Numerical simulation of laser--target interaction and blast wave formation

A numerical hydrodynamics chemistry model to simulate the laser--target interaction experiment at the Naval Research Laboratory's PHAROS Laser interaction and elated Plasma henomena (Plenum, New York, 1986), Vol. 7, p. 857] is presented. Both laser--target and debris--background interactions are modeled, solving mass continuity, total momentum, and separate ion and electron internal energy equations. The model is appropriate for background densities≥1 Torr. To accurately treat both the early-time planar ablation and the later spherical expansion of the blast wave, as well as the rear-side shock front, an oblate spheroidal coordinate system was adopted. The aluminum target ablates into and interacts with an ambient nitrogen gas, filling the facility chamber. The simulation models the target continuously from the solid state to the state of a highly ionized nonequilibrium plasma, including all charge states of aluminum and all charge states of the nitrogen background. The laser beam has a wavelength of 1 μ, a approximately 5 nsec full width at half-maximum (FWHM), an intensity at the target surface approximately 1013 W/cm2, and total energy varying from 20--100 J. The model accurately reproduces the measured time-of-flight profile and the mass of ablated aluminum. Expansion of the blast wave in the model follows the ideal Sedov relation until radiation losses force a deviation due to a failure in the constant energy assumption. In the shock wave region the simulations show electron density of a few times 1018 cm/sup -3/, temperatures ranging from 10--20 eV, and dominant nitrogen species of N+3 and N+4, all in agreement with experimental measurement