Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Within the next few years hard X-ray Free Electron Lasers will come on line. Such systems will have spectral brightnesses ten orders of magnitude greater than any extant synchrotron, with pulse lengths as short as a few femtoseconds. It is anticipated that large-scale optical lasers capable of shock-compressing matter to multi-megabar pressures will be sited alongside the X-ray source. We discuss how such systems can further our knowledge of shocked and isochorically heated matter, in particular investigating the potential to perform poly crystalline diffraction and the creation of warm dense matter. © 2007 American Institute of Physics.

Abstract:

The propagation of shock waves through polycrystalline iron is explored by large-scale atomistic simulations. For large enough shock strengths the passage of the wave causes the body-centered-cubic phase to transform into a close-packed phase with most structure being isotropic hexagonal-close-packed (hcp) and, depending on shock strength and grain orientation, some fraction of face-centered-cubic (fcc) structure. The simulated shock Hugoniot is compared to experiments. By calculating the extended x-ray absorption fine structure (EXAFS) directly from the atomic configurations, a comparison to experimental EXAFS measurements of nanosecond-laser shocks shows that the experimental data is consistent with such a phase transformation. However, the atomistically simulated EXAFS spectra also show that an experimental distinction between the hcp or fcc phase is not possible based on the spectra alone. © 2007 The American Physical Society.

Abstract:

Extended X-ray absorption fine structure (EXAFS) measurements on shocked polycrystalline iron have provided further evidence for the shock induced α - ε phase transition in iron. However, recent molecular dynamics investigation of this system has suggested the presence of fcc material in the shocked region. In this paper we will investigate the difficulties in simulating EXAFS signals from molecular dynamics data. We will aim to show that in the case of the shock induced α - ε transition EXAFS is insensitive to the type of close packing of the product phase. © 2007 American Institute of Physics.