Oxford Centre for High Energy Density Science (OxCHEDS)

Abstract:

We analyze recent experimental results on the increase of fast electron penetration in shock compressed plastic [Phys. Rev. Lett. 81, 1003 (1998)]. It is explained by a combination of stopping power and electric field effects, which appear to be important even at laser intensities as low as 10(16) W cm-2. An important conclusion is that fast electron induced heating must be taken into account, changing the properties of the material in which the fast electrons propagate. In insulators this leads to a rapid insulator to conductor phase transition.

Abstract:

Experiments have been developed using high powered laser facilities to study the response of materials in the solid state under extreme pressures and strain rates. Details of the target and drive development required for solid-state experiments and results from two separate experiments are presented. In the first, thin foils were compressed to a peak pressure of 180 GPa and accelerated. A pre-imposed modulation at the embedded Rayleigh-Taylor unstable interface was observed to grow. The growth rates were fluid-like at early time, but suppressed at later time. This result is suggestive of the theory of localized heating in shear bands, followed by conduction of the heat into the bulk material, allowing for recovery of the bulk material strength. In the second experiment, the response of Si was studied by dynamic x-ray diffraction. The crystal was observed to respond with uni-axial compression at a peak pressure 11.5-13.5 GPa. (C) 2000 American Institute of Physics. [S1070-664X(00)94505-1].

Abstract:

An X-ray drive has been developed to shock compress metal foils in the solid state using an internally shielded hohlraum with a high contrast shaped pulse from the Nova laser. The drive has been characterized, and hydrodynamics experiments designed to study the growth of the Rayleigh-Taylor (R-T) instability in Cu foils at 3 Mbar peak pressures in the plastic how regime have been started. Preimposed modulations with an initial wavelength of 20-50 mu m and amplitudes of 1.0-2.5 mu m show growth consistent with simulations. In the Nova experiments, the fluid and solid states are expected to behave similarly for Cu. An analytic stability analysis is used to motivate an experimental design with an Al foil where the effects of material strength of the R-T growth are significantly enhanced. The conditions reached in the metal foils at peak compression are similar to those predicted at the core of Earth.