Theory and numerical modeling of the accelerated expansion of laser-ablated materials near a solid surface

A self-similar theory and numerical hydrodynamic modeling is developed to i nvestigate the effects of dynamic source and partial ionization on the acce leration of the unsteady expansion of laser-ablated material near a solid t arget surface. The dynamic source effect accelerates the expansion in the d irection perpendicular to the target surface, while the dynamic partial ion ization effect accelerates the expansion in all directions. The vaporized m aterial during laser ablation provides a nonadiabatic dynamic source at the target surface into the unsteady expanding fluid. For studying the dynamic source effect, the self-similar theory begins with an assumed profile of p lume velocity, u = v/v(m) = alpha + (1 - alpha)xi, where v(m) is the maximu m expansion velocity, alpha is a constant, and xi = x/v(m)t. The resultant profiles of plume density and plume temperature are derived. The relations obtained from the conservations of mass, momentum, and energy, respectively , all show that the maximum expansion Velocity is inversely proportional to alpha, where 1 - alpha is the slope of plume velocity profile. The numeric al hydrodynamic simulation is performed with the Rusanov method and the New ton Raphson method. The profiles and scalings obtained from numerical hydro dynamic modeling are in good agreement with the theory. The dynamic partial ionization requires ionization energy from the heat at the expansion front , and thus reduces the increase of front temperature. The reduction of ther mal motion would increase the flow velocity to conserve the momentum. This dynamic partial ionization effect is studied with the numerical hydrodynami c simulation including the Saha equation. With these effects, alpha is redu ced from its value of conventional free expansion. This reduction on alpha increases the flow velocity slope, decreases the flow velocity near the sur face, and reduces the thermal motion of plume, such that the maximum expans ion velocity is significantly increased over that found from conventional m odels. The result may provide an explanation for experimental observations of high-expansion front velocities even at low-laser fluence. [S0163-1829(9 9)08835-9].