Gas dynamics and radiation heat transfer in the vapor plume produced by pulsed laser irradiation of aluminum

The interaction of pulsed laser irradiation of nanosecond duration with a metal surface is studied by numerical simulation. The heat transfer in the solid substrate and the melted liquid is modeled as one-dimensional transient heat conduction using the enthalpy formulation for the solution of phase change problems. A discontinuity layer is assumed just above the liquid surface. Mass, momentum, and energy conservation are expressed across this layer, while the vapor across the discontinuity is modeled as an ideal gas. The compressible gas dynamics is computed numerically by solving the system of Euler equations for mass, momentum, and energy, supplemented with an isentropic equation of state in a two-dimensional axisymmetric system of coordinates. The excimer laser-beam absorption and radiation transport in the vapor phase are modeled using the discrete ordinates method. The rates for ionization are computed using the Saha-Eggert equation assuming conditions of local thermal equilibrium. The inverse bremsstrahlung mechanism is considered as the main mechanism of plasma absorption. Results show that a thin, submicron vapor layer is formed above the target surface in the duration of laser pulse while thermal radiation plays the key role for plume cooling during the period of strong absorption by the plasma. The release of a very strong shock wave, propagating with a speed of 10(4) m/s, is observed in the evaporating plume. (C) 1996 American Institute of Physics.