Publications

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To test the hypothesis that high quality 3D Earth models will produce seismic event locations which are more accurate and more precise, we are developing a global 3D P wave velocity model of the Earth's crust and mantle using seismic tomography. In this paper, we present the most recent version of our model, SALSA3D version 1.5, and demonstrate its ability to reduce mislocations for a large set of realizations derived from a carefully chosen set of globally-distributed ground truth events. Our model is derived from the latest version of the Ground Truth (GT) catalog of P and Pn travel time picks assembled by Los Alamos National Laboratory. To prevent over-weighting due to ray path redundancy and to reduce the computational burden, we cluster rays to produce representative rays. Reduction in the total number of ray paths is {approx}50%. The model is represented using the triangular tessellation system described by Ballard et al. (2009), which incorporates variable resolution in both the geographic and radial dimensions. For our starting model, we use a simplified two layer crustal model derived from the Crust 2.0 model over a uniform AK135 mantle. Sufficient damping is used to reduce velocity adjustments so that ray path changes between iterations are small. We obtain proper model smoothness by using progressive grid refinement, refining the grid only around areas with significant velocity changes from the starting model. At each grid refinement level except the last one we limit the number of iterations to prevent convergence thereby preserving aspects of broad features resolved at coarser resolutions. Our approach produces a smooth, multi-resolution model with node density appropriate to both ray coverage and the velocity gradients required by the data. This scheme is computationally expensive, so we use a distributed computing framework based on the Java Parallel Processing Framework, providing us with {approx}400 processors. Resolution of our model is assessed using a variation of the standard checkerboard method. We compare the travel-time prediction and location capabilities of SALSA3D to standard 1D models via location tests on a global event set with GT of 5 km or better. These events generally possess hundreds of Pn and P picks from which we generate different realizations of station distributions, yielding a range of azimuthal coverage and ratios of teleseismic to regional arrivals, with which we test the robustness and quality of relocation. The SALSA3D model reduces mislocation over standard 1D ak135 regardless of Pn to P ratio, with the improvement being most pronounced at higher azimuthal gaps.

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The ion photon emission microscope (IPEM), a new radiation effects microscope for the imaging of single event effects from penetrating radiation, is being developed at Sandia National Laboratories and implemented on the 88' cyclotron at Lawrence Berkeley National Laboratories. The microscope is designed to permit the direct correlation between the locations of high-energy heavy-ion strikes and single event effects in microelectronic devices. The development of this microscope has required the production of a robust optical system that is compatible with the ion beam lines, design and assembly of a fast single photon sensitive measurement system to provide the necessary coincidence, and the development and testing of many scintillating films. A wide range of scintillating material for application to the ion photon emission microscope has been tested with few meeting the stringent radiation hardness, intensity, and photon lifetime requirements. The initial results of these luminescence studies and the current operation of the ion photon emission microscope will be presented. Finally, the planned development for future microscopes and ion luminescence testing chambers will be discussed.

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Plasma spray coating techniques allow unique control of electrolyte microstructures and properties as well as facilitating deposition on complex surfaces. This can enable significantly improved solid oxide fuel cells (SOFCs), including non-planar designs. SOFCs are promising because they directly convert the oxidization of fuel into electrical energy. However, electrolytes deposited using conventional plasma spray are porous and often greater than 50 microns thick. One solution to form dense, thin electrolytes of ideal composition for SOFCs is to combine suspension plasma spray (SPS) with very low pressure plasma spray (VLPPS). Increased compositional control is achieved due to dissolved dopant compounds in the suspension that are incorporated into the coating during plasma spraying. Thus, it is possible to change the chemistry of the feed stock during deposition. In the work reported, suspensions of sub-micron diameter 8 mol.% Y2O3-ZrO2 (YSZ) powders were sprayed on NiO-YSZ anodes at Sandia National Laboratories (SNL) Thermal Spray Research Laboratory (TSRL). These coatings were compared to the same suspensions doped with scandium nitrate at 3 to 8 mol%. The pressure in the chamber was 2.4 torr and the plasma was formed from a combination of argon and hydrogen gases. The resultant electrolytes were well adhered to the anode substrates and were approximately 10 microns thick. The microstructure of the resultant electrolytes will be reported as well as the electrolyte performance as part of a SOFC system via potentiodynamic testing and impedance spectroscopy.

