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We report new results from the intense laser target interaction experiment that produces relativistic electron-positron pairs. Laser to electron energy transfer, inferred using x-ray and neutron measurements, was found to be consistent with the measured positrons. To increase the number of positrons, one needs to deliver a greater number of relativistic electrons from the laser-plasma interaction to the high Z gold target. A large preplasma was found to have a negative impact for this purpose, while the laser could produce hotter electrons in such preplasma. The peak energy shift in the positron spectrum is confirmed as the post-acceleration in the sheath potential behind the target. The results were supported by a collisional one-dimensional particle-in-cell code. This experiment was performed using the high-power LFEX laser at the Institute of Laser Engineering at Osaka University using a suite of diagnostics measuring electrons, positrons, x-rays and neutrons from the laser-target interaction at the relativistic regime. [ABSTRACT FROM AUTHOR]

The strength of shock-loaded single crystal tantalum [100] has been experimentally determined using in situ broadband x-ray Laue diffraction to measure the strain state of the compressed crystal, and elastic constants calculated from first principles. The inferred strength reaches 35 GPa at a shock pressure of 181 GPa and is in excellent agreement with a multiscale strength model [N. R. Barton et al., J. Appl. Phys. 109, 073501 (2011)], which employs a hierarchy of simulation methods over a range of length scales to calculate strength from first principles. [ABSTRACT FROM AUTHOR]

Observations of a rapid decrease in thermal temperature as a function of depth of solid targets irradiated with a short pulse, ultrahigh-intensity laser are reported. This phenomenon is investigated using the Titan short pulse laser with intensities greater than 1020 W/cm2 interacting with buried layer targets. The longitudinal temperature profile is determined by measuring K-shell spectra from a 0.4 μm copper tracer layer placed at various depths (i.e., 0–1.5 μm) within the 2.4 μm thick target. It is observed that the line ratios (He-like K-shell lines) as a function of temperature require a consideration of at least three parameters to analyze the K-shell spectra: hot electron population, time-dependent plasma conditions, and opacity. Here, the study of the effect of these three parameters on measured spectra in the short pulse high intensity laser-matter interactions using the atomic model FLYCHK [H.-K. Chung et al., High Energy Density Phys. 1, 3 (2005)] is presented. [ABSTRACT FROM AUTHOR]

17–75 keV one- and two-dimensional high-resolution (<10 μm) radiography has been developed using high-intensity short pulse lasers. High energy Kα sources are created by fluorescence from hot electrons interacting in the target material after irradiation by lasers with intensity IL>1017 W/cm2. High-resolution point projection one- and two-dimensional radiography has been achieved using microfoil and microwire targets attached to low-Z substrate materials. The microwire size was 10 μm×10 μm×300 μm on a 300 μm×300 μm×5 μm polystyrene substrate. The radiography experiments were performed using the Titan laser at Lawrence Livermore National Laboratory. The results show that the resolution is dominated by the microwire target size and there is very little degradation from the plasma plume, implying that the high-energy x-ray photons are generated mostly within the microwire volume. There are enough Kα photons created with a 300 J, 1-ω, 40 ps pulse laser from these small volume targets, and that the signal-to-noise ratio is sufficiently high, for single shot radiography experiments. This unique technique will be used on future high energy density experiments at many new high-power laser facilities. [ABSTRACT FROM AUTHOR]

