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, can occasionally resume at a later stage of the pulse, and proceeds nonuniformly in the spot area, and the major contribution to the mass removal occurs in the outer part of the irradiated region. Plasma induced by the 0.248μm laser is colder and partially transparent since it transmits 30%–70% of the incident radiation; therefore evaporation does not stop but continues in the subsonic regime with the Mach number of about 0.1. The amount of evaporated matter that condenses back to the surface is as high as 15%–20% and less than 10% for the 1.06 and 0.248μm radiations, respectively. For a beam radius smaller than ∼100μm, the screening and retarding effect of the plasma weakens because of the lateral expansion, thickness of the removed layer increases, and condensation after the end of the pulse is not observed. Comparison of the numerical and experimental results on the removed layer thickness has shown, in particular, the importance of accounting for the plasma effect to predict the correct trends for radiation intensity and beam radius.

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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 both cases the plasma becomes opaque: for the IR effect it occurs over a very short period of time, 3–5 ns, while for the UV effect the process takes much longer. When the laser intensity is increased, the peak intensity and the duration of the transmitted pulse tend to reach a saturation level.

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.

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