Numerical Simulation of the Laser-Target Interaction and Blast Wave Formation in the DNA/NRL (Defense Nuclear Agency/Naval Research Laboratory) Laser Experiment

摘要

A numerical simulation of the ablation of a planar, aluminum foil target by a neodymium laser in the presence of a 2.5 Torr nitrogen background gas is presented. The simulation includes inverse bremsstrahlung absorption during the laser pulse, the cold isotherm in the equation of state for solid aluminum, non-equilibrium chemistry for both the target material and the background gas, radiation transport of the continuum, ion and electron energy equations, and blast wave formation in the background gas. The simulation is followed out to > 50 nsec from the peak of the laser pulse. Although the Lagrangian code is 1-D, the divergence of the ablated target material expanding toward the laser (forward) as well as the rearward acceleration of the remaining target is accounted for by using an oblate spheroidal coordinate system. The primary results of the simulations are as follows: (i), by about 50 nsec a cavity with T sub e about 60 ev and ne about 2 times 10 to the 17th power 1cc has formed behind the denser and cooler forward moving blast wave; (ii), this cavity is not in chemical equilibrium but highly overionized for its temperature with predominantly Al(+9),Al(+10),Al(+11),N(+5) and N(+6); and (iii), the conditions for a deceleration driven Rayleigh Taylor instability are satisfied at the rear blast wave near one equal mass radius. This last feature may account for the experimentally observed floculli on the rearward side. Keywords: Laser plasmas; Blast wave; Numerical simulation; and Raleigh Taylor instability.