Sources of 5-12 keV thermal Hea x-rays are readily generated by laser irradiation of mid-Z foils at intensities >1014 W/cm², and are widely used as probes for inertial confinement fusion and high-energy-density experiments. Higher energy 17-50 keV x-ray sources are efficiently produced from "cold" Ka emission using short pulse, petawatt lasers at intensities >1018 W/cm² [H.-S. Park, B. R. Maddox et al., "High-resolution 17-75 keV backlighters for high energy density experiments," Phys. Plasmas 15(7), 072705 (2008); B. R. Maddox, H. S. Park, B. A. Remington et al., "Absolute measurements of x-ray backlighter sources at energies above 10 keV," ibid. 18(5), 056709 (2011)]. However, when long pulse (>1 ns) lasers are used with Z > 30 elements, the spectrum contains contributions from both K shell transitions and from ionized atomic states. Here we show that by sandwiching a silver foil between layers of high-density carbon, the ratio of Kα:Heαα in the x-ray spectrum is significant increased over directly illuminated Ag foils, with narrower lines from K-shell transitions. Additionally, the emission volume is more localized for the sandwiched target, producing a more planar x-ray sheet. This technique may be useful for generating probes requiring spectral purity and a limited spatial extent, for example, in incoherent x-ray Thomson scattering experiments. [ABSTRACT FROM AUTHOR]

We report direct lattice-level measurements of plastic relaxation kinetics through time-resolved, in situ Laue diffraction of shock-compressed single-crystal [001 ] Ta at pressures of 27-210 GPa. For a 50-GPa shock, a range of shear strains is observed extending up to the uniaxial limit for early data points (<0.6 ns), and the average shear strain relaxes to a near steady state over ~1 ns. For 80- and 125-GPa shocks, the measured shear strains are fully relaxed already at 200 ps, consistent with rapid relaxation associated with the predicted threshold for homogeneous nucleation of dislocations occurring at shock pressure ~65 GPa. The relaxation rate and shear stresses are used to estimate the dislocation density, and these quantities are compared to the results of other high-pressure work, flow stress models, and molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

Observations of a rapid decrease in thermal temperature as a function of depth of solid targets irradiated with a short pulse, ultrahigh-intensity laser are reported. This phenomenon is investigated using the Titan short pulse laser with intensities greater than 1020 W/cm2 interacting with buried layer targets. The longitudinal temperature profile is determined by measuring K-shell spectra from a 0.4 μm copper tracer layer placed at various depths (i.e., 0–1.5 μm) within the 2.4 μm thick target. It is observed that the line ratios (He-like K-shell lines) as a function of temperature require a consideration of at least three parameters to analyze the K-shell spectra: hot electron population, time-dependent plasma conditions, and opacity. Here, the study of the effect of these three parameters on measured spectra in the short pulse high intensity laser-matter interactions using the atomic model FLYCHK [H.-K. Chung et al., High Energy Density Phys. 1, 3 (2005)] is presented. [ABSTRACT FROM AUTHOR]

We present resonant inelastic x-ray scattering and x-ray emission spectroscopy results on Gd metal to 113 GPa which suggest Kondo-like aspects in the delocalization of 4f electrons. Analysis of the resonant inelastic x-ray scattering data reveals a prolonged and continuous delocalization with volume throughout the entire pressure range, so that the volume-collapse transition at 59 GPa is only part of the phenomenon. Moreover, the Lgamma1 x-ray emission spectroscopy spectra indicate no apparent change in the bare 4f moment across the collapse, suggesting that Kondo screening is responsible for the expected Pauli-like behavior in magnetic susceptibility.

We and others have recently found that mature murine B cells can be induced to undergo apoptosis, in vitro, in a dose-dependent manner, by extensive crosslinking of membrane IgM with polyclonal anti-mu. During the analysis of tolerance in transgenic mice expressing rearranged IgM or IgM + IgD receptors, we observed that, Sp6 anti-TNP Ig and anti-MHC transgenic splenocytes, would undergo receptor-mediated apoptosis in vitro just like their normal, non-transgenic littermates. However, transgenic mice expressing rearranged receptors typical of B1 cells, not only contained large numbers of CD5+ cells in their spleens, but these cells failed to undergo apoptosis under conditions that led to programmed cell death in normal splenocytes. B1 spleen cells also failed to proliferate with anti-IgM, although the responsiveness of cells from the other transgenic lines varied depending on the background strains. These differences are due in part, to strain differences, but they also imply that the response pattern of transgenic B cells reflects not only the subset composition in this organ, but also the transgenic specificity of the receptor.