Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

The past few years have seen a rapid growth in the development and exploitation of X-ray diffraction on ultrafast time-scales. One area of physics which has benefited particularly from these advances is the the field of shock-waves. Whilst it has been known for many years that crystalline matter, subjected to uniaxial shock compression, can undergo plastic deformation and, for certain materials, polymorphic phase transformations, it has hitherto not been possible to observe the rearrangement of the atoms on the pertinent timescales. We have used laser-plasma generated X-rays to study how single crystals of metals (copper and iron) react to uniaxial shock compression, and observed rapid plastic flow (in the case of copper), and directly observed the famous alpha-epsilon transition in Iron. These studies have been complemented by large-scale multi-million atom molecular dynamics simulations, yielding significant information on the underlying physics.

Abstract:

Electron beam hollowing in a plasma is investigated using an analytical, rigid beam model and two different hybrid codes in an attempt to explain observations of hollow plasma formations on the back of plastic targets in experiments carried out on the Vulcan terawatt laser. The relevance of the results to electron transport in fast ignition inertial confinement fusion is considered using dimensionless scaling parameters. © 2006 IOP Publishing Ltd.

Abstract:

A flexible multi-channel neutron spectrometer has been constructed to measure both the ion temperature and acceleration mechanisms in ultra-intense laser-plasma interactions. Angularly resolved neutron spectra are required to deduce the momentum distribution of ions accelerated by the intense electric field. The first neutron spectra have been obtained using this instrument in a 100-TW class laser interaction with deuterated plastic targets. These show a slight Doppler shift to lower energy side of the center of mass energy of d (d, n)3 He reactions at 2.45 MeV. A three-dimensional Monte-Carlo calculation of the neutron generation confirms the fast ion acceleration from the rear side of a solid target which has a Maxwellian momentum distribution. © 2006 Elsevier B.V. All rights reserved.

Abstract:

We propose a modified x-ray form factor that describes the scattering cross section in warm dense matter valid for both the plasma and the solid (crystalline) state. Our model accounts for the effect of lattice correlations on the electron-electron dynamic structure, as well as provides a smooth transition between the solid and the plasma scattering cross sections. In addition, we generalize the expression of the dynamic structure in the case of a two-temperature system (with different electron and ion temperatures). This work provides a unified description of the x-ray scattering processes in warm and dense matter, as the one encountered in inertial confinement fusion, laboratory astrophysics, material science, and high-energy density physics and it can be used to verify temperature relaxation mechanisms in such environments.