Enumerating triangles (3-cycles) in graphs is a kernel operation for social network analysis. For example, many community detection methods depend upon finding common neighbors of two related entities. We consider Cohen's simple and elegant solution for listing triangles: give each node a 'bucket.' Place each edge into the bucket of its endpoint of lowest degree, breaking ties consistently. Each node then checks each pair of edges in its bucket, testing for the adjacency that would complete that triangle. Cohen presents an informal argument that his algorithm should run well on real graphs. We formalize this argument by providing an analysis for the expected running time on a class of random graphs, including power law graphs. We consider a rigorously defined method for generating a random simple graph, the erased configuration model (ECM). In the ECM each node draws a degree independently from a marginal degree distribution, endpoints pair randomly, and we erase self loops and multiedges. If the marginal degree distribution has a finite second moment, it follows immediately that Cohen's algorithm runs in expected linear time. Furthermore, it can still run in expected linear time even when the degree distribution has such a heavy tail that the second moment is not finite. We prove that Cohen's algorithm runs in expected linear time when the marginal degree distribution has finite 4/3 moment and no vertex has degree larger than {radical}n. In fact we give the precise asymptotic value of the expected number of edge pairs per bucket. A finite 4/3 moment is required; if it is unbounded, then so is the number of pairs. The marginal degree distribution of a power law graph has bounded 4/3 moment when its exponent {alpha} is more than 7/3. Thus for this class of power law graphs, with degree at most {radical}n, Cohen's algorithm runs in expected linear time. This is precisely the value of {alpha} for which the clustering coefficient tends to zero asymptotically, and it is in the range that is relevant for the degree distribution of the World-Wide Web.

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This presentation will discuss two tensor decompositions that are not as well known as PARAFAC (parallel factors) and Tucker, but have proven useful in informatics applications. Three-way DEDICOM (decomposition into directional components) is an algebraic model for the analysis of 3-way arrays with nonsymmetric slices. PARAFAC2 is a related model that is less constrained than PARAFAC and allows for different objects in one mode. Applications of both models to informatics problems will be shown.

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For practical quantum computing, it will be necessary to detect the fluorescence from many trapped ions. We describe a design and integration approach using micro-optics to couple this fluorescence into an array of optical fibers.

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Graphene has emerged as a promising material for high speed nano-electronics due to the relatively high carrier mobility that can be achieved. To further investigate electronic transport in graphene and reveal its potential for microwave applications, we employed a near-field scanning microwave microscope with the probe formed by an electrically open end of a 4 GHz half-lambda parallel-strip transmission line resonator. Because of the balanced probe geometry, our microscope allows for truly localized quantitative characterization of various bulk and low-dimensional materials, with the response region defined by the one micron spacing between the two metallic strips at the probe tip. The single- and few-layer graphene flakes were fabricated by a mechanical cleavage method on 300-nm-thick silicon dioxide grown on low resistivity Si wafer. The flake thickness was determined using both AFM and Raman microscopies. We observe clear correlation between the near-field microwave and far-field optical images of graphene produced by the probe resonant frequency shift and thickness-defined color gradation, respectively. We show that the microwave response of graphene flakes is determined by the local sheet impedance, which is found to be predominantly active. Furthermore, we apply a quantitative electrodynamic model relating the probe resonant frequency shift to 2D conductivity of single- and few-layer graphene. From fitting a model to the experimental data we evaluate graphene sheet resistance as a function of thickness. Near-field scanning microwave microscopy can simultaneously image location, geometry, thickness, and distribution of electrical properties of graphene without a need for device fabrication. The approach may be useful for design of graphene-based microwave transistors, quality control of large area graphene sheets, or investigation of chemical and electrical doping effects on graphene transport properties. We acknowledge support from the DOE Center for Integrated Nanotechnologies user support program (grant No.U2008A061), from the NASA NM Space Grant Consortium program, and from the LANL-NMT MOU program supported by UCDRD.

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This report documents the progression of crude oil phase behavior modeling within the U.S. Strategic Petroleum Reserve vapor pressure program during the period 2004-2009. Improvements in quality control on phase behavior measurements in 2006 coupled with a growing body of degasification plant operations data have created a solid measurement baseline that has served to inform and significantly improve project understanding on phase behavior of SPR oils. Systematic tuning of the model based on proven practices from the technical literature have shown to reduce model bias and match observed data very well, though this model tuning effort is currently in process at SPR and based on preliminary data. The current report addresses many of the steps that have helped to build a strong baseline of data coupled with sufficient understanding of model features so that calibration is possible.

