| In Inertial Confinement Fusion (ICF), a spherical shell is accelerated inward by laser irradiation, and the shell outer surface is unstable to the Rayleigh-Taylor (RT) instability. During the acceleration phase, the instability can be seeded by both inner and outer surface nonuniformities. The seeding of the instability by the inner surface nonuniformities (typically referred to as “feedout”) is investigated both analytically and numerically. A simple formula relating the outer surface distortion to the inner surface roughness is derived for long wavelength perturbations satisfying the condition kd < 1, where k is the perturbation wavenumber, d is the shell thickness. The validity of the analytic feedout formula derived for long wavelength modes is extended to the short wavelengths by fitting the results of two-dimensional Lagrangian simulation.; When the laser is turned off and the shell is decelerated by the large pressure building up in the center of the capsule, the shell inner surface is RT unstable. The dynamics of the shell and hot-spot during the deceleration phase is investigated both numerically and analytically. It is shown that mass ablation off the shell inner surface significantly reduces the growth rates of the deceleration phase Rayleigh-Taylor instability. It is also found that for typical direct drive capsules designed for the National Ignition Facility, the instability of Legendre modes with l > 90 is suppressed by the ablative stabilization. |
