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Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the published Hugoniot Elastic Limit (HEL) for these materials. In situ x-ray diffraction has been used to directly measure the response of the shocked lattice during shock loading. Static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. In addition, experiments were conducted using a wide-angle detector to record x rays diffracted from multiple lattice planes simultaneously. These data showed uniaxial compression of Si (100) along the shock direction and three-dimensional compression of Cu (100). In the case of the Si diffraction, there was a multiple wave structure observed. This is evaluated to determine whether there is a phase transition occurring on the time scale of the experiments, or the HEL is much higher than previously reported. Results of the measurements are presented. (C) 2003 American Institute of Physics.

The high energy density of pulsed lasers can be used to generate shockless loading in solids to high pressures and compressions but low temperatures. [J. Edwards, K.T. Lorenz, B.A. Remington, S. Pollaine, J. Colvin, D. Braun, B.F. Lasinski, D. Reisman, J.M. McNaney, J.A. Greenough, R.Wallace, H. Louis, D. Kalantar, Laser-driven plasma loader for shockless compression and acceleration of samples in the solid state, Phys. Rev. Lett. 92 (2004) 075002.] We have used the Omega laser to extend the capabilities of this technique to multi-Mbar pressures and compressions approaching a factor of 2 in aluminum foils. The energy from a 3.7 ns laser pulse is used to drive a strong shock through a 200 μm polystyrene disc. The disc material unloads from a high-pressure state and expands across a 300 μm vacuum gap where it stagnates against the sample to produce a smooth, monotonically increasing load with rise times from a few to ∼20 ns. Ramped compression waves having peak pressures of 14–200 GPa (0.14–2.0 Mbar) and peak compressions ρ / ρ 0 of 1.1–2.0 were generated in the aluminum samples using laser pulse energies of 400 J to 2 kJ. Wave profiles from a series of successively thicker targets loaded to 120 GPa show the evolution of the high-pressure compression wave within the sample. The initial loading in the sample is shockless, and develops into a shock at a depth of 20–25 μm. We compare these wave profiles with hydrodynamic simulations from which we extract material temperatures and plastic strain rates behind the compression wave. Limitations and future prospects for this new shockless loading technique are discussed.

Recent development of a laser-based experimental platform allows loading materials to high pressures in the solid state while controlling both strain rate and peak pressure. The drive utilizes momentum transfer from a plasma generated by the introduction of a strong shock in a reservoir of low-Z material. This study looks at the response of a commercial aluminum alloy (6061-T6) subjected to pressures of 18 and 40 GPa at strain rates of 107/s and 5 × 107/s, respectively. It was found that the depth of the crater formed on the sample surface is a good indicator of the general yield behavior of the material and that a relatively simple strength model prevails under the loading conditions considered here. Metallographic examination of recovered samples showed no evidence of shear-band formation or significant melting due to plasma-surface interactions. Crystal plasticity-based calculations were used to assess the effects of material texture. Lack of shear-band formation during the laser-based drive is rationalized by considering the strain gradient as compared to grain size and texture.

Solid-state dynamics experiments at very high pressures and strain rates are becoming possible with high-power laser facilities, albeit over brief intervals of time and spatially small scales. To achieve extreme pressures in the solid state requires that the sample be kept cool, with T-sample < T-melt. To this end, a shockless, plasma-piston "drive" has been developed on the Omega laser, and a staged shock drive was demonstrated on the Nova laser. To characterize the drive, velocity interferometer measurements allow the high pressures of 10 to 200 GPa (0.1 to 2 Mbar) and strain rates of 10(6) to 10(8) s(-1) to be determined. Solid-state strength in the sample is inferred at these high pressures using the Rayleigh-Taylor (RT) instability as a "diagnostic." Lattice response and phase can be inferred for single-crystal samples from time-resolved X-ray diffraction. Temperature and compression in polycrystalline samples can be deduced from extended X-ray absorption fine-structure (EXAFS) measurements. Deformation mechanisms and residual melt depth can be identified by examining recovered samples. We will briefly review this new area of laser-based materials-dynamics research, then present a path forward for carrying these solid-state experiments to much higher pressures, P > 10(3) GPa (10 Mbar), on the National Ignition Facility (NIF) laser at Lawrence Livermore National Laboratory.