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Thin and small form factor cells have been researched lately by several research groups around the world due to possible lower assembly costs and reduced material consumption with higher efficiencies. Given the popularity of these devices, it is important to have detailed information about the behavior of these devices. Simulation of fabrication processes and device performance reveals some of the advantages and behavior of solar cells that are thin and small. Three main effects were studied: the effect of surface recombination on the optimum thickness, efficiency, and current density, the effect of contact distance on the efficiency for thin cells, and lastly the effect of surface recombination on the grams per Watt-peak. Results show that high efficiency can be obtained in thin devices if they are well-passivated and the distance between contacts is short. Furthermore, the ratio of grams per Watt-peak is greatly reduced as the device is thinned.

We present a newly developed microsystem enabled, back-contacted, shade-free GaAs solar cell. Using microsystem tools, we created sturdy 3 {micro}m thick devices with lateral dimensions of 250 {micro}m, 500 {micro}m, 1 mm, and 2 mm. The fabrication procedure and the results of characterization tests are discussed. The highest efficiency cell had a lateral size of 500 {micro}m and a conversion efficiency of 10%, open circuit voltage of 0.9 V and a current density of 14.9 mA/cm{sup 2} under one-sun illumination.

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Analysis of 50 mm diameter wire arrays at the Z Accelerator has shown experimentally the accretion of mass in a stagnating z pinch and provided insight into details of the radiating plasma species and plasma conditions. This analysis focused on nested wire arrays with a 2:1 (outeninner) mass, radius, and wire number ratio where Al wires were fielded on the outer array and Ni-clad Ti wires were fielded on the inner array.In this presentation, we will present analysis of data from other mixed Al/Ni-clad Ti configurations to further evaluate nested wire array dynamics and mass accretion. These additional configurations include the opposite configuration to that described above (Ni-clad Ti wires on the outer array, with Al wires on the inner array) as well as higher wire number Al configurations fielded to vary the interaction of the two arrays. These same variations were also assessed for a smaller diameter nested array configuration (40 mm). Variations in the emitted radiation and plasma conditions will be presented, along with a discussion of what the results indicate about the nested array dynamics. Additional evidence for mass accretion will also be presented.

The planar wire array research on Zebra at UNR that started in 2005 continues experiments with new types of planar loads with results for consideration and comprehensive analysis [see, for example, Kantsyrev et al, HEDP 5, 115 (2009)]. The detailed studies of radiative properties of such loads are important and spectroscopy and imaging constitute a very valuable and informative diagnostic tool. The set of theoretical codes is implemented which provides non-LTE kinetics, wire ablation dynamic, and MHD modeling. This talk is based on the results of new recent experiments with planar wire arrays on Zebra at UNR. We start with results on radiative properties of a uniform single planar wire array (SPWA) from alloyed Al wires and move to combined triple planar wire arrays (TPWA) made from two materials, Cu and Al. Such combined TPWA includes three planar wire rows that are parallel to each other and made of either Cu or Al alloyed wires. Three different configurations (Al/Cu/Al, Cu/Al/Cu, and Cu/Cu/Al) are considered and compared with each other, and with the results from SPWA of the same materials. X-ray time-gated and time integrated pinhole images and spectra are analyzed together with bolometer, PCD, and XRD measurements, and optical images. Emphasis is made on the radiative properties and temporal and spatial evolution of plasma parameters of such two-component plasmas. The opacity effects are considered and the important question of what causes K-shell Al lines to be optically thin in combined TPWAs is addressed. In conclusion, the new findings from studying multi-planar wire array implosions are summarized and their input to Z-pinch radiation physics is discussed.

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Supercomputers are composed of many diverse components, operated at a variety of scales, and function as a coherent whole. The resulting logs are thus diverse in format, interrelated at multiple scales, and provide evidence of faults across subsystems. When combined with system configuration information, insights on both the downstream effects and upstream causes of events can be determined. However, difficulties in joining the data and expressing complex queries slow the speed at which actionable insights can be obtained. Effectively connecting data experts and data miners faces similar hurdles. This paper describes our experience with applying the Splunk log analysis tool as a vehicle to combine both data, and people. Splunk's search language, lookups, macros, and subsearches reduce hours of tedium to seconds of simplicity, and its tags, saved searches, and dashboards offer both operational insights and collaborative vehicles.

