Publications

Here we present a new approach to optimal controller identification which unifies system identification and optimal control theory. Starting with empirical, open-loop frequency response function (FRF) data from a system, it is shown that the optimal controller can be identified directly without performing the intermediary steps of system identification and controller design. The primary benefit is that we are able to work directly with the measured data and the uncertainties inherent in it. Further, we go on to show a method of incorporating the empirical FRF uncertainty into the cost for robustness against plant uncertainty. This method leads to a more precise identification of H 2 and LQG controllers since it avoids the residual errors associated with performing the traditional intermediary step of system identification, while concurrently accounting for measured system uncertainty. © 2009 IFAC.

Sandia National Laboratories currently utilizes two laser tracking systems to provide time-space-position-information (TSPI) and high speed digital imaging of test units under flight. These laser trackers have been in operation for decades under the premise of theoretical accuracies based on system design and operator estimates. Advances in optical imaging and atmospheric tracking technology have enabled opportunities to provide more precise six degree of freedom measurements from these trackers. Applying these technologies to the laser trackers requires quantified understanding of their current errors and uncertainty. It was well understood that an assortment of variables contributed to laser tracker uncertainty but the magnitude of these contributions was not quantified and documented. A series of experiments was performed at Sandia National Laboratories large centrifuge complex to quantify TSPI uncertainties of Sandia National Laboratories laser tracker III. The centrifuge was used to provide repeatable and economical test unit trajectories of a test-unit to use for TSPI comparison and uncertainty analysis. On a centrifuge, testunits undergo a known trajectory continuously with a known angular velocity. Each revolution may represent an independent test, which may be repeated many times over for magnitudes of data practical for statistical analysis. Previously these tests were performed at Sandia's rocket sled track facility but were found to be costly with challenges in the measurement ground truth TSPI. The centrifuge along with on-board measurement equipment was used to provide known ground truth position of test units. This paper discusses the experimental design and techniques used to arrive at measures of laser tracker error and uncertainty. © 2009 Copyright SPIE - The International Society for Optical Engineering.

Shock and shockless compression methods were used to examine the loading path and rate dependence of single crystal x-cut quartz. Analysis of the transmitted wave profiles show remarkably different behavior between shock and shockless loaded samples. Shock loaded x-cut quartz shows inelastic deformation below 5 GPa. Ramp loaded samples do not show inelastic behavior until approximately 9 GPa, with the onset of this behavior dependent on sample thickness. The results demonstrate that both loading path and rate play important roles in the inelastic behavior of materials. © 2009 American Institute of Physics.

A program is underway at Sandia National Laboratories to predict long-term reliability of photovoltaic (PV) systems. The vehicle for the reliability predictions is a Reliability Block Diagram (RBD), which models system behavior. Because this model is based mainly on field failure and repair times, it can be used to predict current reliability, but it cannot currently be used to accurately predict lifetime. In order to be truly predictive, physics-informed degradation processes and failure mechanisms need to be included in the model. This paper describes accelerated life testing of metal foil tapes used in thin-film PV modules, and how tape joint degradation, a possible failure mode, can be incorporated into the model. © 2009 SPIE Victor Karpov.

Pump-probe transient thermoreflectance (TTR) techniques are powerful tools for measuring thermophysical properties of thin films, such as thermal conductivity, A, or thermal boundary conductance, G. TTR experimental setups rely on lock-in techniques to detect the response of the probe signal relative to the pump heating event. The temporal decays of the lock-in signal are then compared to thermal models to deduce the A and G in and across various materials. There are currently two thermal models that are used to relate the measured signals from the lock-in to the A and G in the sample of interest. In this work, the thermal models, their assumptions, and their ranges of applicability are compared. The advantages and disadvantages of each technique are elucidated from the results of the thermophysical property measurements. Copyright © 2009 by ASME.

Electron-interface scattering during electron-phonon nonequilibrium in thin films creates another pathway for electron system energy loss as characteristic lengths of thin films continue to decrease. As power densities in nanodevices increase, excitations of electrons from sub-conduction-band energy levels will become more probable. These subconduction-band electronic excitations significantly affect the material's thermophysical properties. In this work, the effects of d-band electronic excitations are considered in electron energy transfer processes in thin metal films. In thin films with thicknesses less than the electron mean free path, ballistic electron transport leads to electron-interface scattering. The ballistic component of electron transport, leading to electron-interface scattering, is studied by a ballistic-diffusive approximation of the Boltzmann Transport Equation. The effects of d-band excitations on electron-interface energy transfer is analyzed during electron-phonon nonequilibrium after short pulsed laser heating in thin films. Copyright © 2009 by ASME.

