Joseph B. Stone

Experimental observations on the links between surface perturbation parameters and shock-induced mass ejection

We have assembled together our ejecta measurements from explosively shocked tin acquired over a period of about ten years. The tin was cast at 0.99995 purity, and all of the tin targets or samples were shocked to loading pressures of about 27 GPa, allowing meaningful comparisons. The collected data are markedly consistent, and because the total ejected mass scales linearly with the perturbations amplitudes they can be used to estimate how much total Sn mass will be ejected from explosively shocked Sn, at similar loading pressures, based on the surface perturbation parameters of wavelength and amplitude. Most of the data were collected from periodic isosceles shapes that approximate sinusoidal perturbations. Importantly, however, we find that not all periodic perturbations behave similarly. For example, we observed that sawtooth (right triangular) perturbations eject more mass than an isosceles perturbation of similar depth and wavelength, demonstrating that masses ejected from irregular shaped perturbatio...

Ejected particle size measurement using Mie scattering in high explosive driven shockwave experiments

We report on the development of a diagnostic to provide constraints on the size of particles ejected from shocked metallic surfaces. The diagnostic is based on measurements of the intensity of laser light transmitted through a cloud of ejected particles as well as the angular distribution of scattered light, and the analysis of the resulting data is done using the Mie solution. We describe static experiments to test our experimental apparatus and present initial results of dynamic experiments on Sn targets. Improvements for future experiments are briefly discussed.

Second shock ejecta measurements with an explosively driven two-shockwave drive

We develop and apply an explosively driven two-shockwave tool in material damage experiments on Sn. The two shockwave tool allows the variation of the first shockwave amplitude over range 18.5 to 26.4 GPa, with a time interval variation between the first and second shock of 5 to 7 μs. Simulations imply that the second shock amplitude can be varied as well and we briefly describe how to achieve such a variation. Our interest is to measure ejecta masses from twice shocked metals. In our application of the two-shockwave tool, we observed second shock ejected areal masses of about 4 ± 1 mg/cm2, a value we attribute to unstable Richtmyer-Meshkov impulse phenomena. We also observed an additional mass ejection process caused by the abrupt recompression of the local spallation or cavitation of the twice shocked Sn.

MEASUREMENT OF THE SHOCK-HEATED MELT CURVE OF LEAD USING PYROMETRY AND REFLECTOMETRY

Data on the high-pressure melting temperatures of metals is of great interest in several fields of physics including geophysics. Measuring melt curves is difficult but can be performed in static experiments (with laser-heated diamond-anvil cells, for instance) or dynamically (i.e., using shock experiments). However, at the present time, both experimental and theoretical results for the melt curve of lead are at too much variance to be considered definitive. As a result, we decided to perform a series of shock experiments designed to provide a measurement of the melt curve of lead up to about 50GPa in pressure. At the same time, we developed and fielded a reflectivity diagnostic, and conducted measurements on tin as well. The results show that the melt curve of lead is somewhat higher than the one previously obtained with static compression and heating techniques.

The study of high-speed surface dynamics using a pulsed proton beam

We present experimental results supporting physics based ejecta model development, where we assume ejecta form as a special limiting case of a Richtmyer-Meshkov (RM) instability with Atwood number A = -1. We present and use data to test established RM spike and bubble growth rate theory through application of modern laser Doppler velocimetry techniques applied in a novel manner to coincidentally measure bubble and spike velocities from shocked metals. We also explore the link of ejecta formation from a solid material to its plastic flow stress at high-strain rates (

Estimation of Metal Strength at Very High Rates Using Free-Surface Richtmyer–Meshkov Instabilities

107/s) and high strains (700%).

A class of ejecta transport test problems

Recently, Richtmyer–Meshkov Instabilities (RMI) have been proposed for studying the average strength at strain rates up to at least 107/s. RMI experiments involve shocking a metal interface that has initial sinusoidal perturbations. The perturbations invert and grow subsequent to shock and may arrest because of strength effects. In this work we present new RMI experiments and data on a copper target that had five regions with different perturbation amplitudes on the free surface opposite the shock. We estimate the high-rate, low-pressure copper strength by comparing experimental data with Lagrangian numerical simulations. From a detailed computational study we find that mesh convergence must be carefully addressed to accurately compare with experiments, and numerical viscosity has a strong influence on convergence. We also find that modeling the as-built perturbation geometry rather than the nominal makes a significant difference. Because of the confounding effect of tensile damage on total spike growth, which has previously been used as the metric for estimating strength, we instead use a new strength metric: the peak velocity during spike growth. This new metric also allows us to analyze a broader set of experimental results that are sensitive to strength because some larger initial perturbations grow unstably to failure and so do not have a finite total spike growth.

Explosively driven two-shockwave tools with applications

Hydro code implementations of ejecta dynamics at shocked interfaces presume a source distribution function of particulate masses and velocities, f0(m,u;t). Some properties of this source distribution function have been determined from Taylor- and supported-shockwave experiments. Such experiments measure the mass moment of f0 under vacuum conditions assuming weak particle-particle interactions and, usually, fully inelastic scattering (capture) of ejecta particles from piezoelectric diagnostic probes. Recently, planar ejection of W particles into vacuum, Ar, and Xe gas atmospheres have been carried out to provide benchmark transport data for transport model development and validation. We present those experimental results and compare them with modeled transport of the W-ejecta particles in Ar and Xe.

Infrared emissivity of tin upon release of a 25 GPa shock into a lithium fluoride window

We present the development of an explosively driven physics tool to generate two mostly uniaxial shockwaves. The tool is being used to extend single shockwave ejecta models to account for a second shockwave a few microseconds later. We explore techniques to vary the amplitude of both the first and second shockwaves, and we apply the tool experimentally at the Los Alamos National Laboratory Proton Radiography (pRad)facility. The tools have been applied to Sn with perturbations of wavelength λ = 550 μm, and various amplitudes that give wavenumber amplitude products of kh {3/4,1/2,1/4,1/8}, where h is the perturbation amplitude, and k = 2π/λ is the wavenumber. The pRad data suggest the development of a second shock ejecta model based on unstable Richtmyer-Meshkov physics.

Use of IR pyrometry to measure free-surface temperatures of partially melted tin as a function of shock pressure

We measured the emissivity of a tin sample at its interface with a lithium-fluoride window upon release of a 25 GPa shock wave from the tin into the window. Measurements were made over four wavelength bands between 1.2 and 5.4 μm. Thermal emission backgrounds from the tin, glue, and lithium fluoride were successfully removed from the reflectance signals. Emissivity changes for the sample, which was initially nearly specular, were small except for the longest wavelength band, where uncertainties were high because of poor signal-to-noise ratio at that wavelength. A thin glue layer, which bonds the sample to the window, was found to heat from reverberations of the shock wave between the tin and the lithium fluoride. At approximately 3.4 μm, the thermal emission from the glue was large compared to the tin, allowing a good estimate of the glue temperature from the thermal radiance. The glue appears to remain slightly colder than the tin, thereby minimizing heat conduction into or out of the tin immediately aft...