Laser-based shock experiments have been conducted in thin Si and Cu crystals at pressures above the Hugoniot elastic limit. In these experiments, static film and x-ray streak cameras recorded x rays diffracted from lattice planes both parallel and perpendicular to the shock direction. These data, showed uniaxial compression of Si(100) along the shock direction and three.-dimensional compression of Cu(100). In the case of the Si diffraction, there was a multiple wave structure observed, which may be due to a one-dimensional phase transition or a time variation in the shock pressure. A new film-based detector has been developed for these in situ dynamic diffraction experiments. This large-angle detector consists of three film cassettes that are positioned to record x rays diffracted from a shocked crystal anywhere within a full pi steradian. It records x rays that are diffracted from multiple lattice planes both parallel and at oblique angles with respect to the shock direction. It is a time-integrating measurement, but time-resolved data may be recorded using a short duration laser pulse to create the diffraction source x rays. This new instrument,has been fielded at the OMEGA and Janus lasers to study single-crystal materials shock compressed by direct laser irradiation. In these experiments, a multiple wave structure was observed on many different lattice planes in Si. These data provide information on, the structure under compression. (C) 2003 American Institute of Physics.

State selective differential cross sections for rotationally inelastic scattering of NO (Ji=0.5, 1.5, F1→Jf=2.5–12.5, F1 and Jf=1.5–9.5, F2) from He and D2 measured by crossed molecular beam product imaging are reported. The differential cross sections were extracted from the data images using a new basis image iterative fitting technique. The images typically exhibit a single broad rotational rainbow maximum that shifts from the forward to the backward scattering direction with increasing ΔJ. The angle of the rainbow maximum was lower at a given ΔJ for D2 than for He as a collision partner. At a collision energy of ∼500 cm−1, primarily the repulsive part of the potential surface is probed, which can be modeled with a two-dimensional hard ellipse potential. This model for rotationally inelastic scattering is shown to qualitatively match the experimental differential cross sections. A more advanced correlated electron pair approximation potential energy surface for NO+He does not give substantially better ...

Experimental results showing significant reductions from classical in the Rayleigh-Taylor instability growth rate due to high pressure effective lattice viscosity are presented. Using a laser created ramped drive, vanadium samples are compressed and accelerated quasi-isentropically at $\ensuremath{\sim}1\text{ }\text{ }\mathrm{Mbar}$ peak pressures, while maintaining the sample in the solid state. Comparisons with simulations and theory indicate that the high pressure, high strain rate conditions trigger a phonon drag mechanism, resulting in the observed high effective lattice viscosity and strong stabilization of the Rayleigh-Taylor instability.

A new recovery method for investigating material response to shockless (ramped) loading paths is described. The work makes use of a laser generated plasma piston that produces ramped loading at high strain rates (>≈107/s). Large sample sizes are utilized to prevent reflected wave interactions. The overall deformation path is characterized by two transients: one at very high strain rate on the 5–10 nanosecond time scale and one at a lower strain rate occurring over a 1–2 microsecond timescale. It was found that a sufficiently large region of material experiences shockless loading conditions enabling recovery based characterization. The presence of two strain transients makes the method more applicable to comparative assessments between shockless and shock loading conditions.

Solid state dynamics experiments at extreme pressures, P > 1000 GPa (10 Mbar), and ultrahigh strain rates (1.e6-1.e8 1/s) are being developed for the National Ignition Facility (NIF) laser. These experiments will open up exploration of new regimes of materials science at an order of magnitude higher pressures than have been possible to date. Such extreme, solid state conditions can be accessed with a ramped pressure drive. The experimental, computational, and theoretical techniques are being developed and tested on the Omega laser. Velocity interferometer measurements (VISAR) establish the high pressure conditions generated by the ramped drive. Constitutive models for solid state strength under these conditions are tested by comparing simulations with experiments measuring perturbation growth from the Rayleigh-Taylor instability in solid state samples of vanadium. Radiography techniques using synchronized bursts of x-rays have been developed to diagnose this perturbation growth. Experiments on Omega demonstrating these techniques at peak pressures of {approx}1 Mbar will be discussed. The time resolved observation of foil cracking and void formation show the need for tamped samples and a planar drive.

Solid state experiments at extreme pressures (0.1 – 1 Mbar) and strain rates (106–108 s−1) are being developed on high‐energy laser facilities. The goal is a capability to test constitutive models for high‐pressure, solid‐state strength of materials. Relevant constitutive models are discussed, and our progress in developing a ramped‐pressure, shockless drive is given. Designs to test the constitutive models with experiments measuring perturbation growth due to the Rayleigh‐Taylor instability ‐in solid‐state samples are presented. Results from dynamic diffraction and EXAFS lattice diagnostics are given, showing that compression, phase, and temperature can be inferred on sub‐nsec time scales.

