The ablation-front Rayleigh-Taylor dispersion curve in indirect drive

The Rayleigh-Taylor (RT) instability, which occurs when a lower-density flu id accelerates a higher-density layer, is common in nature. At an ablation front a sharp reduction in the growth rate of the instability at short wave lengths can occur, in marked contrast to the classical case where growth ra tes are highest at the shortest wavelengths. Theoretical and numerical inve stigations of the ablative RT instability are numerous and differ considera bly on the level of stabilization expected. Presented here are the results of a series of laser experiments designed to measure the RT dispersion curv e for a radiatively driven sample. Aluminum foils with imposed sinusoidal p erturbations ranging in wavelength from 10 to 70 mum were ablatively accele rated with a radiation drive generated in a gold cylindrical hohlraum. A st rong shock wave compresses the package followed by an similar to2 ns period of roughly constant acceleration and the experiment is diagnosed via face- on radiography. Perturbations with wavelengths greater than or equal to 20 mum experienced substantial growth during the acceleration phase while shor ter wavelengths showed a sharp drop off in overall growth. These experiment al results compared favorably to calculations with a two-dimensional radiat ion-hydrodynamics code, however, the growth is significantly affected by th e rippled shock launched by the drive. Due to the influence of the rippled shock transit phase of the experiment and ambiguities associated with direc tly extracting the physical amplitude of the perturbations at the ablation front from the simulations, direct comparison to the ablation front RT theo ry of Betti [Phys. Plasmas 5, 1446 (1998)], was difficult. Instead, a numer ical "experiment" was constructed that minimized the influence of the shock and this was compared to the Betti model showing quite good agreement. (C) 2001 American Institute of Physics.