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We present experiments investigating dense carbon at pressures between 100 GPa and 200 GPa and temperatures between 5,000 K and 15,000 K. High-pressure samples with different temperatures were created by laser-driven shock compression of graphite and varying the initial density from 1.53 g/cm3 to 2.21 g/cm3 and the drive laser intensity from 7.1 TW/cm2 to 14.2 TW/cm2. In order to deduce temperatures, spectrally resolved X-ray scattering was applied to determine ion-ion structure factors at a scattering vector of k = 4.12 · 10 10 m − 1 , which shows high sensitivity to temperature for the investigated sample conditions. After comparison to corresponding DFT-MD simulations, we were able to assign each structure factor a temperature. This information is indicative of the expected temperature range for the melting line of carbon at high pressures and can be compared to theoretical predictions. [ABSTRACT FROM AUTHOR]

We present measurements of electron-ion temperature equilibration in proton-heated tantalum, under warm dense matter conditions. Our results agree with theoretical predictions for metals calculated using input data from ab initio simulations. However, the fast relaxation observed in the experiment contrasts with much longer equilibration times found in proton heated carbon, indicating that the energy flow pathways in warm dense matter are far from being fully understood. [ABSTRACT FROM AUTHOR]

Abstract: We evaluate various analytical models for the electron–ion energy transfer and compare the results to data from molecular dynamics (MD) simulations. The models tested include energy transfer via strong binary collisions, Landau–Spitzer rates with different choices for the cut-off parameters in the Coulomb logarithm, rates based on Fermi's golden rule (FGR) and theories taking coupled collective modes (CM) into account. In search of a model easy to apply, we first analyze different approximations of the FGR energy transfer rate. Then, we investigate several numerical studies using MD simulations and try to uncover CM effects in the data obtained. Most MD data published so far, except one study by Murillo et al. [23], show no distinct CM effects and, thus, can be interpreted within a FGR or binary collision approach. We show that this finding is related to the parameter regime, in particular the initial temperature difference, considered in these investigations. [Copyright &y& Elsevier]

Abstract: We have carried out X-ray scattering experiments on iron foil samples that have been compressed and heated using laser-driven shocks created with the VULCAN laser system at the Rutherford-Appleton Laboratory. This is the highest Z element studied in such experiments so far and the first time scattering from warm dense iron has been reported. Because of the importance of iron in telluric planets, the work is relevant to studies of warm dense matter in planetary interiors. We report scattering results as well as shock breakout results that, in conjunction with hydrodynamic simulations, suggest the target has been compressed to a molten state at several 100 GPa pressure. Initial comparison with modelling suggests more work is needed to understand the structure factor of warm dense iron. [Copyright &y& Elsevier]

Abstract: We use a two-fluid model combining the quantum Green's function technique for the electrons and a classical HNC description for the ions to calculate the high-density equation of state of hydrogen. This approach allows us to describe fully ionized plasmas of any electron degeneracy and any ionic coupling strength which are important for the modeling of a variety of astrophysical objects and inertial confinement fusion targets. We have also performed density functional molecular dynamics simulations (DFT-MD) and show that the data obtained agree with our approach in the high-density limit. Good agreement is also found between DFT-MD and quantum Monte Carlo simulations. [Copyright &y& Elsevier]

Abstract: The effective ion–ion potential is extracted from first principle simulations and from experimental structure factors for several warm dense matter systems. Results of three different methods are compared for simple elements like hydrogen or beryllium as well as for composite materials like lithium-hydride and hydrogen–helium plasmas. It is shown that iterative techniques based on the pair distribution function are not unique in their solution and direct force-matching from first principle simulations is subject to finite size effects. Moreover, both methods are not able to provide potentials for small distances. These disadvantages can be avoided by using the static structure factor as input, although higher order correlations are only accounted for within the hypernetted chain approximation in this case. Furthermore, we discuss possibilities to use the extracted effective potentials to investigate the dielectric function beyond linear response. [Copyright &y& Elsevier]

Abstract: We present the theory necessary for analysing Thomson scattering data from plasmas in states far from equilibrium. As an example, we compare with experimental data obtained during the interaction of cryogenic hydrogen with the intense VUV radiation of the FLASH free electron laser at DESY in Hamburg. The component due to inelastic scattering from free electrons is treated within a generalised quantum statistical approach which accounts for the nonequilibrium states produced by the strong pump of the FEL radiation. The elastic scattering component is determined by the structure factor of the ions for the initial conditions of a cold, atomic fluid as the ions do not rearrange during the interaction time. Monte Carlo simulations show that this treatment results in a very small elastic scattering feature. Integrating the full nonequilibrium spectrum over time yields excellent agreement with the measured data. Moreover, this treatment gives quantitatively different plasma conditions as may be inferred using an equilibrium analysis. [Copyright &y& Elsevier]

Abstract: We combine experiments and theoretical models to characterize warm dense deuterium. A shockwave was driven in a planar target by the OMEGA laser without a standard pusher making the analysis independent of a quartz or aluminium pressure standard. The conditions of the shocked material were diagnosed with VISAR and optical pyrometry which yields the shock velocity (16.9 ± 0.9 km/s) and the temperature (0.57 ± 0.05 eV). We find a self-consistent description of the data when using ab initio simulations (DFT-MD), but not for other equation of state (EOS) models tested. [Copyright &y& Elsevier]

Abstract: We present measurements of the changes in the microscopic structure of graphite in a laser-driven shock experiment with X-ray scattering. Laser radiation with intensities of ∼2 × 1013 W/cm2 compressed the carbon samples by a factor of two reaching pressures of ∼90 GPa. Due to the change of the crystalline structure the scattered signals of the probe radiation were modified significantly in intensity and spectral composition compared to the scattering on cold samples. It is shown that the elastic scattering on tightly bound electrons increases strongly due to the phase transition whereas the inelastic scattering on weakly bound electrons remains nearly unchanged for the chosen geometry. [Copyright &y& Elsevier]

The structure of the fluid carbon phase in the pressure region of the graphite, diamond, and BC8 solid phases is investigated. We find increasing coordination numbers with an increase in density. From zero to 30 GPa , the liquid shows a decrease of packing efficiency with increasing temperature. However, for higher pressures, the coordination number increases with increasing temperature. Up to 1.5 eV and independent of the pressure up to 1500 GPa , a double-peak structure in the ion structure factors exists, indicating persisting covalent bonds. Over the whole pressure range from zero to 3000 GPa, the fluid structure and properties are strongly determined by such covalent bonds. [ABSTRACT FROM AUTHOR]