Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

When a crystal is subjected to shock compression beyond its Hugoniot Elastic Limit (HEL), the deformation it undergoes is composed of elastic and plastic strain components. In situ time-dependent X-ray diffraction, which allows direct measurement of lattice spacings, can be used to investigate such phenomena. This paper presents recent experimental results of X-ray diffraction from shocked fcc crystals. Comparison is made between experimental data and simulated X-ray diffraction using a post-processor to Molecular Dynamics (MD) simulations of shocked fcc crystals.

Abstract:

A molecular-dynamic (MD) code for calculating the relaxation of an arbitrary electron energy distribution in a plasma was described. The MD approach provided a more fundamental set of equations, with fewer assumptions. The accuracy of the MD approach was proved by comparing its results with the Monte Carlo and Fokker-Planck codes using a set of plasma parameters for which the Fokker-Planck calculation gave incorrect results. Calculating energy relaxation in plasmas proved important for the understanding of the operation of new types of short-wavelength lasers based on optical field ionization.

Abstract:

A molecular-dynamic (MD) code is used to calculate the temporal evolution of nonequilibrium electron distribution functions in plasmas. To the authors' knowledge, this is the first time that a molecular-dynamic code has been used to treat this problem using a macroscopic number of particles. The code belongs to the class of P3M (particle-particle-particle-mesh) codes. Since the equations solved by the MD code are fundamental, this approach avoids several assumptions that are inherent to alternative methods. For example, the initial energy distribution can be arbitrary, and there is no need to assume a value for the Coulomb logarithm. The advantages of the MD code are illustrated by comparing its results with those of Monte Carlo and Fokker-Planck codes with a set of plasma parameters for which the Fokker-Planck calculation is shown to give incorrect results. As an example, we calculate the relaxation of the electron energy distribution produced by optical field ionization of a mixed plasma containing argon and hydrogen.

Abstract:

Accurate phase diagrams for simple molecular fluids (H2, H 2O, NH3 and CH4) and their constituent elements at temperatures of several thousand Kelvin and pressures of several Mbar are integral to planetary models of the gas giant planets ( Jupiter, Saturn, Uranus and Neptune). Experimental data at high pressure has, until recently, been limited to around 1 Mbar with both dynamic (i.e. two-stage light-gas guns) and static (i.e. diamond anvil cells) techniques. Current high intensity laser facilities can now produce tens of Mbar pressures in these light fluids, reaching the dense plasma states required for understanding the cores of giant planets and low mass stars. This presentation will first describe recent Hugoniot data for water at pressures up to 8 Mbar and carbon up to 30 Mbar. At Hugoniot pressures near 1 Mbar, water transitions from an ionic to electronic conductor as observed from the shock front reflectivity. Pressure-density-temperature data follow the Sesame database up to 8 Mbar where water is a dense plasma. Carbon starting from the diamond phase is shown to metallizes at Hugoniot pressures extending from 6 to 11 Mbar. This insulator-conductor transition appears to be coincident with the melt transition and from P-rho data it appears the Hugoniot crosses the melt with dP/dT>0. To obtain high pressure dense plasma data very close to planetary isentropes, techniques are being developed to generate data off the principal Hugoniot (lower temperature and higher density than standard Hugoniot track). Diamond anvil cell targets are used to precompress planetary fluids and then single and double shocks are launched in this already dense fluid. This technique has been used to map the insulator-conductor transition in both water and hydrogen at densities well above those achieved starting at low pressure, One clear trend in both these fluids is that the insulator conductor transition is pushed to higher pressures with increasing initial density.