Computer Modelling of Hydrodynamic Instability in Spherical Laser-Accelerated Targets

Available from UMI in association with The British Library. The extent to which the hydrodynamic instability known as Rayleigh-Taylor instability disrupts the symmetry of laser imploded targets is a question of crucial importance to the viability of the inertial confinement fusion scheme. The computer code POLLUX has been used to model the implosion of spherical plastic shells in order to study the growth of the instability at the outer (ablation) surface. Such shells would comprise part of a laser fusion target. A detailed study of the non-linear growth stage has been made and the break up of targets due to the penetration of Rayleigh-Taylor bubbles has been examined. The non -linear growth of the instability in spherical targets has been found to obey a scaling law of the form: A = sigma>^2 where A is the width of the mix region, g the acceleration and t is time; the theoretical basis of this law is discussed and simulation results indicate that the scaling parameter (sigma) is of the order of 0.1. Comparison with instability growth in planar targets has revealed differences in the non-linear growth due to convergence effects and these have been identified. A study of the susceptibility to the instability of various laser/target configurations has been carried out in a parameter space relevant to laser fusion. It is argued on a theoretical basis, and confirmed by the simulations, that perturbations with an amplitude greater than about 1/50 of their wavelength assume a non-linear growth behaviour, rather than the faster linear growth rate, almost immediately. Most experiments to investigate the instability fall into this category. Perturbations applied to the target surface are imprinted throughout the entire target by the transit of the initial shock wave. The mechanism of this effect is discussed and it is inferred from this that considerably larger perturbations can be tolerated when introduced at the critical surface of a uniformly formed blowoff plasma than can be applied directly at the target surface, this is supported by simulation results.