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  • American Institute of Physics (AIP)  (8)
  • Oxford University Press  (2)
  • American Chemical Society (ACS)
  • American Geophysical Union
  • Cell Press
  • 2015-2019
  • 1995-1999  (12)
  • 1955-1959
  • 1999  (12)
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  • 2015-2019
  • 1995-1999  (12)
  • 1955-1959
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  • 1
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Microchannel plate intensified (MPI) x-ray detectors are commonly used for imaging and spectral measurements in the 100–1500 eV photon energy range. Using a laser-produced plasma x-ray source, we measured the integrated detector response versus incident x-ray intensity and the relative efficiency versus photon energy of a MPI x-ray detector. Two identical 2000 lines/mm transmission grating spectrometers simultaneously record broadband plasma source emission from a tantalum target. The relative efficiency was determined by comparing the spectrum recorded with an absolutely calibrated x-ray CCD reference detector on one spectrometer to the spectrum recorded with a MPI x-ray detector on the other spectrometer. The integrated detector response versus incident x-ray intensity was measured by simultaneously illuminating the CCD reference detector and the MPI detector with step-wedge-filtered magnesium plasma emission. The aluminum step wedge x-ray filters pass the 1s–2p emission lines of H-like Mg at 1470 eV and the 1s2–1s2p emission lines of He-like Mg at 1350 eV, and provide a four order of magnitude range in incident intensity on the detectors.© 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 1624-1626 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: An instrument is described which can obtain x-ray spectral power measurements utilizing the diffraction pattern produced when x rays pass through a slit. Traditionally, these types of measurements yielding low to moderate spectral resolution have been made with filtered x-ray diodes or with a transmission grating. The instrument described below has several advantages over filtered x-ray diodes in determining the spectral power profile such as an insensitivity to surface contamination. In addition, this instrument does not require the use of filters which can be destroyed during a shot making absolute measurements difficult and very time consuming. The advantages over a transmission grating system include cost, mechanical robustness, and fewer components which require spectral calibration. © 1999 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 5816-5818 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Magnetization reversal modes and coercivities were calculated for a magnetic particle with thickness : width : length aspect ratios 0.1 : 1 : 5 as a function of the reduced particle width d/lex, where d is the particle width and lex is the intrinsic magnetostatic exchange length. With only exchange energy and magnetostatic energy included, the particle corresponds to μMAG standard problem No. 2. The problem is modeled with two-dimensional grids of three-dimensional spins, and the results are compared for two methods of calculating magnetostatic energies, the "constant magnetization" method and the "constant charge" method. For both magnetostatic computational methods, the coercivity decreases from Hc/Ms=0.06±0.003 to 0.014±0.003 over the range 3〈d/lex〈80, where the uncertainties reflect the field step size. Also over this interval, as d/lex increases, the magnetization exhibits three modes of reversal: nearly uniform rotation, transverse switching of end domains followed by propagation of head-to-head domain walls from the ends to the center of the particle, and nucleation and propagation of vortices accompanied by more complex domain structures. © 1999 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 70 (1999), S. 2012-2015 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: A technique is described to determine the spatial x-ray flux emitted from a hohlraum wall and subsequently transmitted through a diagnostic hole. This technique uses x-ray diodes, bolometers, and a time-resolved pinhole camera to determine the spatial flux of x rays emitted through a hohlraum's diagnostic hole. The primary motivation for this analysis was the relatively long duration, nearly 100 ns, of the x-ray drive present in z-pinch driven hohlraums. This radiation causes plasma to ablate from the hohlraum walls surrounding the diagnostic hole and results in a partial obscuration that reduces the effective area over which diagnostics view the radiation. The effective change in area leads to an underestimation of the wall temperature when nonimaging diagnostics such as x-ray diodes and bolometers are used to determine power and later to infer a wall temperature. An analysis similar to the one described below is then necessary to understand the radiation environment present in x-ray driven hohlraums when these diagnostics are used and hole closure is important. © 1999 American Institute of Physics.
