Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

Shocked crystals were investigated by picosecond X-ray diffraction. The response of single crystal silicon to nanosecond timescale uniaxial shock compression was purely elastic. Single crystal Cu shocked along the (200) axis exhibited compression of the unit cells along both (200) and (020), indicating a transition to plastic flow.

Abstract:

Experimental time-resolved X-ray diffraction data that provide firm evidence that the response of single crystal silicon to nanosecond timescale uniaxial shock compression along the (400) axis is anomalous in that it is purely elastic is presented. In addition, the results of a molecular-dynamics simulations of the diffraction of X-rays from shocked samples, which indicate that it may be possible to directly measure dislocation densities by observing the angular shifts associated with sticking faults within face-centred-cubic metals are presented as well.

Abstract:

Accurate phase diagrams for simple molecular fluids (H2, H2O, 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. These pressures are usually achieved dynamically with explosives and two-stage light-gas guns, or statically with diamond anvil cells. Current high intensity laser facilities can produce tens of Mbar pressures in these light fluids. This presentation will first describe recent shock compressed Hugoniot data for water at pressures up to 8 Mbar. At Hugoniot pressures near 1 Mbar, water becomes an electronic conductor as observed through the shock front reflectivity. Similar experimental results will be shown for carbon starting from the diamond phase. Reflectivity data reveal diamond metalizes near the Hugoniot pressure of 11 Mbar and is perhaps coincident with the melt transition. To obtain high pressure data very close to planetary isentropes, techniques are being developed to generate off Hugoniot data (lower temperature and higher density than standard Hugoniot track). Two techniques to achieve such high density states (double shock and precompressed samples) have been tested. These techniques and preliminary data from these experiments will be described.

Abstract:

A one-dimensional dissipative magnetohydrodynamics code is used to investigate the discharge dynamics of a waveguide for high-intensity laser pulses: the gas-filled capillary discharge waveguide. Simulations are performed for the conditions of a recent experimental measurement of the electron density profile in hydrogen-filled capillaries [D. J. Spence et al., Phys. Rev. E 63, 015401 (R) (2001)], and are found to be in good agreement with those results. The evolution of the discharge in this device is found to be substantially different to that found in Z-pinch capillary discharges, owing to the fact that the plasma pressure is always much higher than the magnetic pressure. Three stages of the capillary discharge are identified. During the last of these the distribution of plasma inside the capillary is determined by the balance between ohmic heating, and cooling due to electron heat conduction. A simple analytical model of the discharge during the final stage is presented, and shown to be in good agreement with the magnetohydrodynamic simulations. © 2001 The American Physical Society.