Numerical simulation of laser–target interaction and blast wave formation

Abstract

A numerical hydrodynamics chemistry model to simulate the laser–target interaction experiment at the Naval Research Laboratory’s PHAROS [Laser Interaction and Related Plasma Phenomena (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 ∼5 nsec full width at half-maximum (FWHM), an intensity at the target surface ∼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−3, temperatures ranging from 10–20 eV, and dominant nitrogen species of N+3 and N+4, all in agreement with experimental measurement. A calculated profile of electron density both in the shock and in the cavity region agree closely with experiment and imply an average aluminum charge state of 11 times ionized in the cavity out to late times, as predicted by the simulation described in this paper. The simulation suggests, also, that observed rear-side structuring is a result of a deceleration Rayleigh–Taylor instability. The model is capable of providing detailed predictions, which are presented, as to profiles of charge states, densities, and temperatures as a function of time; these predictions are not yet tested by experimental measurement.

Laser-produced blast wave and numerical simulation using the FLASH code

“…Since a single temperature is only available in FLASH, we cannot comment on the error caused in the simulation due to ion and electron non-equilibrium effects. However, Giuliani et al~1989! did follow both ion and electron temperatures in a similar experiment, and the temperatures appear to be generally similar and in near equilibrium, especially in the blast front region.…”

Charge-exchange pumping of laser-produced plasma colliding with vapour cloud for lasing in XUV

“…The reason for such a low efficiency was clarified in other experiments (for example [3]) and especially in numerical simulations [4]. At first millimetres from the surface of plasma production, the background gas is very rapidly ionized by photons and hot electrons.…”