Femtosecond Diffraction Studies of Shock and Ramp-Compressed Matter

The project aims to use the femtosecond pulses from a hard x-ray free electron laser (the Linac Coherent Light Source - LCLS at SLAC) to measure deformation and structure of matter shock or ramp compressed to multi-megabar pressures, accessing regions of phase space inaccessible to study with diamond anvil cells. The experiments will be augmented with multi-million atom molecular dynamics simulations coupled with diffraction simulations. The research has both applied and fundamental aspects. From the fundamental point of view a significant amount of matter in the visible universe exists in states of pressure and temperature inaccessible to many laboratory experiments - e.g. the interior of the large planets in our own solar system, and those that have been discovered orbiting other stars (exoplanets). Recreating and understanding the conditions existing in these regions is of fundamental interest. From the applied point of view it is as yet unknown what materials, once produced at high temperature and pressure, may be metastable (and of commercial use) at ambient conditions. For example, it is well known that diamond is metastable (it is not the state with the lowest free energy), however, the enthalpy barrier between it and the graphite phase is huge, and it thus exists under ambient conditions, and has considerable industrial impact. Thus the overall field of explore matter under novel conditions, learning its structure, and then bringing it back (if possible) to ambient conditions could be of considerable impact.

The aims of the project are to better understand, at the lattice level, the physics of high-strain rate plasticity and polymorphic phase transformations. This will be undertaken both by theoretical investigation (molecular dynamics simulations) and via experiments employing femtosecond x-ray diffraction of materials shock or ramp compressed by high power laser ablation. The aim is, for several classes of material, to better understand how, under the uniaxial strain conditions applied by laser ablation, the material relieves the shear stresses to flow towards the hydrostatic. We know that, depending upon the material, this can happen via dislocation generation and flow, via twinning, and via polymorphic phase transitions. However, what actually happens at the lattice level, and at the mesoscale of individual grains, has yet to be explored in any detail. In this project the student will study both simple metals and more complex targets to start to unravel this complicated set of phenomena.

This project is directly related to the laser-plasmas and fusion EPSRC theme. For example the physics is encompassed under EPSRC grant EP/J017256/1 The Creation and Diagnosis of Solid-State Matter at Multi-TeraPascal Pressures. This is 50% funded by AWE, and we will also collaborate closely with LLNL in the USA.