Experimental determination of the hydrodynamic instability growth rates in indirect and direct drive ICF

摘要

A comprehensive set of measurements of the growth of ablatively stabilized Rayleigh Taylor instabilities in laser driven direct drive and indirectly drive planar foils is presented. For both cases, imposed single Fourier modes are seen to grow as a planar foil is accelerated. Harmonics of the imposed wavelength are observed as the sinusoidal perturbation becomes a classic bubble and spike. Independent measurements of the foil acceleration are made by sidelighting the foils. For small amplitude initial perturbations, a shaped indirect drive has been used to achieve growth rates as large as 80. Two dimensional simulations of the foils demonstrate excellent agreement with the measured growth rates and non-linear saturation. For both direct and indirect drive, the small amplitude growth rates are seen to be in agreement with simple dispersion relations (gamma) = (alpha) (radical)kg - (beta) kv(sub a) albeit with different values of the constants for the two cases. Further experiments with radiatively accelerated planar foils with two imposed modes of wavenumber k1 and k2 show mode coupling with sum and difference modes, again in agreement with theory. Experiments have also been performed measuring the modulations in areal density of unperturbed foils. The early time non-uniformity of the laser beam can be varied by changing the bandwidth of the SSD smoothing scheme. As the smoothing of the laser drive beam is increased, the degree of non-uniformity of the accelerated foil decreases. For indirect drive there is no measurable non-uniformity of an initially smooth accelerated foil. Indirect drive experiments need a rough foil with a 4 (mu)m surface finish to see any growth of Rayleigh Taylor non-uniformity.