ablation: Analysis of energy and mass transfer

A thermal model of nanosecond laser ablation considering kinetics of surface evaporation is proposed. Equations concerning heat transfer in the target and associated gas dynamics are coupled by mass and energy balances at the surface and Knudsen layer conditions. Rigorous analysis of gas-dynamics related to condensation at the target surface is introduced in this model. Laser energy absorbed by the target is partly spent for evaporation and partly dissipated in the target by thermal conduction. The sum of thermal and kinetic energies of the gas phase is, usually, less than the energy of evaporation. The fraction of energy lost for target heating increases with decrease in laser fluence and attains 100% at the ablation threshold. The dependence of ablated depth on fluence is, thus, determined by energy partition between the solid and gas phases. The gas-dynamic flow accompanying ablation consists of a layer of compressed high-temperature vapor adjacent to the target that expands and pushes the ambient gas ...

Gas dynamics of laser ablation: Influence of ambient atmosphere

A two-stage two-dimensional (2D) gas-dynamic model of laser ablation in an ambient gas atmosphere is proposed. The initial one-dimensional stage of the process is related to the ablation plume formation under the action of a laser pulse (duration of the order of 10 ns; fluence about several J/cm2; laser spot diameter about 1 mm) and describes heating, melting, and evaporation of the target, the target–vapor interaction in the Knudsen layer, and the vapor dynamics. The final 2D stage is responsible for the formation of the energy and angular distributions of the ablated material. Considerable compression of the ambient gas around the expanding plume of the laser-evaporated material and a shock front propagating through the undisturbed ambient gas are found. The pressure of the compressed ambient gas behind the shock may be much higher than the ambient one. However, at the investigated ambient pressures below 100 Pa, it remains still much lower than the vapor pressure during laser evaporation. Therefore, th...

Gas-dynamic boundary conditions of evaporation and condensation: Numerical analysis of the Knudsen layer

The gas-dynamic Euler equations require two boundary conditions to be specified at the surface of evaporated condensed phase and one condition at the surface of condensation. In the commonly considered three-parameter space of the temperature and pressure ratios and the Mach number this corresponds to a three-dimensional curve in the case of evaporation and to a surface in the case of condensation. To obtain the conditions of evaporation and condensation the steady-state Knudsen layer is numerically studied by the discrete velocity method applied to a Boltzmann equation with a relaxation collision term. Simple models of Mott-Smith type based on the conservation laws and analytical approximations of the velocity distribution function in the Knudsen layer may give satisfactory description of the gas-dynamic evaporation and condensation conditions while in general they inadequately represent the detailed structure of the distribution function. One of the reasons why the models deviate from the calculations is that they do not allow different parallel and perpendicular temperatures of the velocity distribution. Under evaporation, the Knudsen layer thickness increases with the Mach number M. Under condensation, it is inversely proportional to M when M is low. Numerical results are obtained and an analytical model is proposed for the vapor temperature considerably less than the condensed phase one (up to 10 times) what is typical for back condensation under pulsed laser ablation.

Ion-assisted deposition of MoSx films from laser-generated plume under pulsed electric field

The thickness profiles and compositional distributions of MoSx films deposited from a plume generated by pulsed laser irradiation of the MoS2 target were investigated at a varying fluence and constant laser pulse energy. It was shown that films with stoichiometric composition were formed at sufficiently low fluence (near the ionization threshold), and increasing fluence caused intricate nonmonotonic variations of the compositional distribution. A substantial deviation of the film composition from stoichiometric and a significant radial gradient of the sulfur concentration over the substrate surface (1<x<3, where x is the ratio of concentrations of S and Mo atoms, x=S/Mo) were found. These phenomena were caused by: (1) the incongruent target evaporation; (2) the mass dependence of the angular distribution of the ablated particles; and (3) the selective sulfur sputtering and desorption induced by energetic particles (ions, excited atoms) of the laser-generated plume. When the laser fluence was low, films of...

A model for nanoparticles synthesis by pulsed laser evaporation

A one-dimensional model of nanoscale particle formation by the exposure of solid targets to millisecond laser pulses of about 104-105 W cm-2 energy density flux is proposed. A `soft regime of vapour diffusion from a laser-heated surface into an ambient atmosphere is studied. Laser flux is presumed to be relatively low, so that the surface temperature does not exceed the boiling point. Heat transport in the solid, liquid and gas phases, and evaporation and vapour diffusion in ambient atmosphere are analysed. The nuclei formation and growth rates are estimated and size distributions of synthesized nanoparticles are found. The influence of operating conditions, such as ambient gas pressure, laser pulse duration and properties of target material, on mean particle size is studied.

