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

Abstract

The Rayleigh–Taylor (RT) instability, which occurs when a lower-density fluid 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 wavelengths can occur, in marked contrast to the classical case where growth rates are highest at the shortest wavelengths. Theoretical and numerical investigations of the ablative RT instability are numerous and differ considerably on the level of stabilization expected. Presented here are the results of a series of laser experiments designed to measure the RT dispersion curve for a radiatively driven sample. Aluminum foils with imposed sinusoidal perturbations ranging in wavelength from 10 to 70 μm were ablatively accelerated with a radiation drive generated in a gold cylindrical hohlraum. A strong shock wave compresses the package followed by an ∼2 ns period of roughly constant acceleration and the experiment is diagnosed via face-on radiography. Perturbations with wavelengths ⩾20 μm experienced substantial growth during the acceleration phase while shorter wavelengths showed a sharp drop off in overall growth. These experimental results compared favorably to calculations with a two-dimensional radiation-hydrodynamics code, however, the growth is significantly affected by the rippled shock launched by the drive. Due to the influence of the rippled shock transit phase of the experiment and ambiguities associated with directly extracting the physical amplitude of the perturbations at the ablation front from the simulations, direct comparison to the ablation front RT theory of Betti et al. [Phys. Plasmas 5, 1446 (1998)], was difficult. Instead, a numerical “experiment” was constructed that minimized the influence of the shock and this was compared to the Betti model showing quite good agreement.