We have designed an experimental technique to use on the National Ignition Facility (NIF) laser to achieve very high pressure (P max > 10 Mbar = 1000 GPa), dense states of matter at moderate temperatures (T < 0.5 eV = 6000 K), relevant to the core conditions of the giant planets. A discussion of the conditions in the interiors of the giant planets is given, and an experimental design that can approach those conditions is described.

This report describes work done using isentropic drives based on high-explosives and lasers, along with recovery and ex-situ examination, to characterize material deformation behavior.

A new method for shockless compression and acceleration of solid materials is presented. A plasma reservoir pressurized by a laser-driven shock unloads across a vacuum gap and piles up against an Al sample thus providing the drive. The rear surface velocity of the Al was measured with a line VISAR, and used to infer load histories. These peaked between approximately 0.14 and 0.5 Mbar with strain rates approximately 10(6)-10(8) s(-1). Detailed simulations suggest that apart from surface layers the samples can remain close to the room temperature isentrope. The experiments, analysis, and future prospects are discussed.

Laser‐based experiments are being developed to study the response of solids under high pressure loading. Diagnostic techniques that have been applied include dynamic x‐ray diffraction, VISAR wave profile measurements, and post‐shock recovery and analysis. These techniques are presented with some results from shocked Si, Al, and Cu experiments.

Microwave interferometry is a useful technique for understanding the development and propagation of detonation waves. The velocity of the front can be determined directly with the dielectric constant of the explosive and the instantaneous phase difference of the reflected microwave signal from the detonation front. However, the dielectric constant of HMX-based explosives has been measured only over a small range of wavelengths. Here we employ an open-ended coaxial probe to determine the complex dielectric constant for LX-10 and other HMX-based explosives over the 5-20 GHz range. The propagation of a detonation wave in a lightly-confined cylindrical charge geometry is described where the microwave-reflective properties of the detonation front are characterized with a waveguide. For comparison, piezoelectric pins were used to measure the detonation velocity and indirectly estimate the dielectric constant of LX-10 at 26.5 GHz. Future work in this area will also be discussed.

Chemical dynamics is the exacting study of individual details of chemical reactions. This involves studying both intramolecular and intermolecular motion of energy and mass. A new tool in the study of chemical dynamics is ion imaging. Unimolecular photodissociation [1, 2, 3], reactive scattering [4, 5] and inelastic scattering [6, 7] are areas that have been investigated over the last several years using ion imaging. Many of these experiments were performed at the Chemical Dynamics Visitors Laboratory at Sandia National Laboratories. Two-dimensional imaging techniques allow the measurement of the velocity of a state-selectively photoionized product of a unimolecular dissociation or a bimolecular interaction. In the last few years there have been significant improvements in these techniques with the further development of velocity mapping and the accelerated application of the techniques to the study of bimolecular reactions. In this paper we present an overview of some experiments that have been performed at Sandia that highlight the present experimental capabilities available.

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...

The use of in situ extended x-ray absorption fine structure (EXAFS) for characterizing nanosecond laser-shocked vanadium, titanium, and iron has recently been demonstrated. These measurements are extended to laser-driven, quasi-isentropic compression experiments (ICE). The radiation source (backlighter) for EXAFS in all of these experiments is obtained by imploding a spherical target on the OMEGA laser [T. R. Boehly et al., Rev. Sci. Instrum. 66, 508 (1995)]. Isentropic compression (where the entropy is kept constant) enables to reach high compressions at relatively low temperatures. The absorption spectra are used to determine the temperature and compression in a vanadium sample quasi-isentropically compressed to pressures of up to ∼0.75Mbar. The ability to measure the temperature and compression directly is unique to EXAFS. The drive pressure is calibrated by substituting aluminum for the vanadium and interferometrically measuring the velocity of the back target surface by the velocity interferometer sy...

A new approach to the study of material strength of metals at extreme pressures has been developed on the Omega laser, using a ramped plasma piston drive. The laser drives a shock through a solid plastic reservoir that unloads at the rear free surface, expands across a vacuum gap, and stagnates on the metal sample under study. This produces a gently increasing ram pressure, compressing the sample nearly isentropically. The peak pressure on the sample, inferred from interferometric measurements of velocity, can be varied by adjusting the laser energy and pulse length, gap size, and reservoir density, and obeys a simple scaling relation [J. Edwards et al., Phys. Rev. Lett. 92, 075002 (2004)]. In an important application, using in-flight x-ray radiography, the material strength of solid-state samples at high pressure can be inferred by measuring the reductions in the growth rates (stabilization) of Rayleigh–Taylor unstable interfaces. This paper reports the first attempt to use this new laser-driven, quasi-i...