Research on Absorption Effects of Laser Energy Due to Plasma Plume Induced by Laser Ablation

Ablaiton of solid target with high power energy can induce laser vapor plasma, which would impart reverse impulse to the target. The physical processes include target heating, melting, vaporization and formation of plasma plume. In this paper, we presented a new numerical model, which described target heating, melting and evaporation. Meanwhile, the ejection of material formed a plasma plume above the surface and expanded into the ambient vacuum. The formed plasma absorbed the laser energy passing through it. The heating of the target was described with a heat conduction equation, which led to the temperature distribution inside the target, as a function of time. When the temperature rises further, vaporization would appear. The vapor velocity and temperature at the surface were used as input for the boundary conditions of plasma plume, which was described with Navier-Stokes equations, for conservation of total vapor mass density, momentum and energy. We considered two dominant absorption mechanisms in the process of plasma shielding, which were electron-ion and electron-neutral inverse Bremsstrahlung. Based on above assumptions, the left laser energy because of plasma shielding was calculated. Results for an aluminum target with Gaussian profile laser pulse with duration of nanosecond were obtained, including the plasma plume temperature, ionization degree, densities of neutral, ions and electrons and laser absorption energy. Results showed that the energy absorption by plasma plume played an important role in the coupling of laser energy and target.