Publications

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Eighty years ago, it was proposed that solid hydrogen would become metallic at sufficiently high density. Despite numerous investigations, this transition has not yet been experimentally observed. More recently, there has been much interest in the analog of this predicted metallic transition in the dense liquid, due to its relevance to planetary science. Here, we show direct observation of an abrupt insulator-to-metal transition in dense liquid deuterium. Experimental determination of the location of this transition provides a much-needed benchmark for theory and may constrain the region of hydrogen-helium immiscibility and the boundary-layer pressure in standard models of the internal structure of gas-giant planets.

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Mixtures of light elements with heavy elements are important in inertial confinement fusion. We explore the physics of molecular scale mixing through a validation study of equation of state (EOS) properties. Density functional theory molecular dynamics (DFT-MD) at elevated temperature and pressure is used to obtain the thermodynamic state properties of pure xenon, ethane, and various compressed mixture compositions along their principal Hugoniots. To validate these simulations, we have performed shock compression experiments using the Sandia Z-Machine. A bond tracking analysis correlates the sharp rise in the Hugoniot curve with the completion of dissociation in ethane. The DFT-based simulation results compare well with the experimental data along the principal Hugoniots and are used to provide insight into the dissociation and temperature along the Hugoniots as a function of mixture composition. Interestingly, we find that the compression ratio for complete dissociation is similar for several compositions suggesting a limiting compression for C-C bonded systems.

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We use Sandia's Z machine and magnetically accelerated flyer plates to shock compress liquid krypton to 850 GPa and compare with results from density-functional theory (DFT) based simulations using the AM05 functional. We also employ quantum Monte Carlo calculations to motivate the choice of AM05. We conclude that the DFT results are sensitive to the quality of the pseudopotential in terms of scattering properties at high energy/temperature. A new Kr projector augmented wave potential was constructed with improved scattering properties which resulted in excellent agreement with the experimental results to 850 GPa and temperatures above 10 eV (110 kK). Finally, we present comparisons of our data from the Z experiments and DFT calculations to current equation of state models of krypton to determine the best model for high energy-density applications.

We present an improved first-principles description of melting under pressure based on thermodynamic integration comparing density functional theory (DFT) and quantum Monte Carlo (QMC) treatments. The method is applied to address the longstanding discrepancy between DFT calculations and diamond anvil cell (DAC) experiments on the melting curve of xenon, a noble gas solid where van der Waals binding is challenging for traditional DFT methods. The calculations show agreement with data below 20 GPa and that the high-pressure melt curve is well described by a Lindemann behavior up to at least 80 GPa, in contrast to DAC data.

The fifth Fundamental Science with Pulsed Power: Research Opportunities and User Meeting was held in Albuquerque, NM, July 20-­23, 2014. The purpose of the workshop was to bring together leading scientists in four research areas with active fundamental science research at Sandia’s Z facility: Magnetized Liner Inertial Fusion (MagLIF), Planetary Science, Astrophysics, and Material Science. The workshop was focused on discussing opportunities for high-­impact research using Sandia’s Z machine, a future 100 GPa class facility, and possible topics for growing the academic (off-Z-campus) science relevant to the Z Fundamental Science Program (ZFSP) and related projects in astrophysics, planetary science, MagLIF- relevant magnetized HED science, and materials science. The user meeting was for Z collaborative users to: a) hear about the Z accelerator facility status and plans, b) present the status of their research, and c) be provided with a venue to meet and work as groups. Following presentations by Mark Herrmann and Joel Lash on the fundamental science program on Z and the status of the Z facility where plenary sessions for the four research areas. The third day of the workshop was devoted to breakout sessions in the four research areas. The plenary-­ and breakout sessions were for the four areas organized by Dan Sinars (MagLIF), Dylan Spaulding (Planetary Science), Don Winget and Jim Bailey (Astrophysics), and Thomas Mattsson (Material Science). Concluding the workshop were an outbrief session where the leads presented a summary of the discussions in each working group to the full workshop. A summary of discussions and conclusions from each of the research areas follows and the outbrief slides are included as appendices.

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