Session D25: Focus Session: Matter at Extreme Conditions - New Experimental Capabilities

Ultrafast, high resolution, phase contrast imaging of shock response with synchrotron radiation: opportunities and challenges

Designing materials that function at dynamic extremes and predicting dynamic materials response require experimental investigations of their time, rate and microstructure dependences. Key to such experiments are {\it in situ}, in-volume, temporally and spatially resolved measurements (e.g., x-ray imaging and diffraction). Here we report ultrafast ($<$100 ps), high resolution ($\sim$3 $\mu$m), dynamic phase contrast imaging (PCI) measurements during high strain-rate loading (100 ns scale). A gas gun was installed at 32ID beamline of the Advanced Photon Source for dynamic loading, and dynamic PCI measurements were performed with a single x-ray pulse on representative materials/processes, including cylinder impact and penetration, large-cell foam compaction, cerium jet formation and granular material compression. We present overall experimental scheme and opportunities for dynamic materials research as seen from the preliminary results, as well as challenges both for photon sources and detectors.   

Probing Matter at an Atomic Unit of Pressure using convergent compression

Geometric confinement significantly increases the shock pressure as a spherically-converging shock approaches the central focus. Inertial confinement fusion is one area where this technique enables the ~100 MBar ablation pressure to multiply to the several-GBar pressure required for fusion. We are using x-ray radiography of a spherically convergent shock wave in a solid sphere to explore material equations of state at pressures which exceed the atomic unit of pressure (Eh/a0 = 300MBar); the energy density of a hydrogen atom. Measuring materials properties above this pressure will breach yet another significant barrier in our quest to understand extreme states, and will open a completely new realm where the atomic nature of matter is very strongly perturbed. We will discuss initial experiments preformed on the Omega laser facility and plans for future experiments on the NIF.   

High-resolution phase contrast imaging of brittle failure during impact loading

Heterogeneous processes involved in brittle failure necessitate in situ, spatially resolved observation. An impact capability has recently been developed in which synchrotron phase contrast imaging (PCI), at the 32-ID beamline of the Advanced Photon Source, can be used to resolve crack interfaces during dynamic deformation. The imaging is both fast and high-resolution as images with approximately 3 micrometer resolution are obtained from single x-ray pulses ($<$100 ps duration). Experiments have been performed to investigate questions regarding velocimetry interpretation, the effect of stress states, and whether cracking can occur under uniaxial compression. Uniaxial compression and tension in planar impact configurations and cylindrical impact penetration has been used to vary stress states and observe failure. PCI and velocimetry results from these experiments will be presented for a range of brittle materials spanning glasses and ceramics.   

Deformation and material dynamics under ultrafast compression

For decades, dynamic compression experiments have been used to determine the equation of state of materials, and examine material deformation at high strain rates. Within the last 15 years, ultrafast optical methods have been used to characterize deformation at strain rates in excess of 10$^{10}$/s. Recently such experiments have found broad consistency with empirical laws formulated at orders of magnitude lower strain rates, but have also discovered intriguing phenomena on short time scales, such as elastic stress orders of magnitude beyond the yield strength. These experiments explore the ultimate limits of material relaxation via deformation, and the results suggest exciting possibilities for practical and scientific application of ultrafast compression, including nonequilibrium material synthesis, determination of the equation of state with a small scale experiment, and the investigation of ultrahigh density with a table top laser. Here we will talk about our experiments on the ultrafast deformation of metals, including aluminum and iron, and the ultrafast compression of deuterium.   

High-resolution phase contrast imaging of jet formation in shocked cerium to examine material strength

Understanding the dynamic properties of metals has been a long-standing scientific challenge. Experiments are needed to locate phase boundaries, to obtain equation-of-state data within those boundaries, and to examine properties such as material strength in the relevant phases. Efforts have been underway in recent years to examine the multiphase equation-of-state for cerium largely because of its complex phase diagram that exists at relatively moderate pressures and temperatures. To date, experiments have been performed to determine the Hugoniot, the shock-melt transition, and to examine the low-pressure phases through the critical point. In the current work, we present novel data that uses ultrafast, high-resolution phase contrast imaging (PCI) to examine jet-formation in cerium for impact stresses that span the alpha-phase up to the melt boundary. These experiments were performed using a recently developed capability at the 32-ID beamline of the Advanced Photon Source that couples the PCI method with an impact system to obtain real-time, spatially resolved images during dynamic compression. Experimental results will be presented and compared with recent efforts that use more traditional shock-release and double-shock loading to examine strength.   

