An understanding of the timing and dynamics of hohlraum filling by laser-induced gold wall ablation is critical to the performance of indirectly-driven fusion ignition designs for the National Ignition Facility [E. Moses and C. Wuest, Fusion Science and Technology, 43, 420 (2003)]. Hohlraum wall ablation negatively affects ignition hohlraum performance by (1) reducing laser coupling by increasing backscatter by laser plasma instabilities, e.g., stimulated Brillouin scattering, (2) altering where lasers couple by moving the critical surface away from the walls and changing the refractive index, and (3), in the case of vacuum hohlraums, ablating directly into contact with the ablation layer of the fuel capsule. We report on measurements of gold-filling of hohlraums from a series of OMEGA laser [T.R. Boehly, R.L. McCrory, C.P. Verdon et al., Fusion Engineering and Design, 44, 35 (1999)] experiments involving vacuum and gas-filled hohlraums. On-axis x-ray imaging of gold self-emission shows delayed filling for gas-filled hohlraums, as expected. In addition, we present data on the hohlraum temperature penalty incurred with the use of a 1-atmosphere methane-fill. We discuss data and simulation predictions for 1-atmosphere neopentane filled hohlraums driven with a modified laser pulse.
Shocks extending across crystals' grain boundaries can nucleate and grow velocity fluctuations on the order of 5-10% when the shock speeds differ in the adjacent grains. Dynamic materials experiments at the Los Alamos National Laboratory Trident Laser Laboratory aim to examine this phenomenon by temporally- and spatially-resolving free surface velocity over a large region of interest. While line-imaged velocimetry can serve as a quantitative method for examining the velocity fluctuations across a single boundary, it is more desirable to resolve the velocity field around an entire embedded grain. We present a novel diagnostic design that utilizes a four-frame gated-optical-imaging interferometric velocimeter in combination with a streaked line-imaging interferometric velocimeter. This diagnostic will provide high-spatial resolution velocigraphs of a shock as it hits a free surface in multigrain crystals.
Confined plasma ablation is an efficient method to accelerate 1-D metal plates, with the impact of the plate resulting in a well-defined shock being generated in a target material. By using confined plasma to accelerate a plate, some details of the laser parameters are decoupled from the plate impact. Several types of experiments and related diagnostics to evaluate the performance parameters of the laser beam, flyer plate acceleration, and plate conditions are described. Several experiments using the flyer plates to generate shocks in materials to determine pressure-velocity relations, and dynamic spall strength of various metals are presented.
X-ray imaging is one essential tool for capturing phenomena that occur when high-irradiance lasers interact with complex, optically thick targets. We use x-ray backlighting and emission to measure the result of such interactions at experiments on the Omega laser and the Z-machine z-pinch facilities. In this presentation, we will show some of the images collected with a variety of experiments, we will discuss some of the difficulties we overcame, and look to issues that will arise with higher-energy lasers and larger objects.