Laser Simulation of High P-T Planetary Processes

During the accretion and differentiation of large planetary bodies like the Moon and terrestrial planets their matter experienced physical and chemical transformations at very high pressures and temperatures that are beyond the limits of most modern experimental techniques. We describe experimental techniques developed at the SNL pulsed power laser facilities that use high energy density (0.5 - 20 TW/cm2) laser pulses to create several 100 GPa-range pressures above the front surfaces of ~ 6.3 mm diameter targets allowing measurements of target EOS parameters such as shock wave and particle velocities, rear surface Hugoniot pressure, and momentum coupling coefficients as well as mineralogical and chemical studies of recovered shocked target materials. The experiments on combinations of synthetic and natural solid and powdered targets ~ 1 to 3 mm thick were performed in vacuum (< 10-4 Torr). Laser pulse widths varied from ~ 0.15 to 1 ns and wavelengths were 1064 and 527 nm. Corona pressures and temperatures above the target front surfaces were estimated by (hydrocode) computer modeling and analysis to be ~ 200 GPa and ~ 5 x 106 K, respectively. These very rapidly dissipated to ~20 GPa and ~800,000 K upon traversing ~ 0.1 mm of the target where an ablated layer ~ 1 μm was deposited, suggesting (opacity driven) pressure and temperature gradients of ~1,000 GPa/mm and ~107 K/mm, respectively. Solid and powdered targets show different behavior, with the momentum coupling coefficient being 2-3 orders of magnitude smaller in the powdered targets. These results suggest distinctive Hugoniot pressure, temperature and ablation gradients for different materials depending on whether they are solid metals, solid dielectrics (silicates), or powders of various compositions. Further details of the experiments along with the measured EOS parameters will be reported at the meeting. Some targets contain small amounts of shock-induced melt which is described in the accompanying abstract by Petaev et al.