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Circuit simulation codes, such as SPICE, are invaluable in the development and design of electronic circuits in radiation environments. These codes are often employed to study the effect of many thousands of devices under transient current conditions. Device-scale simulation codes are commonly used in the design of individual semiconductor components, but computational requirements limit their use to small-scale circuits. Analytic solutions to the ambipolar diffusion equation, an approximation to the carrier transport equations, may be used to characterize the transient currents at nodes within a circuit simulator. We present new analytic transient excess carrier density and photocurrent solutions to the ambipolar diffusion equation for 1-D abrupt-junction pn diodes. These solutions incorporate low-level radiation pulses and take into account a finite device geometry, ohmic fields outside the depleted region, and an arbitrary change in the carrier lifetime due to neutron irradiation or other effects. The solutions are specifically evaluated for the case of an abrupt change in the carrier lifetime during or after, a step, square, or piecewise linear radiation pulse. Noting slow convergence of the Fourier series solutions for some parameters sets, we evaluate portions of the solutions using closed-form formulas, which result in a two order of magnitude increase in computational efficiency.

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A2BLnX6 elpasolites (A, B: alkali; Ln: lanthanide; X: halogen), LaBr3 lanthanum bromide, and AX alkali halides are three classes of the ionic compound crystals being explored for {gamma}-ray detection applications. Elpasolites are attractive because they can be optimized from combinations of four different elements. One design goal is to create cubic crystals that have isotropic optical properties and can be grown into large crystals at lower costs. Unfortunately, many elpasolites do not have cubic crystals and the experimental trial-and-error approach to find the cubic elpasolites has been prolonged and inefficient. LaBr3 is attractive due to its established good scintillation properties. The problem is that this brittle material is not only prone to fracture during services, but also difficult to grow into large crystals resulting in high production cost. Unfortunately, it is not always clear how to strengthen LaBr3 due to the lack of understanding of its fracture mechanisms. The problem with alkali halides is that their properties decay rapidly over time especially under harsh environment. Here we describe our recent progress on the development of atomistic models that may begin to enable the prediction of crystal structures and the study of fracture mechanisms of multi-element compounds.

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Automated analysis of unstructured text documents (e.g., web pages, newswire articles, research publications, business reports) is a key capability for solving important problems in areas including decision making, risk assessment, social network analysis, intelligence analysis, scholarly research and others. However, as data sizes continue to grow in these areas, scalable processing, modeling, and semantic analysis of text collections becomes essential. In this paper, we present the ParaText text analysis engine, a distributed memory software framework for processing, modeling, and analyzing collections of unstructured text documents. Results on several document collections using hundreds of processors are presented to illustrate the exibility, extensibility, and scalability of the the entire process of text modeling from raw data ingestion to application analysis.

Goal - design methods to characterize and identify a low dimensional representation of graphs. Impact - enabling predictive simulation; monitoring dynamics on graphs; and sampling and recovering network structure from limited observations. Areas to explore are: (1) Enabling technologies - develop novel algorithms and tailor existing ones for complex networks; (2) Modeling and generation - Identify the right parameters for graph representation and develop algorithms to compute these parameters and generate graphs from these parameters; and (3) Comparison - Given two graphs how do we tell they are similar? Some conclusions are: (1) A bad metric can make anything look good; (2) A metric that is based an edge-by edge prediction will suffer from the skewed distribution of present and absent edges; (3) The dominant signal is the sparsity, edges only add a noise on top of the signal, the real signal, structure of the graph is often lost behind the dominant signal; and (4) Proposed alternative: comparison based on carefully chosen set of features, it is more efficient, sensitive to selection of features, finding independent set of features is an important area, and keep an eye on us for some important results.

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Threshold stress intensity factors were measured in high-pressure hydrogen gas for a variety of low alloy ferritic steels using both constant crack opening displacement and rising crack opening displacement procedures. The sustained load cracking procedures are generally consistent with those in ASME Article KD-10 of Section VIII Division 3 of the Boiler and Pressure Vessel Code, which was recently published to guide design of high-pressure hydrogen vessels. Three definitions of threshold were established for the two test methods: K{sub THi}* is the maximum applied stress intensity factor for which no crack extension was observed under constant displacement; K{sub THa} is the stress intensity factor at the arrest position for a crack that extended under constant displacement; and K{sub JH} is the stress intensity factor at the onset of crack extension under rising displacement. The apparent crack initiation threshold under constant displacement, K{sub THi}*, and the crack arrest threshold, K{sub THa}, were both found to be non-conservative due to the hydrogen exposure and crack-tip deformation histories associated with typical procedures for sustained-load cracking tests under constant displacement. In contrast, K{sub JH}, which is measured under concurrent rising displacement and hydrogen gas exposure, provides a more conservative hydrogen-assisted fracture threshold that is relevant to structural components in which sub-critical crack extension is driven by internal hydrogen gas pressure.

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