High Resolution Diagnostics for Simultaneous Measurements of Velocity and Density in Shock-Driven Instabilities

The interaction between a shock wave and the interface between two fluids of different density might induce macroscopic mixing of the fluids. It is generally accepted that baroclinic vorticity, resulting from misalignments between the density gradient across the interface and the pressure gradient of the shock wave, impels this macroscopic mixing. So far, the Extreme Fluids Team at Los Alamos National Laboratory has conducted the only detailed studies of the structure of the developing instability. These studies encompass simultaneous measurements of velocity and density via combined Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). Using this approach, the above mentioned Team has conducted extensive studies over a varicose curtain of heavy gas (SF$_6$). Since a curtain implies two succeeding interfaces, a new Vertical Shock Tube (VST) was developed for simultaneous characterization of velocity and density fields of single-interface shock-driven flows. This talk is intended to present some of the results obtained for double-interface shock driven flows, as well as describing the characteristics, challenges, and range of possibilities of the laser diagnostics incorporated in the VST.   

LCLS: A new frontier for time-resolved in situ lattice measurements in materials at extreme conditions

While laser-based pump-probe experiments have provided significant insights into the processes induced by dynamic loading (e.g. shock-induced phase transitions, elastic-plastic response), the material behavior at the lattice level has been extremely difficult to unveil due to the temporal and signal-to-noise limitations of laser-based x-ray techniques. Here we present recent dynamic x-ray diffraction measurements on the shock-induced behavior of polycrystalline Mg in the 10-40 GPa regime, followed by future experiments in the field of dynamic high pressure science using the free electron laser at LCLS as an ultrafast x-ray probe.   

Progress towards Single Shot Spectroscopic Techniques for Time-Resolved Measurements in the Diamond Anvil Cell

We will discuss how we are bridging the gap between static diamond anvil cell and dynamic shock experiments using various spectroscopic techniques which utilize nonlinear optics. Using pulsed laser techniques, we can achieve extreme temperatures while probing optical and chemical changes on fast time scales. Recent developments incorporating broadband spectroscopy into the laser heated diamond anvil cell have indicated that probing phase transitions while measuring temperature is possible [1]. Various methods for incorporating nonlinear vibrational spectroscopy (such as CARS) into the diamond anvil cell will be discussed. The application of these optical diagnostics to pulsed laser heating and table-top shock experiments [2] will be presented. \\[4pt] [1] R.S. McWilliams et al., in preparation. \\[0pt] [2] M.R. Armstrong et al., J. Appl. Phys., \textbf{108}, 023511, (2010).   

Phase diagram of shock and ramp-compressed tin

We will present powder x-ray diffraction results on laser-ramp-compressed solid tin up to 600 GPa, and discuss new methods for detecting the melting transition. Tin has a complex phase diagram with multiple observed and predicted high pressure phases and a moderate melting temperature, making it an ideal subject for a fundamental study of material properties using new techniques. Ramp compression in the solid allows access to extremely dense condensed phases and in the liquid the possibility for dynamically freezing molten tin. With newly developed x-ray diffraction methods we examine crystal structure, strength and texture in the dynamically compressed phases, and explore the possibility of a new method for mapping out melting curves.   

Ultrafast Shock Interrogation of Hydrogen Peroxide/Water Mixtures: Thermochemical Predictions of Shock Condition Chemistry

Hydrogen peroxide is a powerful oxidizer and its concentrated aqueous solutions exhibit very high reactivity, even sustaining detonation under strong enough confinement. Due to its simple composition and basic expected decomposition kinetics hydrogen peroxide is very suitable for studying the interplay of high pressures, temperatures and reactivity and their effect on the equation of state, particularly at the boundary between detonating and non-detonating behavior. To this end we performed speed of sound and picosecond time resolved shock measurements on solutions of hydrogen peroxide of concentrations from 30 to 90 percent, and analyzed the results in terms of common assumptions of chemical equilibrium in reactive fluid mixtures. Experimental shock states were achieved up to a maximum pressure of 20 GPa with corresponding shock velocities of 6-7 km/sec.   

Simulation of Forward and Inverse X-ray Scattering From Shocked Materials

The next generation of high-intensity, coherent light sources should generate sufficient brilliance to perform in-situ coherent x-ray diffraction imaging (CXDI) of shocked materials. In this work, we present beginning-to-end simulations of this process. This includes the calculation of the partially-coherent intensity profiles of self-amplified stimulated emission (SASE) x-ray free electron lasers (XFELs), as well as the use of simulated, shocked molecular-dynamics-based samples to predict the evolution of the resulting diffraction patterns. In addition, we will explore the corresponding inverse problem by performing iterative phase retrieval to generate reconstructed images of the simulated sample. The development of these methods in the context of materials under extreme conditions should provide crucial insights into the design and capabilities of shocked in-situ imaging experiments.