Late-time radiography of beryllium ignition-target ablators in long-pulse gas-filled hohlraums

A multiple-laboratory campaign is underway to qualify beryllium as a fusion capsule ablator for the National Ignition Facility [Moses and Wuest, Fusion Sci. Technol. 43, 420 (2003)]. Although beryllium has many advantages over other ablator materials, individual crystals of beryllium have anisotropic properties, e.g., sound speed, elastic constants, and thermal expansion coefficients, which may seed hydrodynamic instabilities during the implosion phase of ignition experiments. Experiments based on modeling have begun at the OMEGA laser [Boehly, McCrory, Verdon et al., Fusion Eng. Design 44, 35 (1999)] to create a test bed for measuring instability growth rates with face-on radiography of perturbed beryllium samples with the goal of establishing a specification for microstructure in beryllium used as an ablator. The specification would include the size and distribution of sizes of grains and voids and the impurity content. The experimental platform is a 4kJ laser-heated (for ∼6ns) hohlraum that is well modeled for radiation temperature and for shock pressure and breakout timing through the driven beryllium sample. A 1atm methane gas fill has been used to maintain a clear line of sight through the hohlraum for radiography with acceptable plasma backscatter losses. The peak radiation temperature is 145eV; the pressure early in the laser pulse is 1Mbar for over 1ns. Radiographs of sinusoidally perturbed copper-doped (0.9% by atom) beryllium samples have been obtained more than 10ns after drive initiation. With the current laser drive, a growth factor approaching ten has been measured for initial 2.5μm perturbations with on-axis radiography.