With component sizes approaching the mesoscale, conventional size microstructures offer insufficient homogeneity in mechanical properties, forcing microstructures to be reduced to the nanoscale. This work examines the effect of a nanocrystalline surface layer on the dynamic consolidation response of two different morphology Al 6061-T6 powders. Shock-propagation through equiaxed and needle morphology Al 6061-T6 powder beds initially at 73.5 and 75.0% theoretical density, respectively, is simulated at constant particle velocities ranging between 150 and 850 m/s. Shock velocity-particle velocity relationships are determined for powders both with and without the presence of a 2 μm high strength surface layer, which is representative of a nanocrystalline surface layer. Significant deviations in dynamic response are observed with the presence of the surface layer, especially at lower particle velocities. The equation of state (EOS) for both the homogeneous particles and those with a high strength surface layer are found to be best represented by a piecewise EOS. © 2009 American Institute of Physics.

In order to generate new properties of metals exposed to high pressure states, it is desirable to study samples loaded in one-dimensional strain. Previous work to obtain these ideal conditions, involve a technique where the sample was recovered at late times to examine its microstructure. In those experiments, the shock-loading was produced by impacting the sample with a flyer plate. In the present work, we modified the sample recovery assembly and optimized it for ramp wave loading. We describe the 2-D calculations performed with the ALEGRA MHD code that led to improved recovery assembly efficiency. Preliminary comparisons of the simulations with measurements of the sample deformation from an experiment indicate excellent agreement. © 2009 American Institute of Physics.

Films of the high explosive PETN (pentaerythritol tetranitrate) up to 500-μm thick have been deposited through physical vapor deposition, with the intent of creating well-defined samples for shock-initiation studies. PETN films were characterized with microscopy, x-ray diffraction, and focused ion beam nanotomography. These high-density films were subjected to strong shocks in both the out-of-plane and in-plane orientations. Initiation behavior was monitored with high-speed framing and streak camera photography. Direct initiation with a donor explosive (either RDX with binder, or CL-20 with binder) was possible in both orientations, but with the addition of a thin aluminum buffer plate (in-plane configuration only), initiation proved to be difficult. Initiation was possible with an explosively-driven 0.13-mm thick Kapton flyer and direct observation of initiation behavior was examined using streak camera photography at different flyer velocities. Models of this configuration were created using the shock physics code CTH. © 2009 American Institute of Physics.

Xenon is not only a technologically important element used in laser technologies and jet propulsion, but it is also one of the most accessible materials in which to study the metal-insulator transition with increasing pressure. Because of its closed shell electronic configuration, xenon is often assumed to be chemically inert, interacting almost entirely through the van der Waals interaction, and at liquid density, is typically modeled well using Leonard-Jones potentials. However, such modeling has a limited range of validity as xenon is known to form compounds under normal conditions and likely exhibits considerably more chemistry at higher densities when hybridization of occupied orbitals becomes significant. We present DFT-MD simulations of shocked liquid xenon with the goal of developing an improved equation of state. The calculated Hugoniot to 2 MPa compares well with available experimental shock data. Sandia is a mul-tiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. © 2009 American Institute of Physics.

Three-dimensional shock simulations of energetic materials have been conducted to improve our understanding of initiation at the mesoscale. Vapor-deposited films of PETN and pressed powders of HNS were characterized with a novel three-dimensional nanotomographic technique. Detailed microstructures were constructed experimentally from a stack of serial electron micrographs obtained by successive milling and imaging in a dual-beam FIB/SEM. These microstructures were digitized and imported into a multidimensional, multimaterial Eulerian shock physics code. The simulations provided insight into the mechanisms of pore collapse in PETN and HNS samples with distinctly different three-dimensional pore morphology and distribution. This modeling effort supports investigations of microscale explosive phenomenology and elucidates mechanisms governing initiation of secondary explosives. © 2009 American Institute of Physics.

