ALBERT

Notizen: Ohmic contacts comprised of the layer sequence of W/WC/TaC/SiC showed excellent thermal stability after testing at 600 °C for 1000 h. No degradation of the specific contact resistance nor a reaction of the film with the SiC substrate was observed. From Auger electron spectroscopy depth profiles, it was found that W and WC reacted to form W2C on TaC after annealing. After annealing for several hundred hours at 1000 °C, the specific contact resistance of the W/WC/TaC/SiC contacts displayed noticeable degradation. According to secondary ion mass spectrometry (SIMS) analysis, after annealing at 1000 °C for 600 h, small but measurable changes in the electrical characteristics were associated with O incorporation at the interface between TaC and SiC. Investigation of the W/WC/TaC/SiC interface by transmission electron microscopy (TEM) indicated that a reaction between the W and WC had occurred, but there was no observed reaction with the SiC substrate. After annealing for 1000 h, substantial changes in the chemistry, the microstructure, the specific contact resistance, and the spreading resistance of the contacts were observed. To understand the mechanisms associated with the degradation of the electrical properties, SIMS and TEM analyses were performed on samples annealed for 600, 700, 800, 900, and 1000 h at 1000 °C. These findings indicated that both oxidation and metallurgical reactions played important roles in the degradation of the electrical properties. © 2001 American Institute of Physics.

Notizen: A two-dimensional micromagnetic model is extended to support simulation of films with nonuniform thickness. Zeeman and crystalline anisotropy energies of each cell scale with the cell thickness, while the exchange energy of a pair of neighbor cells scales by a weight dependent on the thicknesses of both cells. The self-magnetostatic energy is computed by scaling the moment of each cell by its thickness, and adding a local correction to the out-of-plane field. The calculation of the magnetostatic field for a 10×10×1 oblate spheroid is shown to be more accurate by the nonuniform thickness model than by a uniform thickness model. With the extended model a 530×130×10 nm film in the shape of a truncated pyramid with tapering over the 15 nm nearest the edges is shown to have smaller switching field and different reversal mechanism compared with uniform thickness films of similar size and shape. © 2001 American Institute of Physics.

Notizen: We report results for μMAG standard problem No. 4, a 500 nm×125 nm×3 nm rectangle of material with properties to mimic Permalloy. Switching dynamics are calculated for fields applied instantaneously to an initial s state: Field 1 at 170° and Field 2 at 190° (−170°) from the positive long axis. Reversal in Field 1 proceeds by propagation of end domains toward the sample center. Reversal in Field 2 involves rotation of the end domains in one direction while the center of the particle rotates in the opposite direction, resulting in collapsing 360° walls with complex dynamics on fine length scales. Approaching the static coercivity, Hc, in small field steps, we find that the ring down frequency, f, and susceptibility, χ, are in approximate agreement with a single spin model that predicts f∝(Hc−H)1/4 and χ∝(Hc−H)−1/2. We show a correlation between the modes of oscillation that become unstable at the critical field and the switching behavior. © 2001 American Institute of Physics.

Notizen: For a uniformly magnetized rectangular particle with dimensions in the ratio 5 : 1 : 0.1, the coercive and switching fields in the (1,1,1) direction are determined to be Hc/Ms=0.057069478 and Hs/Ms=0.057142805. Previous micromagnetic computations of coercive and switching fields that did not approach these values for small particles are analyzed. It is shown that the disagreement was primarily due to a disparity in the method of calculating demagnetization energy. Corrected simulations are shown to agree with analytically determined values. © 2000 American Institute of Physics.

Notizen: The 100 ns, 20 MA pinch-driver Z is surrounded by an extensive set of diagnostics. There are nine radial lines of sight set at 12° above horizontal and each of these may be equipped with up to five diagnostic ports. Instruments routinely fielded viewing the pinch from the side with these ports include x-ray diode arrays, photoconducting detector arrays, bolometers, transmission grating spectrometers, time-resolved x-ray pinhole cameras, x-ray crystal spectrometers, calorimeters, silicon photodiodes, and neutron detectors. A diagnostic package fielded on axis for viewing internal pinch radiation consists of nine lines of sight. This package accommodates virtually the same diagnostics as the radial ports. Other diagnostics not fielded on the axial or radial ports include current B-dot monitors, filtered x-ray scintillators coupled by fiber optics to streak cameras, streaked visible spectroscopy, velocity interferometric system for any reflector, bremsstrahlung cameras, and active shock breakout measurement of hohlraum temperature. The data acquisition system is capable of recording up to 500 channels and the data from each shot is available on the Internet. A major new diagnostic presently under construction is the BEAMLET backlighter. We will briefly describe each of these diagnostics and present some of the highest-quality data from them. © 2001 American Institute of Physics.

Notizen: Three x-ray spectrometers, each with a transmission grating dispersion element, are routinely used at the Z soft x-ray facility to measure the spectrum and temporal history of the absolute soft x-ray power emitted from z-pinch and hohlraum radiation sources. Our goal is to make these measurements within an accuracy of ±10%. We periodically characterize the efficiency of the gratings used in the spectrometers by using an electron-impact soft x-ray source, a monochromator, grazing-incidence mirrors, thin filters, and an x-ray charge-coupled device (CCD) detector. We measure the transmission efficiency of the gratings at many photon energies for several grating orders. For each grating, we calculate efficiency as a function of photon energy using published optical constants of gold and multiple-slit Fraunhofer diffraction theory and fit the calculation to the measurements using the physical parameters of the grating as variables. This article describes the measurement apparatus and calibration techniques, discusses the grating efficiency calculation and fitting procedure, and presents recent results.