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  • 5
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A two-dimensional calculation is presented for the transport of plasma in the edge region of a divertor tokamak solving continuity, momentum, and energy balance fluid equations. The model uses classical processes of parallel transport along the magnetic field and cross-field drifts together with anomalous radial diffusion, including perpendicular ion viscosity. The self-consistent electrostatic potential is calculated on both sides of the magnetic separatrix via quasineutrality and current continuity. Outside the separatrix, the model extends to material divertor plates where the incident plasma is recycled as neutral gas and where the plate sheath and parallel currents dominate the potential structure. Inside the separatrix, various radial current terms—from anomalous viscosity, collisional damping, inertia, and ∇B drifts—contribute to determining the potential. The model rigorously enforces cancellation of gyroviscous and magnetization terms from the transport equations. The results emphasize the importance of E×B particle flow under the X-point which depends on the sign of the toroidal magnetic field. Radial electric field profiles at the outer midplane show strong variation with the magnitude of the anomalous diffusion coefficients and the core toroidal rotation velocity, indicating that shear stabilization of edge turbulence can likewise be sensitive to these parameters.
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 75 (1999), S. 775-777 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: This letter reports the experimental characterization of a z-pinch-driven vacuum hohlraum. We have measured soft x-ray fluxes of 5×1012 W/cm2 radiating from the walls of hohlraums which are 2.4–2.5 cm in diameter by 1 cm tall. The x-ray source used to drive these hohlraums was a z pinch consisting of a 300 wire tungsten array driven by a 20 MA, 100 ns current pulse. In this hohlraum geometry, the z-pinch x-ray source can produce energies in excess of 800 kJ and powers in excess of 100 TW to drive these hohlraums. The x rays released in these hohlraums represent greater than a factor of 25 in energy and more than a factor of 3 in x-ray power over previous laboratory-driven hohlraums. © 1999 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 75 (1999), S. 2674-2676 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The role of clay minerals in the prebiotic synthesis of nucleotide oligomers has received considerable attention in recent years. Scanning force microscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry are used to identify oligomers of adenylic acid formed on the clay mineral Cu(II)-exchanged hectorite in simulated prebiotic cycling experiments. Electron-spin resonance and x-ray diffraction data indicate that the monomer (5′-phosphorimidazolide of adenosine, or ImpA) penetrates into the intergallery regions of the mineral substrate, and complexes the gallery Cu(II) cations. It is postulated that polymerization of the monomer is initiated in the clay intergallery regions, producing oligomers of adenylic acid up to 8 units in length or more. © 1999 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 75 (1999), S. 3956-3958 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Tantalum carbide contacts with and without Au, Pt, and W/WC overlayers on n-type 6H–SiC (0001) were ohmic after annealing at temperatures between 800 and 1075 °C. Specific contact resistivities (SCRs) were calculated from current–voltage measurements of transmission line model patterns at temperatures ranging from 20 to 400 °C in air. The minimum SCRs at room temperature on SiC (2.3×1019 cm−3) for TaC and for TaC with Pt and Au overlayers were 2.1×10−5, 7.4×10−6, and 1.4×10−6 Ω cm2, respectively. The SCRs for both the Au/TaC/SiC (5.3×10−7Ω cm2) and the Pt/TaC/SiC (7.5×10−7 Ω cm2) samples decreased with measurement temperature to 200 and 400 °C, respectively, while the latter samples showed reversibility after heating to 400 °C. W/WC/TaC/SiC samples showed the best stability after annealing at 400 °C for 144 h in vacuum. Changes in the electrical characteristics were correlated with increases in O incorporation in the contacts as a result of annealing. Investigation of the TaC/SiC interface by transmission electron microscopy indicated that there was little or no reaction between the materials. © 1999 American Institute of Physics.
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  • 9
    Publication Date: 1999-10-01
    Print ISSN: 0962-8924
    Electronic ISSN: 1879-3088
    Topics: Biology , Medicine
    Published by Cell Press
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  • 10
    Publication Date: 1999-04-01
    Print ISSN: 0305-1048
    Electronic ISSN: 1362-4962
    Topics: Biology
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