Beecroft Building, Department of Physics, University of Oxford, Parks Road, Oxford, OX1 3PU
Professor Malcolm McMahon, University of Edinburgh
Abstract
Although dynamic-compression studies of materials at extreme pressure-temperature (P-T) states date back to the Manhattan Project, the field has been transformed over the last 15 years by the advent of X-ray free-electron lasers (XFELs). In these experiments, a nanosecond optical laser pulse ablates the surface of a target, launching shock or ramp compression waves that generate pressures from tens of GPa to beyond a TPa, at temperatures from a few hundred to tens of thousands of kelvin depending on the loading path: a range extending far beyond the reach of static compression techniques. Because such states survive for only nanoseconds, the extreme brightness and brevity of the XFEL pulse are essential, with a single 20 fs pulse capable of recording a diffraction snapshot of the compressed lattice. In this seminar I will introduce the field for a general physics audience, describing how these experiments are performed at facilities including the LCLS and the European XFEL, how pressure is determined by velocimetry, and why temperature remains, for now, the least well-constrained state variable. I will then illustrate these ideas with examples from my own research on the structural behaviour of simple elemental metals at extreme P-T. I will close by asking how far nanosecond-timescale measurements represent equilibrium behaviour, given the influence of strain rate and material strength, and what high-repetition-rate drivers and novel XFEL pulse structures will make possible.
A recent perspective on the field can be found in Wark, McMahon and Eggert, J. Appl. Phys. 132, 080902 (2022).