Traditionally modal and vibration tests have been performed separately because their classical purposes require different inputs and outputs. However, motivation exists in some instances to be able to perform a modal test on a shaker table, if the boundary conditions could be accounted for appropriately. This is especially a concern for large test articles mounted on large tables because the table has flexible dynamics in the frequency range of interest for the modal test. For the past thirty years various attempts have been made to develop a method that would allow the two tests to both be conducted on a shaker table requiring only one setup. However, in most cases the table is assumed to be rigid. When the table cannot be assumed rigid the remaining approaches usually require that all six forces and all six degrees of freedom of motion at every attachment points be measured. Most approaches neglect moments and rotation measurements. Even measuring the translational forces and accelerations is rarely done. In the method employed here, the boundary condition is constrained mathematically. However, a measure of the shaker force is required. In addition, the classical mathematical constraints to produce a fixed base result are augmented in a way that alleviates the ill conditioning that almost always results when using the classical constraint equations. The two major advances here are a method to estimate the shaker force, and improved conditioning of the constrained equations. The effect of improving the conditioning is demonstrated with a modal test of hardware on a base that is not fixed. The full process is demonstrated with a random vibration test on a simple flexible horizontal slip table with a cantilevered beam mounted as the test article. A general outline of the method proceeds as follows: 1) characterize the modes of the bare shaker table attached to the shaker; 2) mount and instrument the test article; 3) attach a portable shaker to the tip of the shaker table with a force gage and measure a specific frequency response function (FRF); 4) detach the portable shaker and run the typical random vibration test; 5) calculate transmissibilities to the tip accelerometer; 6) create acceleration/force FRFs from reciprocity by multiplying the FRF in step 3 times every transmissibility; 7) extract modal parameters from FRFs; 8) finally apply augmented constraint equations with FRFs synthesized from the modal parameters and extract the fixed base modes. © 2009 Society for Experimental Mechanics Inc.

Uncertainty quantification in climate models is challenged by the sparsity of the available climate data due to the high computational cost of the model runs. Another feature that prevents classical uncertainty analyses from being easily applicable is the bifurcative behavior in the climate data with respect to certain parameters. A typical example is the Meridional Overturning Circulation in the Atlantic Ocean. The maximum overturning stream function exhibits discontinuity across a curve in the space of two uncertain parameters, namely climate sensitivity and CO2 forcing. We develop a methodology that performs uncertainty quantification in this context in the presence of limited data. © 2009 IEEE.

Using a line-imaging VISAR to infer the position (x) and time (t) dependent distension of a spalling sample requires two assumptions: (1) a calculated velocity surface v[no spall] (x, t) for the no-spall case to compare with the observed v [observed](x, t) surface, and (2) a lack of significant wave processing by the near-surface microstructure. We have designed and are conducting a matrix of experiments to evaluate these assumptions. In each experiment, we use a line-imaging VISAR to measure the velocity history of carefully characterized tantalum and copper samples taken to an incipient spall condition. The pre-shot characterization included spatially resolved mapping of grain locations and orientations by electron backscatter diffraction (EBSD). These samples are then soft-recovered and sectioned along the same line as monitored by the line-imaging VISAR. An initial pair of experiments provided ∼1 mm of spall separation; we are preparing further experiments with incipient spall conditions. © 2009 American Institute of Physics.

Developing and evaluating exterior physical security system alternatives can be a daunting task. Once alternatives are identified, they must be evaluated not to only the set of threats they provide against, but also to other factors such as cost, performance, schedule and environmental impact. This article describes a systematic approach of applying decision analysis tools and techniques to develop, and quantitatively evaluate a set of design alternatives and its associated security technologies. The intent is to provide an annotated checklist to guide security system designers when dealing with uncertainties in exterior physical security system design. ©2009 IEEE.

The first approach-to-critical experiment in the Seven Percent Critical Experiment series was recently completed at Sandia. This experiment is part of the Seven Percent Critical Experiment which will provide new critical and reactor physics benchmarks for fuel enrichments greater than five weight percent. The inverse multiplication method was used to determine the state of the system during the course of the experiment. Using the inverse multiplication method, it was determined that the critical experiment went slightly supercritical with 1148 fuel elements in the fuel array. The experiment is described and the results of the experiment are presented.

