LA-UR-99-6286 AWE Experiments on Laser-Driven Mix in Planar and Convergent Geometry

AWE has had an active laser experimental program studying the physics of compressible turbulent mix for many years. Here we report on progress using the HELEN and OMEGA laser facilities. Recent work on HELEN has centred around obtaining well characterised data on shock-induced mixing at a foam-foam interface of varying roughness. A thin, opaque tracer layer is used as a radiographic diagnostic to determine mix widths. Use of OMEGA has allowed us to investigate mix evolution directly in convergent geometry in a compressible plasma regime for the first time. The experiments comprise a plastic cylindrical shell imploded by direct laser irradiation. The cylindrical shell surrounds a lower density plastic foam which provides sufficient back pressure to allow the implosion to stagnate at a sufficiently high radius to permit quantitative radiographic diagnosis of the interface evolution near turnaround. The Atwood number of the shell-foam interface is varied by choosing different density material for the inner shell surface. Experiments to date have concentrated on a target design which undergoes significant acceleration during the laser pulse, leading to dominance of Rayleigh-Taylor growth from the ablation surface. Subsequent experiments will use a modified target design which minimises the acceleration period, allowing the study of shock-induced Richtmyer-Meshkov growth during the coasting phase, and Rayleigh-Taylor growth during the stagnation phase. Here we will concentrate on the calculational predictions using various radiation hydrodynamics codes. The consequences of using direct laser illumination rather than radiative drive (as on NOVA) will be discussed.