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Center for Nonlinear Studies |
Abstract: The use of x-ray diffraction to study materials under rapid dynamic compression has a long history, going back well over half a century. The last two decades have seen particularly spectacular advances, with high power lasers and other drivers coupled to synchrotrons and x-ray free electron lasers (FELs), such that single-shot diffraction patterns from shocked and ramp-compressed samples can now be obtained on a timescale shorter than the period of even the fastest phonon in the system. Experiments using such systems have started to lead to important advances in our understanding of phase transformations and plasticity at the lattice level, although there is clearly much more information that x-ray techniques can allow us to obtain about the atomic rearrangements and thermodynamic state of matter in extreme environments. In this talk I will give a brief history of the field, and provide some overview of some of the initial experiments that have started to lead to lattice level understanding of plasticity and deformation. I will then summarise recent results from the European XFEL where we have gone beyond elastic scattering, and used inelastic thermal diffuse scattering to measure the temperature of copper along the Hugoniot in a regime where pyrometry would be inapplicable. Thermal motion is not the only mechanism that can give rise to diffuse scattering, and it is of interest to question how high resolution spectroscopy may allow us to make some statements about in situ defect densities, where measurements are sorely lacking. Finally, I will discuss how other technical advances, such as RIXS (Resonant Inelastic-X-Ray Scattering), hold potential to provide information about the electronic structure of materials of interest. Bio: Justin Wark is Professor of Physics in the Atomic and Laser Physics subdepartment at the University of Oxford, and has been a William Penney Fellow of AWE since 2019. With partners at AWE, he established the Oxford Centre for High Energy Density Science (OxCHEDS) in early 2013, and he remains its Principal Director. Professor Wark received his degree in physics from the University of Oxford in 1982, and his Ph.D. in plasma physics from Imperial College in 1985. He was awarded a Royal Society University Research Fellowship at the University of Oxford, where he set up a research group working in high power laser-matter interactions. His research includes high harmonic generation, XUV lasers, x-ray spectroscopy, and the development and use of novel x-ray sources in studying shock and isentropic compression of solid-state matter via x-ray diffraction. His research group has recently been highly active in exploiting so-called 4th generation light sources – XUV and X-Ray Free Electron Lasers which have a spectral brightness over a billion times greater than those of any synchrotron. He has used such sources to both create and diagnose matter under extremes of density, temperature, and pressure. This summer, Prof. Wark will receive the 2025 APS George E. Duvall Shock Compression Science Award “For the initial instigation and subsequent development of time-resolved X-ray diffraction from laser-shocked materials, leading to major advances in our understanding of shock-induced plasticity and phase transformations at the lattice level.
Host: Tim Germann (tcg@lanl.gov) and Romain Perriot (rperriot@lanl.gov)