Laura Robin Benedetti

Electronic conduction in shock-compressed water

The optical reflectance of a strong shock front in water increases continuously with pressure above 100 GPa and saturates at ∼45% reflectance above 250 GPa. This is the first evidence of electronic conduction in high pressure water. In addition, the water Hugoniot equation of state up to 790 GPa (7.9 Mbar) is determined from shock velocity measurements made by detecting the Doppler shift of reflected light. From a fit to the reflectance data we find that an electronic mobility gap ∼2.5u2009eV controls thermal activation of electronic carriers at pressures in the range of 100–150 GPa. This suggests that electronic conduction contributes significantly to the total conductivity along the Neptune isentrope above 150 GPa.

Transport of water into the lower mantle: Role of stishovite

When subjected to lower-mantle pressures and temperatures, natural anhydrous basalt containing 0.2 wt.% H{sub 2}O forms a phase assemblage in which SiO{sub 2} stishovite is a significant carrier of hydrogen (up to 500 ppm H{sub 2}O by weight, as hydroxide), whereas the coexisting (Mg, Fe, Al, Ca)SiO{sub 3} perovskite appears to be not (upper bound of 50 ppm (wt) H{sub 2}O). Contrary to the devolatilization characteristically observed at lower pressures, we find that the abundance of H{sub 2}O in residual stishovite increases from {approx}100 to {approx}400 ppm by weight upon partially melting the high-pressure mineral assemblage at 28-60 GPa. We infer that the trace concentration of Al within residual stishovite increases upon partial melting, thereby increasing the coupled abundance of H in this crystalline phase. The anhydrous component of subducted oceanic crust can thus recycle a significant amount of water into the lower mantle over the age of the Earth, with subducted stishovite potentially returning {approx} 10{sup 2} times the amount of water present in todays atmosphere.

Equation of state of stishovite and interpretation of SiO2 shock-compression data

[1]xa0We present an isothermal equation of state for the high-pressure stishovite phase of SiO2 based on synchrotron X-ray diffraction at 300 K to 60 GPa. The inferred equation of state is the same for stishovite synthesized from pure SiO2 glass as from a natural, basalt glass that forms stishovite in association with other mineral phases, yielding a zero-pressure bulk modulus K0T = 312.9(±3.4) GPa and pressure derivative K′0 = 4.8(±0.2). Our measurements of stishovite at static, high pressures predict a shock-compressed (Hugoniot) state that is 2.4 (±1.6)% denser than observed for SiO2, suggesting that an amorphous phase, rather than stishovite, is produced under impact loading.

X-ray penumbral imaging diagnostic developments at the National Ignition Facility

X-ray penumbral imaging has been successfully fielded on a variety of inertial confinement fusion (ICF) capsule implosion experiments on the National Ignition Facility (NIF). We have demonstrated sub-5 μm resolution imaging of stagnated plasma cores (hot spots) at x-ray energies from 6 to 30 keV. These measurements are crucial for improving our understanding of the hot deuterium-tritium fuel assembly, which can be affected by various mechanisms, including complex 3-D perturbations caused by the support tent, fill tube or capsule surface roughness. Here we present the progress on several approaches to improve x-ray penumbral imaging experiments on the NIF. We will discuss experimental setups that include penumbral imaging from multiple lines-of-sight, target mounted penumbral apertures and variably filtered penumbral images. Such setups will improve the signal-to-noise ratio and the spatial imaging resolution, with the goal of enabling spatially resolved measurements of the hot spot electron temperature and material mix in ICF implosions.