Session E3: NT.2 Novel Techniques: PCI/2D Visar

Compression and Shear of Tantalum to 64 GPa

It has been reported that the $\beta $-phase (body-centered-cubic) of tantalum (Ta) is stable to pressures over mega-bar under hydrostatic compression.[1] However, the shock compression clearly indicates its transformation to the $\omega $-phase (hexagonal) at 45 GPa.[2] Theoretical work suggests that the shear might have played an important role in inducing this phase transformation.[3] Here we report our experimental results on the effects of pressure and shear by use of a rotational diamond anvil cell and the synchrotron X-ray diffraction. The results indicate that under extensive shear and pressures over 60 GPa, Ta remains stable in the $\beta $ phase. \\[4pt] [1] H. Cynn and C.-S. Yoo, ``Phy. Rev. B'' \textbf{59}, 8526 (1999)\\[0pt] [2] L. M. Hsiung and D. H. Lassila, ``Scr. Mater.'' \textbf{38}, 1371 (1998)\\[0pt] [3] D. Mukherjee, K. D. Joshi, S. C. Gupta, ``J. Phys. Conf. Ser.'' 377, 012072 (2012)   

Laser Interferometry Measurements in Starphire$\textregistered$ Soda-Lime Glass Shocked to 14 GPa

Symmetric plate impact experiments were conducted on Starphire$\textregistered$ samples at peak stresses varying between 4-14 GPa, and laser interferometry was used to monitor the particle velocity histories at the sample-impactor interface. A few hundred ns after impact, the lowest stress experiments exhibited either decreasing particle velocity or fluctuations in particle velocity. The higher stress experiments did not show these features and exhibited a constant particle velocity throughout. Sample compression causes a change in the optical path length, resulting in the observed particle velocity to be offset from the actual particle velocity at the sample-impactor interface. The actual particle velocities were known independently from the projectile velocity measurements, due to the symmetric nature of the impact in the present work. Observed and actual particle velocities were compared to obtain velocity corrections for the range of compressions examined. The present results were compared to published velocity correction data for Starphire$\textregistered$. While the present data agree with the published data at the lowest stresses, the two sets of results do not agree at higher stresses. Potential reasons for this disagreement are presented. Work supported by the DOE/NNSA.   

Shock wave viscosity measurements

Several decades ago a method was proposed and demonstrated to measure the viscosity of fluids at high pressure by observing the oscillatory damping of sinusoidal perturbations on a shock front [1]. A detailed mathematical analysis of the technique carried out subsequently by Miller and Ahrens [2] revealed its potential, as well as a deep level of complexity in the analysis. We revisit the ideas behind this technique in the context of a recent experimental development: two-dimensional imaging velocimetry. The new technique allows one to capture a broad spectrum of perturbations down to few micron scale-lengths imposed on a shock front from an initial perturbation. The detailed evolution of the perturbation spectrum is sensitive to the viscosity in the fluid behind the shock front. Initial experiments are aimed at examining the viscosity of shock compressed SiO$_2$ just above the shock melting transition.\\[4pt] [1] A. D. Sakharov et al., Sov. Phys. Dokl. {\bf 9} 1091 (1965);\\[0pt] [2] G. H. Miller and T. J. Ahrens Rev. Mod. Phys {\bf 63} 919 (1991).   

Dynamic Loading to Study Damage Evolution in Heterogeneous Microstructures using IMPULSE at the Advanced Photon Source

The performance, safety, and thermo-mechanical response of heterogeneous materials such as plastic-bonded explosives (PBXs) is inherently linked to microstructural phenomena. Experimental resolution of the physics and chemistry of the microstructure at appropriate length scales, both at ambient conditions and under dynamic loading, are highly desirable to develop new materials and models to predict their behavior. Here, the dynamic response of several heterogeneous materials is studied with real-time, \textit{in situ}, spatially resolved measurements using the IMPULSE platform at the Advanced Photon Source (APS) at Argonne National Laboratory. Known PBX damage mechanisms such as void collapse, crack propagation, and plasticity or material flow are imaged at ultrafast speeds under shock loading conditions with simultaneous X-ray phase contrast imaging (PCI). PCI at APS beam line 32-ID is an improvement over radiography because it detects phase shifts in the transmitted X-ray beam, making PCI an ideal technique to image interfaces (i.e. heterogeneity) with high spatial resolution (2um) in-plane. IMPULSE experiments are compared with similar experiments at other length and time scales to discern relevant processing-structure-properties relationships for several PBX materials.   

Numerical re-focusing of 2d-VISAR data

Two dimensional velocity interferometer (2d-VISAR) data can be treated as a kind of hologram, since fringes recorded by the interferometer manifest both phase and magnitude information about changes in the optical field of the target, over an image. By the laws of diffraction, knowledge of the optical field at one focal plane can be used to calculate the optical field at another focal plane. Hence a numerical re-focusing operation can be performed on the data post-experiment, which can bring into focus narrow features that were recorded in an out of focus configuration. Demonstration on shocked Si data and theoretical models are shown.   

Imaging the propagation of shock waves with both high temporal and high spatial resolution using XFELs

The emergence of x-ray sources of the fourth generation, so called x-ray free-electron lasers (XFELs), comes along with completely new research opportunities in various scientific fields. During the last year we developed an x-ray microscope based on beryllium compound refractive lenses (Be-CRLs), which is especially optimized for the XFEL environment and provides focusing capabilities down to 100nm and even below. Based on magnified x-ray phase contrast imaging, this new setup enables us to pursue high-resolution x-ray imaging experiments with single XFEL-pulses. In a first experiment, carried out at the Matter in Extreme Conditions (MEC) endstation of the LCLS, the performance of the instrument was investigated by direct imaging of shock waves in different materials. The shock wave was induced by an intense 150ps optical laser pulse. The evolution of the shock wave was then monitored with the XFEL-beam. In this contribution we report on first analysis results of phase contrast imaging of shock waves in matter.\\[4pt] In collaboration with Brice Arnold, Eric Galtier, Hae Ja Lee, Bob Nagler, Jerome Hastings, SLAC National Accelerator Laboratory, 2575 Sand Hill Rd., Menlo Park, CA 94025, USA; Damian Hicks, Yuan Ping, Gilbert Collins, Lawrence Livermore National Laboratory, P.O. Box 808, Livermore, CA 94551, USA; and Robert Hoppe, Vivienne Meier, Jens Patommel, Frank Seiboth, Christian Schroer, Institute of Structural Physics, Technische Universit\"at Dresden, D-01062 Dresden, Germany.