We previously reported [Colvin et al., J. Appl. Phys. 101, 084906 (2007)] on the microstructure morphology of pure Bi metal subjected to rapid laser-shock-driven melting and subsequent resolidification upon release of pressure, where the estimated effective undercooling rates were of the order of 109–1010 K/s. More recently, we repeated these experiments, but with a Bi/Zn alloy (Zn atomic fraction of 2%–4%) instead of elemental Bi and with a change in target design to suppress spall in the Bi/Zn samples. We observed a similar microstructure morphology in the two sets of experiments, with initially columnar grains recrystallizing to larger equiaxed grains. The Bi samples, however, exhibited micron-scale dendrites on the spall surfaces, whereas there were no dendritic structures anywhere in the nonspalled Bi/Zn, even down to the nanometer scale as observed by transmission electron microscopy. We present the simulations and the interferometry data that show that the samples in the two sets of experiments followed nearly identical hydrodynamic and thermodynamic paths apart from the presence of (probably partially liquid) spall in pure Bi. Simulations also show that the spall occurs right at the moving phase front and, hence, the spall itself cuts off the principal direction for latent heat dissipation across the phase boundary. We suggest that it is the liquid spall itself that creates the conditions for dendrite formation.

The mechanisms and kinetics of plastic flow in body‐centered cubic materials are of current interest in the development of fundamental theories of dynamic strength, applicable at high strain rates such as are found in high explosive and laser loading. We have performed dynamic loading experiments with the Janus and Trident lasers, using tailored pulse shapes to induce shock or ramp loading. The response of the sample was investigated through the surface velocity history, and in some cases with in‐situ x‐ray diffraction. The velocity histories exhibited clear elastic waves, from which the flow stress was deduced and compared with the elastic strain as determined by diffraction. We compare the deduced flow stress with models calibrated to samples millimeters thick, and to theoretical studies.

The distance radiation waves that supersonically propagate in optically thick, diffusive media are energy sensitive. A blast wave can form in a material when the initially diffusive, supersonic radiation wave becomes transonic. Under specific conditions, the blast wave is visible with radiography as a density perturbation. [Peterson , Phys. Plasmas 13, 056901 (2006)] showed that the time-integrated drive energy can be measured using blast wave positions with uncertainties less than 10% at the Z Facility. In some cases, direct measurements of energy loss through diagnostic holes are not possible with bolometric and x-ray radiometric diagnostics. Thus, radiography of high compression blast waves can serve as a complementary technique that provides time-integrated energy loss through apertures. In this paper, we use blast waves to characterize the energy emerging through a 2.4 mm aperture and show experimental results in comparison to simulations.

The plastic response of beryllium was investigated during loading by laser-induced shock waves, using surface velocimetry and in-situ x-ray diffraction. Results from loading by thermal x-rays (hohlraum) were consistent with more extensive studies using laser ablation. Strong elastic waves were observed, up to ~1 km/s in free surface speed, with significant structure before the arrival of the plastic shock. The magnitude and shape of the precursor could be reproduced with a plasticity model based on dislocation dynamics. Changes in lattice spacing measured from the x-ray diffraction pattern gave a direct measurement of uniaxial compression in the elastic wave, triaxial flow from the decay of the precursor, and triaxial compression in the plastic shock; these were consistent with the velocity data. The dynamic strength behavior deduced from the laser experiments was used to help interpret surface velocity data around the onset of shock-induced melting. A model of heterogeneous mixtures is being extended to treat anisotropic components, and spall.

Blast waves (BWs) form when the wave speed of an initially diffusive, supersonic radiation wave becomes subsonic and creates a radiographically-visible, hydrodynamic shock wave. BWs are a novel diagnostic in radiation-flow, code validation experiments that use Sandia’s Z-accelerator’s dynamic hohlraum (DH) as a radiative source. The physics models being tested are sensitive to delivered energy and power changes of better than ±10%; therefore, precise in-situ radiative power and energy measurements are required for quantitative comparisons between simulation and experiment. The energy sensitive BW diagnostic complements bolometric and x-ray radiometric diagnostics in providing these measurements. Recent comparisons between BW qualification experiments and simulations have revealed a spatial dependence on the radiation source. We discuss the experimental design and sensitivities for the BW diagnostic and experimental results in comparison to simulations and other diagnostics.

With the growing importance of nanotechnology, there is increased emphasis on rapid solidification processing to produce materials microstructures with a finer length scale. However, few studies have focused on the question of how a material restructures itself on the microstructural scale when it refreezes at very high cooling rates. Here we report on the development of microstructures in pure bismuth metal as it is subjected to rapid shock-driven melting and subsequent resolidification (on release of pressure), where the estimated effective undercooling rates are on the order of 1010K∕s, orders of magnitude faster than any achieved before in bulk material. Microscopic examination of the recovered material indicates that the melting transformation was far from homogeneous, and substantial morphological changes are observed compared to the starting microstructure.

Dynamic bohlraum (DH) z-pinches on the Z machine at Sandia National Laboratories have proven to be an excellent radiation source for driving high energy density experiments. The Z DH source produces as much as 100 W with powers up to 17 TW into a 4-mm diameter, axially-located radiation exit hole above the source. The Sandia Laboratories' ZR upgrade of Z may be able to similarly produce 200 U. The complex Z DH radiation source varies in spectral power and timing from shot-to-shot. We are characterizing the source physics through simulation and experiment in order to minimize this variability. We are studying how the complex spatially and temporally-dependent behavior of the DH source affects the performance of a number of axially-locatcd experiments. Using the Z DH as a source, we developed the new blast wave diagnostic, and applied it in a number of novel experiments. A large number of physics issues can be addressed by the Z DH and blast wave diagnostic, including experiments to study opacity, radiation transport and hydrodynamics. We will show how some experiments are quite sensitive to the source details while others are much less so. A synopsis and analysis of recently completed experiments as well as proposals for future work are presented.

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 lagers 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 I-atmosphere methane-fill. We discuss data and simulation predictions for I-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.

Dynamic loading experiments are described using nanosecond scale laser pulses of 2 to 1000 GW/cm2 over a region 5 mm in diameter. The laser irradiance was tailored to generate shocks or quasi‐isentropic compression. The experiments include novel diagnostic techniques with high temporal resolution: transient x‐ray diffraction (TXD) and polarization‐dependent reflectivity (ellipsometry). TXD uses a laser‐produced plasma to form an x‐ray source. These x rays are collimated by a pinhole to form a Bragg scattering source, which allowed powder lines to be detected from polycrystalline samples such as beryllium foils. Ellipsometry has been demonstrated with 50 ps resolution using the reflectance of a pulsed 660 nm laser from silicon and tin samples through lithium fluoride windows. Ellipsometry can indicate phase changes and potentially yields estimates of surface temperature via the dielectric conductivity. The combination of TXD and ellipsometry with VISAR measurements provides precision characterization of dy...