Colin Danson

Abstract:

The optical reflectance of a strong shock front in water, which increases with pressure above 100 GPa, was discussed. The water Hugoniot equation of state up to 790 GPa was also determined. The shock velocity measurements, made by detecting the Doppler shift of reflected light, were used for the purpose. It was found from a fit to the reflectance data, that an electronic mobility gap ∼2.5 eV controls thermal activation of electronic carriers at pressures in the range of 100-150 GPa. The results show that above 150 GPa, electronic conduction contributes significantly to the total conductivity along the Neptune isentrope.

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.

Abstract:

The Vulvan Nd:glass laser at the Central Laser Facility (CLF) has recently been upgraded to the Petawatt level (1015 Watts). The three year upgrade project was contracted to deliver 500 J in a near diffraction limited pulse of 500 fs duration. The Petawatt facility will deliver an irradiance on target of 1021 W.cm-2 for a wide ranging experimental programme in fundamental physics and advanced applications. This includes the interaction of super-high intensity light with matter, fast ignition fusion research, photon induced nuclear reactions, electron and ion acceleration by light waves and the exploration of the exotic world of plasma physics dominated by relativity. Of particular relevance to high speed photography, the Petawatt beam will be used to create a source for advanced high-speed imaging using protons, neutrons and X-rays on an hitherto inaccessible time-scale.

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:

The possibility of using high-intensity laser-produced plasmas as a source of energetic ions for heavy ion accelerators is addressed. Experiments have shown that neon ions greater than 6 MeV can be produced from gas jet plasmas, and well-collimated proton beams greater than 20 MeV have been produced from highintensity laser solid interactions. The proton beams from the back of thin targets appear to be more collimated and reproducible than are high-energy ions generated in the ablated plasma at the front of the target and may be more suitable for ion injection applications. Lead ions have been produced at energies up to 430 MeV. © 2000 IEEE.