Biomedical subjects

J S Wark

Publications and source records attributed to J S Wark.

12 recordsLinked to original sources

PubMed2006

Shock deformation of face-centred-cubic metals on subnanosecond timescales.

Despite its fundamental importance for a broad range of applications, little is understood about the behaviour of metals during the initial phase of shock compression. Here, we present molecular dynamics (MD) simulations of shock-wave propagation through a metal allowing a detailed analysis of the dynamics of high strain-rate plasticity. Previous MD simulations have not seen the evolution of the strain from one- to three-dimensional compression that is observed in diffraction experiments. Our large-scale MD simulations of up to 352 million atoms resolve this important discrepancy through a detailed understanding of dislocation flow at high strain rates. The stress relaxes to an approximately hydrostatic state and the dislocation velocity drops to nearly zero. The dislocation velocity drop leads to a steady state with no further relaxation of the lattice, as revealed by simulated X-ray diffraction.

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PubMed2006

Radiation transfer effects on the spectra of laser-generated plasmas.

Experimental x-ray spectra of the H-like 2p --> 1s (Lyman-alpha) doublet have been obtained using time-integrated high-resolution spectroscopy of a constrained-flow, laser-generated aluminum plasma. These spectra show monotonic alteration of the relative intensities of the doublet components with distance from the target surface. Excellent agreement between experiment and theory is found only if the modeling includes both ion collisional rates between the fine-structure components of the level and, more importantly, the radiative pumping of one Lyman-alpha component by the other component along the direction of the major velocity gradient (i.e., perpendicular to the direction of spectra observation). Understanding radiation transfer in plasmas with high velocity gradients is important in modeling many astrophysical objects, and this experiment acts as a benchmark for such complex calculations.

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PubMed2005

Observation of structural anisotropy and the onset of liquidlike motion during the nonthermal melting of InSb.

The melting dynamics of laser excited InSb have been studied with femtosecond x-ray diffraction. These measurements observe the delayed onset of diffusive atomic motion, signaling the appearance of liquidlike dynamics. They also demonstrate that the root-mean-squared displacement in the [111] direction increases faster than in the [110] direction after the first 500 fs. This structural anisotropy indicates that the initially generated fluid differs significantly from the equilibrium liquid.

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PubMed2005

Direct observation of the alpha-epsilon transition in shock-compressed iron via nanosecond x-ray diffraction.

In situ x-ray diffraction studies of iron under shock conditions confirm unambiguously a phase change from the bcc (alpha) to hcp (epsilon) structure. Previous identification of this transition in shock-loaded iron has been inferred from the correlation between shock-wave-profile analyses and static high-pressure x-ray measurements. This correlation is intrinsically limited because dynamic loading can markedly affect the structural modifications of solids. The in situ measurements are consistent with a uniaxial collapse along the [001] direction and shuffling of alternate (110) planes of atoms, and are in good agreement with large-scale nonequilibrium molecular dynamics simulations.

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PubMed2005

Atomic-scale visualization of inertial dynamics.

The motion of atoms on interatomic potential energy surfaces is fundamental to the dynamics of liquids and solids. An accelerator-based source of femtosecond x-ray pulses allowed us to follow directly atomic displacements on an optically modified energy landscape, leading eventually to the transition from crystalline solid to disordered liquid. We show that, to first order in time, the dynamics are inertial, and we place constraints on the shape and curvature of the transition-state potential energy surface. Our measurements point toward analogies between this nonequilibrium phase transition and the short-time dynamics intrinsic to equilibrium liquids.

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PubMed2005

Picosecond x-ray studies of coherent folded acoustic phonons in a multiple quantum well.

Coherent folded acoustic phonons in a multilayered GaSb/InAs epitaxial heterostructure were generated by femtosecond laser pulses and studied by means of ultrafast x-ray diffraction. Coherent phonons excited simultaneously in the fundamental acoustic branch and the first back-folded branch were detected. This represents the first clear evidence for phonon branch folding based directly on the atomic motion to which x-ray diffraction is sensitive. From a comparison of the measured phonon-modulated x-ray reflectivity with simulations, evidence was found for a reduction of the laser penetration depth. This reduction can be explained by the self-modulation of the refractive index due to photogenerated free carriers.

