Capsules with beryllium ablators have long been considered as alternatives to plastic for the National Ignition Facility laser ; now the superior performance of beryllium is becoming well substantiated . Beryllium capsules have the advantages of relative insensitivity to instability growth, low opacity, high tensile strength, and high thermal Zimmerman and W. L. h e r , Comments Plasmas Phys. Controlled Thermonucl. Fusion, 2 , 5 1 (2975)l results that particular beryllium capsule designs are several times less sensitive than the CH point design to instability growth from DT ice roughness. These capsule designs contain more ablator mass and leave some beryllium unablated at ignition. By adjusting the level of copper dopant, the unablated mass can increase or decrease, with a corresponding decrease or increase in sensitivity to perturbations . A plastic capsule with the same ablator mass as the beryllium and leaving the same unablated mass also shows this reduced perturbation sensitivity. Beryllium's low opacity permits the creation of 250 eV capsule designs. Its high tensile strength allows it to contain DT fuel at room temperature. Its high thermal conductivity simplifies cryogenic fielding.

Observation of perturbation coupling between a Richtmyer-Meshkov-unstable interface on the cold surface of a radiatively-driven foil and the Rayleigh-Taylor-unstable hot surface is reported. For the 50 m wavelength studied, the combination of pulse length and foils thickness was found to affect the strength of instability coupling. Thick ͑86 m͒ foils with a 2.2 ns long pulse showed weak coupling between the two instabilities, while thin ͑35 m͒ foils showed strong, fast coupling. An intermediate ͑50 m͒ foil thickness with a cooler, 4.5 ns pulse showed a transition from weak to strong coupling during the pulse duration. Radiation-hydrodynamic simulations are in agreement with the experiments and provide insight into the coupling phenomenon.

Coupling between Richtmyer–Meshkov- and Rayleigh–Taylor-unstable surfaces of indirectly driven, stratified, planar foils composed of an aluminum ablator and a beryllium payload carrying a machined initial perturbation was studied using radiographic techniques. The beryllium was transparent to the x-ray backlighter, allowing clear observation of the perturbation transport into the aluminum. The maximum perturbation amplitude (areal–mass difference) was smaller in the aluminum/beryllium foils than in equivalent pure aluminum foils, which could have been the result of shorter growth time, a lower differential acceleration coupling, or the density discontinuity at the aluminum/beryllium interface. Second and third harmonics were not observed in the composite foils.

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