Showing papers by "Christina Back published in 2000"

Diffusive, supersonic x-ray transport in radiatively heated foam cylinders

Abstract: Diffusive supersonic radiation transport, where the ratio of the diffusive radiation front velocity to the material sound speed >2 has been studied in experiments on low density (40 mg/cc to 50 mg/cc) foams. Laser-heated Au hohlraums provided a radiation drive that heated SiO2 and Ta2O5 aerogel foams of varying lengths. Face-on emission measurements at 550 eV provided clean signatures of the radiation breakout. The high quality data provides new detailed information on the importance of both the fill and wall material opacities and heat capacities in determining the radiation front speed and curvature. The Marshak radiation wave transport is studied in a geometry that allows direct comparisons with analytic models and two-dimensional code simulations. Experiments show important effects that will affect even nondiffusive and transonic radiation transport experiments studied by others in the field. This work is of basic science interest with applications to inertial confinement fusion and astrophysics.

Detailed measurements of a diffusive supersonic wave in a radiatively heated foam

Abstract: We have made the first detailed measurements of a diffusive supersonic radiation wave in the laboratory. A 10 mg/cm(3) SiO2 foam is radiatively heated by the x-ray flux from a laser-irradiated hohlraum. The resulting radiation wave propagates axially through the optically thick foam and is measured via time-resolved x-ray imaging as it breaks out the far end. The data show that the radiation wave breaks out at the center prior to breaking out at the edges, indicating a significant curvature in the radiation front. This curvature is primarily due to energy loss into the walls surrounding the foam.

Experimental results on line shifts from dense plasmas

Abstract: The dynamics of the implosion of a deuterium-filled microsphere has been investigated via the detailed analysis of the Ar XVII 1s 2 –1s3p 1 P line shape Ar is doped into the deuterium core for diagnostic purposes For the analysis calculations of Ar XVII 1–3 line shape including lithium-like dielectronic satellites were compared with time-resolved data Three fitting parameters were used: (a) electron temperature, (b) electron density, and (c) relative shift of the wavelength axis between calculation and data The temporal evolution of the core electron temperature and density were derived, and the shot-to-shot formation of the core plasma was shown to be reliable and reproducible We report on the wavelength shift of the Ar XVII 1s 2 –1s3p 1 P line shape between electron densities of 10 23 – 10 24 cm −3 , results indicate a systematic red shift with increasing density

X-Ray Backlighting for the National Ignition Facility

Abstract: X-ray backlighting is a powerful tool for diagnosing a large variety of high-energy-density phenomena. Traditional area backlighting techniques used at Nova and Omega cannot be extended efficiently to NIF-scale. New, more efficient backlighting sources and techniques are required and have begun to show promising results. These include a backlit-pinhole point projection technique, pinhole and slit arrays, distributed polychromatic sources, and picket fence backlighters. In parallel, there have been developments in improving the data SNR and hence quality by switching from film to CCD-based recording media and by removing the fixed-pattern noise of MCP-based cameras.

Implosions: An Experimental Testbed for High Energy Density Physics

Abstract: The Nova laser facility has been used to produce matter in extreme conditions in the laboratory. The plasmas are produced by imploding spherical capsules filled with deuterium and trace amounts of Ar. A spectroscopic study of these indirectly driven, inertially confined plasmas provides measurements of the plasma parameters as a function of time. Multiple diagnostics measure peak ne ~ 1 × 1024 cm-3 and Te ~ 1000 eV. A series of experiments have demonstrated that the results are reliable and reproducible. These experiments are designed to produce laboratory implosions that can serve as a "testbed" for high energy density matter. Measuring temperature gradients are the next step so that they can become sources suitable for studying physics such as high-density plasma effects or radiative cooling.

On the accuracy of x-ray spectra modeling of inertial confinement fusion plasmas

Abstract: X-ray spectroscopic measurements of the helium-like and lithium-like argon emission from gas bag targets are presented. These data are supported by Thomson scattering diagnostics providing critical tests of plasma spectroscopic K-shell models. The x-ray spectra show lithium-like dielectronic satellites on the red wing of the He-β line that are temperature sensitive and are known to influence the shape of the Stark-broadened line profiles observed from implosions. We find that the satellite intensities compare well to kinetics modeling that uses the independently measured electron temperature for satellites whose upper states are populated by dielectronic capture, but discrepancies are observed for inner-shell collisionally excited transitions.