R. M. Cavallo

A multiscale strength model for extreme loading conditions

We present a multiscale strength model in which strength depends on pressure, strain rate, temperature, and evolving dislocation density. Model construction employs an information passing paradigm to span from the atomistic level to the continuum level. Simulation methods in the overall hierarchy include density functional theory, molecular statics, molecular dynamics, dislocation dynamics, and continuum based approaches. Given the nature of the subcontinuum simulations upon which the strength model is based, the model is particularly appropriate to strain rates in excess of 104 s−1. Strength model parameters are obtained entirely from the hierarchy of simulation methods to obtain a full strength model in a range of loading conditions that so far has been inaccessible to direct measurement of material strength. Model predictions compare favorably with relevant high energy density physics (HEDP) experiments that have bearing on material strength. The model is used to provide insight into HEDP experimental ...

Strong stabilization of the Rayleigh-Taylor instability by material strength at megabar pressures

Experimental results showing significant reductions from classical in the Rayleigh–Taylor (RT) instability growth rate due to high pressure effective lattice viscosity in metal foils are presented. Stabilization of RT instability (RTI) by ablation and density gradients has been studied for decades. The regime of stabilized RTI due to material strength at high pressure is new. On the Omega Laser in the Laboratory for Laser Energetics, University of Rochester, target samples of polycrystalline vanadium are compressed and accelerated quasi-isentropically at ∼1 Mbar pressures, while maintaining the samples in the solid-state. Provided strong shocks are avoided, the higher the applied peak pressure, the higher the predicted foil strength, and hence, the higher the degree of strength stabilization of RTI. Several experiments were conducted where the amount of RT growth is measured by face-on radiography. The vanadium samples are probed by a laser driven He-α x-ray backlighter which produced 5.2 keV radiation. C...

Hydra Observations of Aluminum Abundances in the Red Giants of the Globular Clusters M80 and NGC 6752

Aluminum and other metal abundances were determined in 21 red giants in the globular clusters NGC 6752 and M80 as part of a larger study to determine whether the aluminum distribution on the red giant branch is related to the second-parameter effect that causes clusters of similar metallicity to display different horizontal-branch morphologies. The observations were obtained of the Al I lines near 6700 A with the Cerro Tololo Inter-American Observatory Blanco 4 m telescope and Hydra multiobject spectrograph. The spectra have a resolving power of 18,000 or 9400, with typical signal-to-noise ratios of 100–200. Mean [Fe/H] values obtained from the spectra are -1.58 for NGC 6752 and -1.73 for M80; this represents the first spectroscopic iron abundance determination for M80. Both NGC 6752 and M80 display a spread in aluminum abundance, with mean [Al/Fe] ratios of +0.51 and +0.37, respectively. No trend in the variation of the mean Al abundance with position on the giant branch is discernible in either cluster with our small sample.

Tailored ramp-loading via shock release of stepped-density reservoirsa)

The concept of a gradient piston drive has been extended from that of a single component reservoir, such as a high explosive, to that of a multi-component reservoir that utilizes low density foams and large shocks to achieve high pressures (∼3.5 mbar) and controlled pressure vs. time profiles on a driven sample. Simulated and experimental drives shaped through the use of multiple component (including carbonized resorcinol formaldehyde and SiO2 foam) reservoirs are compared. Individual density layers in a multiple component reservoir are shown to correlate with velocity features in the measured drive which enables the ability to tune a pressure drive by adjusting the components of the reservoir. Pre-shot simulations are shown to be in rough agreement with the data, but post-shot simulations involving the use of simulated plasma drives were needed to achieve an exact match. Results from a multiple component reservoir shot (∼3.5 mbar) at the National Ignition Facility are shown.

Preliminary abundance analysis of galactic bulge main sequence, subgiant, and giant branch stars observed during microlensing with Keck/HIRES

We present an abundance analysis of six main sequence turnoff, subgiant, and giant branch stars toward the Galactic bulge that were observed with Keck/HIRES during microlensing events. This is an early look at the first detailed chemical analysis of main sequence stars in the Galactic bulge. Lensing events allow the effective aperture of Keck to be increased beyond its current dimensions; although, some events still stretched its spectroscopic capabilities. Future large telescopes with high resolution and high throughput spectrometers will allow the study of abundances in distant stellar populations and in less evolved stars with greater ease.