Notizen: X-ray powers on the order of 10 TW over an area of 4.5 mm2 are produced in the axial direction from the compression of a low-density foam target centered within a z-pinch on the Z generator.1 The x rays from this source are used for high-energy–density physics experiments, including the heating of hohlraums for inertial confinements fusion studies.2 In this article, detailed characteristics of this radiation source measured using an upgraded axial-radiation-diagnostic suite3 together with other on- and off-axis diagnostics are summarized and discussed in terms of Eulerian and Lagrangian radiation–magnetohydrodynamic code simulations. The source, characterized here, employs a nested array of 10-mm-long tungsten wires, at radii of 20 and 10 mm, having a total masses of 2 and 1 mg, and wire numbers of 240 and 120, respectively. The target is a 14 mg/cc CH2 foam cylinder of 5 mm diameter. The codes take into account the development of the Rayleigh–Taylor instability in the r–z plane, and provide integrated calculations of the implosion together with the x-ray generation. The radiation exiting the imploding target through the 4.5 mm2 aperture is measured primarily by the axial diagnostic suite that now includes diagnostics at an angle of ∼30° to the z axis. The near on-axis diagnostics include: (1) a seven-element filtered silicon-diode array,4 (2) a five-element filtered x-ray diffraction (XRD) array,5 (3) a six-element filtered PCD array,6 (4) a three-element bolometer,7 (5) time-resolved and time-integrating crystal spectrometers, and (6) two fast-framing x-ray pinhole cameras having 11 frames each. The filtered silicon diodes, XRDs, and PCDs are sensitive to 1–200, 140–2300, and 1000–4000 eV x rays, respectively. They (1) establish the magnitude of the prepluse generated during the run in of the imploding wire arrays, (2) measure the Planckian nature of the dominant thermal, and (3) nonthermal component of the emission. The bolometers and XRDs mounted on the near-normal and 30° LOS (line-of-sight) measure the total power and check the Lambertian nature of the emission. Additionally, a suite of filtered fast-framing x-ray pinhole cameras and silicon-diode arrays behind a transmission grating, mounted on LOSs nearly normal to the z axis, quantify the plasma plume exiting the aperture. The hard bremsstrahlung generated is estimated with both on- and off-axis shielded scintillator photomultiplier diagnostics. © 2001 American Institute of Physics.

Notizen: Measurements of the hohlraum wall temperature in Z-pinch driven hohlraum experiments require looking through small (2–4 mm diameter) diagnostic holes that undergo some degree of hole closure. The existing soft x-ray diagnostics on Z measure the total flux exiting this diagnostic hole and are therefore affected by this hole closure. To avoid having to measure the effective diagnostic hole area we have designed and constructed an imaging diode array (IDA) that incorporates pinhole imaging and an array of filtered silicon diodes to measure the absolute x-ray intensity from a spatially resolved region of a target. The instrument uses silicon diodes with subnanosecond time response that are sensitive to soft x rays in the range 100–3000 eV. An image of the target area is projected onto the silicon diodes using pinholes. Between each pinhole and it's respective diode is a soft x-ray filter. The material and thickness of the filter are selected to allow unfolding of spectral information in the 100–3000 eV spectral region. We plan to insert a set of grazing-incidence mirrors between each of the filter/diode pairs in a future version of this instrument to better define the spectral bandpass of each diode channel. Radiation from the target region is monitored by a gated microchannel-plate-intensified image recording device that is located immediately behind the diode array. A small shadow in the recorded image corresponds to the specific area of the target that is imaged onto each silicon diode. We are presently fielding this instrument in experiments on the Z facility located at Sandia National Laboratories in Albuquerque, NM. The instrument is located on the same line-of-sight and measures the same spatial region as a filtered fast-framing x-ray pinhole camera and a transmission grating spectrometer. This article describes the design of the IDA diagnostic and presents the results of measurements obtained in hohlraum experiments conducted on Z. © 2001 American Institute of Physics.

Notizen: We investigate the efficient construction of guiding wave functions for use in diffusion Monte Carlo calculations of electronic excited states. We test guiding wave functions obtained from singles-only configuration interaction, time-dependent density functional theory, and complete active space self-consistent field methods. The techniques are used to study the first ionization potentials and excited states of silane and methane. © 2001 American Institute of Physics.

Notizen: Initial experiments to study the Z-pinch-driven hohlraum high-yield inertial confinement fusion (ICF) concept of Hammer, Tabak, and Porter [Hammer et al., Phys. Plasmas 6, 2129 (1999)] are described. The relationship between measured pinch power, hohlraum temperature, and secondary hohlraum coupling ("hohlraum energetics") is well understood from zero-dimensional semianalytic, and two-dimensional view factor and radiation magnetohydrodynamics models. These experiments have shown the highest x-ray powers coupled to any Z-pinch-driven secondary hohlraum (26±5 TW), indicating the concept could scale to fusion yields of 〉200 MJ. A novel, single-sided power feed, double-pinch driven secondary that meets the pinch simultaneity requirements for polar radiation symmetry has also been developed. This source will permit investigation of the pinch power balance and hohlraum geometry requirements for ICF relevant secondary radiation symmetry, leading to a capsule implosion capability on the Z accelerator [Spielman et al., Phys. Plasmas 5, 2105 (1998)]. © 2001 American Institute of Physics.