Modeling of plasma-controlled evaporation and surface condensation of Al induced by 1.06 and 0.248μm laser radiations

Phase transition on the surface of an aluminum target and vapor plasma induced by laser irradiation in the nanosecond regime at the wavelengths of 1.06μm in the infrared range and 0.248μm in the ultraviolet range with an intensity of 108–109W∕cm2 in vacuum are analyzed. Special attention is paid to the wavelength dependence of the observed phenomena and the non-one-dimensional effects caused by the nonuniform (Gaussian) laser intensity distribution and the lateral expansion of the plasma plume. A transient two-dimensional model is used which includes conductive heat transfer in the condensed phase, radiative gas dynamics, and laser radiation transfer in the plasma as well as surface evaporation and back condensation at the phase interface. It was shown that distinctions in phase transition dynamics for the 1.06 and 0.248μm radiations result from essentially different characteristics of the laser-induced plasmas. For the 1.06μm radiation, evaporation stops after the formation of hot optically thick plasma,...

Optical breakdown in aluminum vapor induced by ultraviolet laser radiation

Theoretical analysis of the evolution of nonequilibrium plasma induced by ultraviolet laser radiation is carried out. Intensity threshold values are studied by mathematical modeling as a function of laser-pulse wavelength. Basic mechanisms of nonequilibrium ionization of aluminum vapor are analyzed and the dominant role of photo-processes, namely, resonant and nonresonant photoexcitation and photoionization, is shown. The modeling results are in good agreement with experimental data on optical breakdown in aluminum vapor by the excimer laser radiation in nanosecond and microsecond range.

Modeling of plasma dynamics at the air-water interface: Application to laser shock processing

The gas-dynamic expansion stage of the plasma at the air-water interface is studied numerically for the setup corresponding to the laser shock processing of materials in the water-confined regime. The plasma is induced by a laser radiation of the intensity range 4–17 GW/cm2 at the 1.06 and 0.353 μm laser wavelength. A mathematical description of the plasma is performed in the frame of transient two-dimensional radiative gas dynamics, which incorporates the system of gas-dynamic equations and the radiation transfer equation. The studies performed indicate that the plasma evolution significantly depends on the laser wavelength. For the IR laser effect the expansion mechanism is the fast propagation of the ionization wave toward the laser source, and for the UV laser effect the laser supported detonation wave is formed. The plasma radiation contributes significantly to the redistribution of energy inside the plasma domain and, for the UV effect, forms the domain of preionization ahead of the shock wave. In b...

Overheated metastable states in pulsed laser action on ceramics

Volume overheating of solid and liquid phases in pulsed laser evaporation of superconducting ceramics is analyzed by numerical simulation. The mathematical model includes the processes of heating (with a volume energy release), melting‐solidification, and evaporation. It is shown that the maximum values of overheating of the solid phase (with respect to its melting point) exceed 100 degrees and those of the liquid phase exceed several hundred degrees (with respect to the surface temperature). The times of existence of these metastable states are tens and hundreds of nanoseconds, respectively. The dynamics of the processes are analyzed in a wide range of variation of the absorption coefficient (i.e., laser wavelength). It is shown that the probability of explosive decay of the metastable states in the solid phase increases with laser wavelength, whereas for the metastable states in the liquid phase the overheating parameters exhibit a maximum versus laser wavelength.

Particles synthesis in erosive laser plasma in a high pressure atmosphere

Synthesis of ultrafine particles as a result of condensation in an erosive jet at laser vaporization of materials (metals, metal oxides, carbon) in gases (hydrogen, oxygen, helium, argon, xenon and air) at high pressures is investigated. The granulometric, phase, and chemical composition of the condensate particles is analyzed in relation to the elaboration conditions. It is found that the mean dimensions of the condensate particles increase with increasing the ambient gas pressure. A particle nucleation and growth theory is used to describe the formation of the dispersed condensate in the erosive plasma. It is shown that the resulting particle size distribution is determined by the cooling rate dependence of the vapor-gas mixture on the pressure of the surrounding gas.