Oxford Centre for High Energy Density Science (OxCHEDS)

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:

Intense laser-plasma interactions produce high brightness beams of gamma rays, neutrons and ions and have the potential to deliver accelerating gradients more than 1000 times higher than conventional accelerator technology, and on a tabletop scale. This paper demonstrates one of the exciting applications of this technology, namely for transmutation studies of long-lived radioactive waste. We report the laser-driven photo-transmutation of long-lived 129I with a half-life of 15.7 million years to 128I with a half-life of 25 min. In addition, an integrated cross-section of 97±40 mbarns for the reaction 129I(γ,n)128I is determined from the measured ratio of the (γ,n) induced 128I and 126I activities. The potential for affordable, easy to shield, tabletop laser technology for nuclear transmutation studies is highlighted.