Professor Justin Wark

Abstract:

We present molecular dynamics simulations of shock and release in micron-scale tantalum crystals that exhibit postbreakout temperatures far exceeding those expected under the standard assumption of isentropic release. We show via an energy-budget analysis that this is due to plastic-work heating from material strength that largely counters thermoelastic cooling. The simulations are corroborated by experiments where the release temperatures of laser-shocked tantalum foils are deduced from their thermal strains via in situ x-ray diffraction and are found to be close to those behind the shock.

Abstract:

The free-free opacity in dense systems is a property that both tests our fundamental understanding of correlated many-body systems, and is needed to understand the radiative properties of high energy-density plasmas. Despite its importance, predictive calculations of the free-free opacity remain challenging even in the condensed matter phase for simple metals. Here we show how the free-free opacity can be modelled at finite-temperatures via time-dependent density functional theory, and illustrate the importance of including local field corrections, core polarization, and self-energy corrections. Our calculations for ground-state Al are shown to agree well with experimental opacity measurements performed on the Artemis laser facility across a wide range of extreme ultraviolet wavelengths. We extend our calculations across the melt to the warm-dense matter regime, finding good agreement with advanced plasma models based on inverse bremsstrahlung at temperatures above 10 eV.

Abstract:

While experimental and computational studies abound demonstrating the diverse range of phenomena caused by grain interactions under quasistatic loading conditions, far less attention has been given to these interactions under the comparatively dramatic conditions of shock compression. The consideration of grain interactions is essential within the context of contemporary shock-compression experiments that exploit the distinctive x-ray diffraction patterns of highly textured (and therefore strongly anisotropic) targets in order to interrogate local structural evolution. We present here a study of grain interaction effects in shock-compressed, body-centered cubic tantalum nanocrystals characterized by a columnar geometry and a strong fiber texture using large-scale molecular dynamics simulations. Our study reveals that contiguous grains deform cooperatively in directions perpendicular to the shock, driven by the gigapascal-scale stress gradients induced over their boundaries by the uniaxial compression, and in so doing are able to reach a state of reduced transverse shear stress. We compare the extent of this relaxation for two different columnar geometries (distinguished by their square or hexagonal cross-sections), and quantify the attendant change in the transverse elastic strains. We further show that cooperative deformation is able to replace ordinary plastic deformation mechanisms at lower shock pressures, and, under certain conditions, activate new mechanisms at higher pressures.

Abstract:

Ultrafast x-ray diffraction at the LCLS x-ray free electron laser has been used to resolve the structural behavior of antimony under shock compression to 59 GPa. Antimony is seen to transform to the incommensurate, host-guest phase Sb-II at ∼11 GPa, which forms on nanosecond timescales with ordered guest-atom chains. The high-pressure bcc phase Sb-III is observed above ∼15 GPa, some 8 GPa lower than in static compression studies, and mixed Sb-III/liquid diffraction are obtained between 38 and 59 GPa. An additional phase which does not exist under static compression, Sb-I', is also observed between 8 and 12 GPa, beyond the normal stability field of Sb-I, and resembles Sb-I with a resolved Peierls distortion. The incommensurate Sb-II high-pressure phase can be recovered metastably on release to ambient pressure, where it is stable for more than 10 ns.