Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Nonlinear wave-driven processes in plasmas are normally described by either a monochromatic pump wave that couples to other monochromatic waves or as a random phase wave coupling to other random phase waves. An alternative approach involves a random or broadband pump coupling to monochromatic and/or coherent structures in the plasma. This approach can be implemented through the wave kinetic model. In this model, the incoming pump wave is described by either a bunch (for coherent waves) or a sea (for random phase waves) of quasiparticles. This approach has been applied to both photon acceleration in laser wakefields and drift wave turbulence in magnetized plasma edge configurations. Numerical simulations have been compared to experiments, varying from photon acceleration to drift mode-zonal flow turbulence, and good qualitative correspondences have been found in all cases. © 2009 American Institute of Physics.

Abstract:

Measurements of the microscopic response of warm dense matter have been demonstrated by multi-keV inelastic x-ray scattering using laser-based sources. These techniques have been used to study the high frequency electron correlations (plasmons) in low to mid- Z plasmas. The advent of fourth generation light sources will provide high fluxes of narrowband and coherent x rays that will allow to look at the low frequency correlations (the ion-acoustic waves). In this paper we present an analysis of such low frequency modes by calculating the frequency dependent ion-ion structure factor. Our model includes all the relevant multibody contributions arising from strong coupling and nonideal plasma effects. In particular, the ion-ion structure factor is obtained within the memory function formalism by satisfying a finite number of sum rules. This work could be used as a basis to a direct experimental test of dense plasma model as soon as keV free electron laser sources will become available. © 2009 American Institute of Physics.

Abstract:

A variety of targets with different dimensions and materials was irradiated using the VULCAN PW laser [C. N. Danson, Nucl. Fusion 44, S239 (2004)]. Using transverse optical shadowgraphy in conjunction with a one-dimensional radiation hydrodynamics code it was possible to determine a longitudinal temperature gradient. It was demonstrated for thick targets with a low Z substrate and a thin higher Z tracer layer at the rear that the boundary between the two materials was Rayleigh-Taylor unstable. By including a simple bubble growth model into the calculations it was possible to correct for the associated behavior with regard to temperature. The resulting temperature gradient was in good agreement with the previously published data using two different methods of determining the temperature. © 2009 American Institute of Physics.

Abstract:

Simulated EXAFS signals from ab initio models and configurational averaging of molecular dynamics (MD) data are compared for α-Fe, and configurationally averaged MD EXAFS signals are compared with experimental data for iron shock compressed to pressures above the α-ε{lunate} transition pressure. It is shown that molecular dynamics potentials and ab initio models capable of recreating similar vibrational density of states lead to EXAFS signals in good mutual agreement. The effects of the classical nature of the phonon distribution in the MD and the anharmonicity of the potential give rise to noticeable differences between ab initio models and configurational averaging of MD data. However, the greatest influence on the spectra is the form of the potential itself. We discuss the importance of these effects in simulating EXAFS spectra for shock compressed polycrystalline iron. It is shown that EXAFS is an insensitive probe for determining the nature of the close packed product phase in this system. © 2009 Elsevier B.V.