Session E6: Ultra-High-Density Plasmas

Matter under Extreme Conditions: Advances Based on Static Compression

Current technological advances make it possible to perform experiments on materials at static or sustained conditions to multimegabar pressures (several hundred GPa) and several thousand degree ($\sim $1 eV) temperatures. Densities of condensed matter can now be increased over an order of magnitude, causing novel transformations and new physical and chemical phenomena to occur. Growth in this area has been made possible by advances in diamond-anvil cell methods coupled with a wide range of probes, including x-ray diffraction, spectroscopy, inelastic scattering, radiography, and infrared spectroscopy using synchrotron radiation. Examples include investigations of dense hydrogen; transformations in molecular materials; novel ceramics; new types of superconductors, electronic, and magnetic materials; and liquids and amorphous materials. Particularly exciting are new developments in time resolved methods and coupling of static and dynamic compression techniques made possible by the creation of new large-scale facilities and novel technologies.   

To See the Inside of a Planet in a Drop of Deuterium

Despite their proximity, the giant planets of the solar system, Jupiter and Saturn, still present a number of fundamental puzzles. The discovery of nearly 300 planets around other stars in all manners of orbital and physical parameters has further complicated the picture. Giant planets are primarily composed of hydrogen and helium but also contain higher Z elements. The global abundances and the radial variation of the composition of a giant planet represents the end state of its formation process. Observations that pertain to the present-day internal structure of giant planets are the primary thread to the physical mechanisms that where at play when they formed several billion years ago. The interior structure is inferred from observations and is not uniquely determined. The modeled structures of Jupiter and Saturn are quite sensitive to the equation of state (EOS) of hydrogen/helium mixtures in the regime of warm dense matter. Unresolved EOS questions of great importance to planetary science include: Is there a first order metallization transition in hydrogen? What is the EOS of H/He mixtures? Is there a phase separation in H/He mixtures that could provide an additional energy source to slow down the cooling of giant planets, as suggested by observations of Saturn? Can a better EOS provide an explanation for the large radii of several extrasolar giant planets? The last decade has seen a tremendous experimental effort focused on shocked deuterium EOS measurements complemented by a renewed interest in ab initio simulations of H and very recently, He. Those experiments have been crucial in improving EOS models that had been mostly unencumbered by data. New experimental techniques, such as isentropic compression, will reproduce conditions closer to planetary interiors than current data EOS data. When combined with ab initio simulations of H/He mixtures, a much clearer understanding of the interiors of giant planets and of their formation should emerge.   

Dense plasma physics accessible experimentally via new, pulsed x-ray probing techniques.

We have developed accurate x-ray scattering techniques to measure the physical properties of dense matter produced in high-energy density physics experiments. Powerful penetrating x-ray sources are employed to probe these short-lived hot dense states of matter with electron densities in the range of solid density and higher. The back-scattering spectrum accesses the non-collective Compton scattering regime, which provides accurate diagnostic information on the temperature, density and ionization states. The forward scattering spectrum has been shown to measure the collective plasmon oscillations. Besides extracting the standard plasma parameters, density and temperature, forward scattering yields new observables such as a direct measure of collisions, quantum effects and structure factors. In this talk, experiments on shock-compressed matter will be presented that have demonstrated the direct measurement of compression and heating. In addition ultra-fast scattering measurements with K-alpha x rays are presented that probe conditions in coalescing shocks with 10 ps temporal resolution. These data test the radiation-hydrodynamic modeling of shock-compressed dense matter and can be further developed to measure the equation of state at high pressures.