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PubMed2005

Clocking femtosecond X rays.

Linear-accelerator-based sources will revolutionize ultrafast x-ray science due to their unprecedented brightness and short pulse duration. However, time-resolved studies at the resolution of the x-ray pulse duration are hampered by the inability to precisely synchronize an external laser to the accelerator. At the Sub-Picosecond Pulse Source at the Stanford Linear-Accelerator Center we solved this problem by measuring the arrival time of each high energy electron bunch with electro-optic sampling. This measurement indirectly determined the arrival time of each x-ray pulse relative to an external pump laser pulse with a time resolution of better than 60 fs rms.

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PubMed2003

Transient strain driven by a dense electron-hole plasma.

We measure transient strain in ultrafast laser-excited Ge by time-resolved x-ray anomalous transmission. The development of the coherent strain pulse is dominated by rapid ambipolar diffusion. This pulse extends considerably longer than the laser penetration depth because the plasma initially propagates faster than the acoustic modes. X-ray diffraction simulations are in agreement with the observed dynamics.

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PubMed2003

Investigation of the onset and development of forward scattering in an underdense plasma.

We investigate the distortion of a spatial intensity modulation imposed on a 527 nm f/10 probe beam as it transmits through an underdense plasma characterized with Thomson scattering. Combining the measurements with full wave simulations of beam propagation through the entire plasma show that the key features of the data can be reproduced using the Kaiser thermal transport model.

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PubMed2002

K-shell spectroscopy of an independently diagnosed uniaxially expanding laser-produced aluminum plasma.

We present detailed spectroscopic analysis of the primary K-shell emission lines from a uniaxially expanding laser-produced hydrogenic and heliumlike aluminum plasma. The spectroscopic measurements are found to be consistent with time-dependent hydrodynamic properties of the plasma, measured using Thomson scattering and shadowgraphy. The K-shell population kinetics code FLY with the measured hydrodynamic parameters is used to generate spectra that are compared to the experimental spectra. Excellent agreement is found between the measured and calculated spectra for a variety of experimental target widths employed to produce plasmas with different optical depths. The peak emission from the hydrogenic Lyman series is determined to be from a temporal and spatial region where the hydrodynamic parameters are essentially constant. This allows a single steady-state solution of FLY to be used to deduce the electron temperature and density, from the measured line ratios and linewidths, for comparison with the Thomson and shadowgraphy data. These measurements are found to agree well with time-dependent calculations, and provide further validation for the FLY calculations of the ionization and excitation balance for a K-shell aluminum plasma. We also discuss the possible application of this data as a benchmark for hydrodynamic simulations and ionization/excitation balance calculations.

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PubMed2001

Probing impulsive strain propagation with X-ray pulses.

Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden reflections in InSb is used to follow the propagation of a coherent acoustic pulse generated by ultrafast laser excitation. The surface and bulk components of the strain could be simultaneously measured due to the large x-ray penetration depth. Comparison of the experimental data with dynamical diffraction simulations suggests that the conventional model for impulsively generated strain underestimates the partitioning of energy into coherent modes.

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PubMed2001

Anomalous elastic response of silicon to uniaxial shock compression on nanosecond time scales.

We have used x-ray diffraction with subnanosecond temporal resolution to measure the lattice parameters of orthogonal planes in shock compressed single crystals of silicon (Si) and copper (Cu). Despite uniaxial compression along the (400) direction of Si reducing the lattice spacing by nearly 11%, no observable changes occur in planes with normals orthogonal to the shock propagation direction. In contrast, shocked Cu shows prompt hydrostaticlike compression. These results are consistent with simple estimates of plastic strain rates based on dislocation velocity data.

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