Hydrodynamical evolution and radiation confinement of rough surfaces exposed to thermal radiation in indirectly driven ICF targets

Abstract

2–D hydrodynamical simulations have been used to study the effects of surface finish on the evolution of planar analogs for the components of indirect-driven inertial confinement fusion (ICF) targets (cavity surfaces and capsules) when exposed to the same uniform, assigned, temporally profiled thermal radiation bath. For the time-dependence of the radiation temperature was assumed to be the one resulting from 1-D simulations of a highgain indirectly driven target. The perturbation wavelength has been taken in the interval 2.5–40 μm, as suggested by the typical lengths appearing in the 1-D hydrodynamic evolution. The results for the evolution of 300 Å initial perturbation amplitude will be presented, because the results obtained for other amplitudes (somewhat smaller or larger) are qualitatively the same. The results do not seem very encouraging for the indirect-drive approach, at least in the form presented in the current literature, as the simulations show the onset of fast-growing hydrodynamical unstable modes, both on the capsule surface and on the cavity surface, producing dramatic effects on symmetry and radiation confinement.With regard to the (gold) cavity surface, for the shortest wavelengths the evolution enters the nonlinear phase in the early stages of the interaction, producing (due to surface area increase and thermal field rippling) a decrease by a factor of 3–4 of a conventionally defined radiation confinement parameter. For these same wavelengths stabilization mechanisms due to ablative polishing, finite gradient, or overpressure tend to be ineffectual. Similar results are obtained for the capsule surface at short wavelengths. In this case, in addition to an enhancement (up to factor of 10) of the ablation rate (and energy absorption) due to surface area increase and ripple effects, the flow behind the first shock wave turns out to be turbulent, with “eddies” having typical products of angular velocity x time of the order of unity or greater. The situation is not encouraging even for longer wavelengths; in this case, in addition to the generation of eddies in the fuel, the flow exhibits a transverse matter pile-up with consequent generation of holes in the thermal shield and premature fuel preheating.