Interpretation of laser-driven V and TA Rayleigh-Taylor strength experiments

We present theoretical and computational analysis of the deformation regimes accessed by recent Rayleigh-Taylor (RT) material strength experiments in vanadium (V) and tantalum (Ta) done at the Omega laser at high pressures (>1 Mbar) and high strain rates (106 - 108 sec-1). Within the context of the LLNL multiscale models, the V-RT experiment appears to be dominated by deformation in the drag regime, whereas the Ta-RT experiment resides largely within the thermal activation regime.

Rayleigh-Taylor strength experiments of the pressure-induced α→ε→α′ phase transition in iron

We present here the first dynamic Rayleigh-Taylor (RT) strength measurement of a material undergoing solid-solid phase transition. Iron is quasi-isentropically driven across the pressure-induced bcc (α-Fe) → hcp (e-Fe) phase transition and the dynamic strength of the α, e and reverted α′ phases have been determined via proton radiography of the resulting Rayleigh-Taylor unstable interface between the iron target and high-explosive products. Simultaneous velocimetry measurements of the iron free surface yield the phase transition dynamics and, in conjunction with detailed hydrodynamic simulations, allow for determination of the strength of the distinct phases of iron. Forward analysis of the experiment via hydrodynamic simulations reveals significant strength enhancement of the dynamically-generated e-Fe and reverted α′-Fe, compareable in magnitude to the strength of austenitic stainless steels.

Designfor solid-state Rayleigh-Taylor experiments in tantalum at Omega

We have designed an experiment for the Omega – EP laser facility to measure the Rayleigh-Taylor (RT) growth rate of solid-state Ta samples at ~1 Mbar pressures and very high strain rates, 107–108 s−1. A thin walled, hohlraum based, ramp-wave, quasi-isentropic drive has been developed for this experiment. Thick samples (~50 um) of Ta, with a pre-imposed sinusoidal rippled on the driven side, will be accelerated. The ripple growth due to the RT instability is greatly reduced due to the dynamic material strength. We will show detailed designs, and a thorough error analysis used to optimize the experiment and minimize uncertainty.

Modeling of grain size strengthening in tantalum at high pressures and strain rates

Laser-driven ramp wave compression experiments have been used to investigate the strength (flow stress) of tantalum and other metals at high pressures and high strain rates. Recently this kind of experiment has been used to assess the dependence of the strength on the average grain size of the material, finding no detectable variation with grain size. The insensitivity to grain size has been understood theoretically to result from the dominant effect of the high dislocation density generated at the extremely high strain rates of the experiment. Here we review the experiments and describe in detail the multiscale strength model used to simulate them. The multiscale strength model has been extended to include the effect of geometrically necessary dislocations generated at the grain boundaries during compatible plastic flow in the polycrystalline metal. We use the extended model to make predictions of the threshold strain rates and grain sizes below which grain size strengthening would be observed in the las...

Rayleigh–Taylor instabilities in high-energy density settings on the National Ignition Facility

Significance We present research results on the Rayleigh–Taylor (RT) instability at an unstable interface under high-energy density conditions using the National Ignition Facility at Lawrence Livermore National Laboratory. We can reach pressures in the 100-TPa regime on the Hugoniot, or ∼500-GPa regime along a quasi-isentrope, allowing the sample under study to remain solid, at planetary interior pressures. We observe RT stabilization (i) at an ablation front; (ii) in the presence of a strongly radiative shock; and (iii) in a unique regime of quasi-isentropic, high pressure, solid-state material flow, where the material strength significantly affects the evolution of a hydrodynamically unstable interface. The Rayleigh–Taylor (RT) instability occurs at an interface between two fluids of differing density during an acceleration. These instabilities can occur in very diverse settings, from inertial confinement fusion (ICF) implosions over spatial scales of ∼10−3−10−1 cm (10–1,000 μm) to supernova explosions at spatial scales of ∼1012 cm and larger. We describe experiments and techniques for reducing (“stabilizing”) RT growth in high-energy density (HED) settings on the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory. Three unique regimes of stabilization are described: (i) at an ablation front, (ii) behind a radiative shock, and (iii) due to material strength. For comparison, we also show results from nonstabilized “classical” RT instability evolution in HED regimes on the NIF. Examples from experiments on the NIF in each regime are given. These phenomena also occur in several astrophysical scenarios and planetary science [Drake R (2005) Plasma Phys Controlled Fusion 47:B419–B440; Dahl TW, Stevenson DJ (2010) Earth Planet Sci Lett 295:177–186].