Megabar Shocks in Beryllium-Copper Driven by a NIF-Like Foot Pulse in a Vacuum Halfraum

Summary form only given. An understanding of the material dynamics and EoS characteristics of copper-doped beryllium (Be-Cu) ablators is critical for several ignition designs for the National Ignition Facility (NIF). Beryllium offers several advantages as an ablator: (a) high density, (b) low heat capacity, and (c) low opacity. In addition, dopants can be used to optimize these characteristics, for example, copper increases the density and opacity. In such a design, the Be-Cu ablator is subjected to thermal and soft X-ray radiation, delivering a sequence of shocks which compress a deuterium-tritium capsule. The first shock is expected to be between 0.5 and 8.0 Mbar. Be-Cu is predicted to melt on the Hugoniot between 1 and 2 Mbar and may depend upon the time scale. The anisotropic mechanical and thermal expansion properties of Be-Cu may introduce velocity fluctuations as the microstructure is loaded, and may then seed Rayleigh-Taylor instabilities. The uncertainty in Be-Cu melt and instability-growth dependence upon microstructure impacts the predictive capabilities for capsule performance. We have developed and fielded a series of experiments at the Omega laser facility that closely replicate the NIF-foot hohlraum conditions expected for a Be-Cu ablator. The experiments aim to investigate the seeding of instabilities by microstructure. In these experiments, a holhraum is rapidly brought up to a radiation temperature of 70-90 eV and held there for 2.5-3.2 ns. After ~3.0 ns, the temperature is raised to 120-150 eV in order to drive instabilities. Using 25-70 micron thick Be-0.9%Cu samples mounted on one end of the hohlraum, we performed free-surface velocimetry and pyrometry measurements during the foot of the pulse. We will present the velocity and pyrometry data along with inferred pressure, and discuss potential implications for Be-Cu capsule performance at NIF