The structure of laser-shock-compressed polycrystalline iron was probed using in situ x-ray diffraction over a pressure range spanning the α-phase transition. Measurements were also made of the c/a ratio in the phase, which, in contrast with previous in situ x-ray diffraction experiments performed on single crystals and large scale molecular dynamics (MD) simulations are close to those found in high pressure diamond anvil cell experiments. This is consistent with the observation that significant plastic flow occurs within the nanosecond timescale of the experiment. Furthermore, within the sensitivity of the measurement technique, the FCC phase that had been predicted by MD simulations was not observed.

The magnetic-pressure drive technique allows single-shot measurements of compression isentropes. We have used this method to measure the isentropes in the pressure–volume space of bulk and single-crystal lead, and lead–antimony alloy to ∼400 kbar.The isentrope pressure–volume curves were found from integration of the experimentally deduced Lagrangian sound speed as a function of particle velocity. A characteristics calculation method was used to convert time-resolved free-surface velocity measurements to corresponding in situ particle-velocity histories, from which the Lagrangian sound speed was determined from the times for samples of different thicknesses to reach the same particle velocity. Use of multiple velocity interferometry probes decreased the uncertainty due to random errors by allowing multiple measurements.Our results have errors of from 4% to 6% in pressure, ∼1% to 1.5% in volume, depending on the number of measurements, and are consistent with existing isotherm and Hugoniot data and models for lead.

Bismuth has long been a prototypical system for investigating phase transformations and melting at high pressure. Despite decades of experimental study, however, the lattice-level response of Bi to rapid (shock) compression and the relationship between structures occurring dynamically and those observed during slow (static) compression, are still not clearly understood. We have determined the structural response of shock-compressed Bi to 68 GPa using femtosecond X-ray diffraction, thereby revealing the phase transition sequence and equation-of-state in unprecedented detail for the first time. We show that shocked-Bi exhibits a marked departure from equilibrium behavior - the incommensurate Bi-III phase is not observed, but rather a new metastable phase, and the Bi-V phase is formed at significantly lower pressures compared to static compression studies. We also directly measure structural changes in a shocked liquid for the first time. These observations reveal new behaviour in the solid and liquid phases of a shocked material and give important insights into the validity of comparing static and dynamic datasets.

The Rayleigh-Taylor instability is an important limitation in inertial confinement fusion capsule designs. Significant work both theoretically and experimentally has been done to demonstrate the stabilizing effects of material flow through the unstable region. The experimental verification has been done predominantly in planar geometry. Convergent geometry introduces effects not present in planar geometry such as shell thickening and accelerationless growth of modal amplitudes ͑e.g., Bell-Plesset growth͒. Amplitude thresholds for the nonlinear regime are reduced, since the wavelength of a mode m decreases with convergence ϳR/m, where R is the radius. Convergent effects have been investigated using an imploding cylinder driven by x-ray ablation on the NOVA laser ͓J. L. Emmet, W. F. Krupke, and J. B. Trenholme, Sov. J. Quantum Electron. 13, 1 ͑1983͔͒. By doping sections of the cylinder with opaque materials, in conjunction with x-ray backlighting, the growth and feedthrough of the perturbations from the ablation front to the inner surface of the cylinder for various initial modes and amplitudes from early time through stagnation was measured. Mode coupling of illumination asymmetries with material perturbations is observed, as well as phase reversal of the perturbations from near the ablation front to the inner surface of the cylinder. Perturbation growth is observed due to convergence and compressibility alone, without the effects of acceleration, and scales as ϳ1/R, where is the mass density. Imaging is performed with an x-ray pinhole camera coupled to a gated microchannel plate detector.

Richtmyer–Meshkov (RM) mixing seeded by multimode initial surface perturbations in a convergent, compressible, miscible plasma system is measured on the OMEGA [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] laser system. A strong shock (Mach 12–20), created by 50 laser beams, is used to accelerate impulsively a thin aluminum shell into a lower density foam. As the system converges, both interfaces of the aluminum are RM unstable and undergo mixing. Standard x-ray radiographic techniques are employed to survey accurately the zero-order hydrodynamics, the average radius and overall width, of the marker. LASNEX [G. B. Zimmerman et al., Comments on Plasma Physics 2, 51 (1975)] simulations are consistent with the zero-order behavior of initially smooth markers. In experiments with smooth aluminum markers, the measured marker width shortly after shock passage behaves incompressibly and thickens due to Bell–Plesset effects. At high convergence (>4), the marker begins to compress as the rebounding shock passes back through the marker. When an initial multimode perturbation is introduced to the outer surface of the marker, the measured marker width is observed to increase by 10–15 μm, and is substantially smaller than as-shot simulations using RAGE [R. M. Baltrusaitis et al., Phys. Fluids 8, 2471 (1996)] would predict.