Computer Modelling of Laser-Plasma Ablation.

Available from UMI in association with The British Library. The general aim of this research was to try to characterise the roles of refraction and lateral smoothing in the laser-plasma ablation process. Using a computer model which is capable of treating refraction, absorption and hydrodynamic processes self-consistently. The model includes a specially developed non-uniform grid quasi-Lagrangian rezoning procedure which is used to direct computational resolution to the appropriate flow region as dictated by the current flow state. For computational reasons, the model simulates the irradiation of a planar, rather than a spherical target, and ponderomotive and self-generated magnetic field effects are excluded. Modelling results using laser and target parameters appropriate to fusion studies have shown that significant thermal self-focussing (a refractive instability) occurs. The location of the peak in absorbed energy was not fixed, nor was the magnitude of the peak constant, instead clear fluctuations between strong self-focussing and net defocussing were observed. The origin of these oscillations is as yet unclear. Comparison with results from a similar test in which refraction effects were excluded indicated that self-focussing leads to an increase in ablation pressure non-uniformity and hence that refraction effects could impair implosion symmetry. Lateral thermal conduction was seen to help to reduce this deleterious effect. Parametric studies have shown that strong electron heat flux inhibition produced high ablation pressure non -uniformity. Furthermore, strong self-focussing was favoured by high I, long lambda and short R, and DeltaI and the target thickness were relatively unimportant. However detailed examination shows that thermal smoothing mechanisms were also most effective at high I, long lambda and short R in accordance with earlier studies which neglected refraction. No clear trend in ablation pressure non-uniformity was observed for these parameters due to the conflict between the above two factors, and hence it is not possible to give a simple prescription for the optimum laser and target parameters to use in a laser fusion scheme. (Abstract shortened by UMI.).