Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We have studied fast heating of highly compressed plasmas using multi 100 TW laser light. Efficient propagation of the ultra-intense laser light and heating of the imploded plasmas were realized with a cone-attached shell target. Energy deposition rate of the ultra-intense laser pulse into high-density plasmas was evaluated from neutron measurements. Generation and propagation property of energetic electrons in the ultra-intense laser interactions were also investigated with solid density targets. About 40% of the laser energy converted to mega electron volts energetic electrons in the interactions with solid targets at intensities of 1019W cm-2. These electrons propagated in the high-density plasmas with a divergence of 20-30° or jet-like collimation. Taking account of these experimental results, heating laser spot size is optimized for laser fusion ignition with a simple estimation.

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.