Plasma dynamics in air during laser shock processing of materials

The gas-dynamic expansion stage of plasma at the air-water interface is studied numerically for the set-up corresponding to the Laser Shock Processing (LSP) of materials in the water-confined regime. Plasma is induced by a laser radiation of the intensity range 4–17 GW/cm$^2$ at the 1.06 $\mu$m and 0.353 $\mu$m laser wavelength. A mathematical description of plasma is performed in the frame of transient 2D 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 ionisation wave toward the laser source, and for the UV laser effect the Laser Supported Detonation (LSD) 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 pre-ionisation ahead of the shock wave. In both cases 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.