Sandia's Large Optics Coating Operation provides laser damage resistant optical coatings on meter-class optics required for the ZBacklighter Terawatt and Petawatt lasers. Deposition is by electron beam evaporation in a 2.3 m x 2.3 m x 1.8 m temperature controlled vacuum chamber. Ion assisted deposition (IAD) is optional. Coating types range from antireflection (AR) to high reflection (HR) at S and P polarizations for angle of incidence (AOI) from 0° to 47°. This paper reports progress in meeting challenges in design and deposition of these high laser induced damage threshold (LIDT) coatings. Numerous LIDT tests (NIF-MEL protocol, 3.5 ns laser pulses at 1064 nm and 532 nm) on the coatings confirm that they are robust against laser damage. Typical LIDTs are: at 1064 nm, 45° AOI, Ppol, 79 J/cm2 (IAD 32 layer HR coating) and 73 J/cm2 (non-IAD 32 layer HR coating); at 1064 nm, 32° AOI, 82 J/cm2 (Ppol) and 55 J/cm2 (Spol ) (non-IAD 32 layer HR coating); and at 532 nm, Ppol, 16 J/cm2 (25° AOI) and 19 J/cm2 (45° AOI) (IAD 50 layer HR coating). The demands of meeting challenging spectral, AOI and LIDT performances are highlighted by an HR coating required to provide R > 99.6% reflectivity in Ppol and Spol over AOIs from 24° to 47° within ∼ 1% bandwidth at both 527 nm and 1054 nm. Another issue is coating surface roughness. For IAD of HR coatings, elevating the chamber temperature to ∼ 120°C and turning the ion beam off during the pause in deposition between layers reduce the coating surface roughness compared to runs at lower temperatures with the ion beam on continuously. Atomic force microscopy and optical profilometry confirm the reduced surface roughness for these IAD coatings, and tests show that their LIDTs remain high. © 2009 Copyright SPIE - The International Society for Optical Engineering.

Single chamber electrochemical cells were fabricated by patterning working and counter electrodes of Ni and Pt on single-crystal Y2O 3-stabilized ZrO2. Cells were characterized in mixed atmospheres of H2 and H2O at ratios of 1:1 and 1:20 at nominally 923 K and 67 Pa total pressure. Potential sweep and impedance measurements were conducted simultaneously with ambient-pressure x-ray photoelectron spectroscopy (APXPS), which is a unique synchrotron-based probe designed for in-situ chemical characterization of surfaces using photoemission at gas pressures large enough to achieve realistic densities of faradic current. Electrochemically induced oxidation of Ni was observed under anodic polarization and could be reversed by applying a cathodic bias. The thin-film microstructure could also be manipulated electrochemically in that pores exposing underlying electrolyte would open through the Ni film after polarization. Application of APXPS to resolve fundamental details of high-temperature electrochemical process in-situ is discussed. ©The Electrochemical Society.

Coded aperture neutron imaging detectors have the potential to be a powerful tool for the detection of special nuclear material at long range or under heavy shielding, using the signature of fast neutrons from spontaneous fission. We are building a prototype system using liquid scintillator cells, measuring 20'' x 2.5'' x 2.5'' each, in a reconfigurable arrangement. A cross-calibration of the observed detector data with the output of Monte Carlo simulation can both improve the sensitivity of the detector to fast neutron sources and increase the simulation accuracy, allowing the study of next-generation detector designs. Here we describe the tools and procedures developed to calibrate and simulate the detector response, including energy scale and resolution, interaction position, and gamma-neutron separation using pulse shape discrimination. Detector data and simulation are in good agreement for a test configuration. ©2009 IEEE.

Mappings from a master element to the physical mesh element, in conjunction with local metrics such as those appearing in the Target-matrix paradigm, are used to measure quality at points within an element. The approach is applied to both linear and quadratic triangular elements; this enables, for example, one to measure quality within a quadratic finite element. Quality within an element may also be measured on a set of symmetry points, leading to so-called symmetry metrics. An important issue having to do with the labeling of the element vertices is relevant to mesh quality tools such as Verdict and Mesquite. Certain quality measures like area, volume, and shape should be label-invariant, while others such as aspect ratio and orientation should not. It is shown that local metrics whose Jacobian matrix is non-constant are label-invariant only at the center of the element, while symmetry metrics can be label-invariant anywhere within the element, provided the reference element is properly restricted.

Because of their penetrating power, energetic neutrons and gamma rays (>-1 MeV) offer the best possibility of detecting highly shielded or distant special nuclear material (SNM). Of these, fast neutrons offer the greatest advantage due to their very low and well understood natural background. We are investigating a wholly new approach to fast-neutron imaging - an active coded-aperture system that uses a coded mask composed of neutron detectors. The only previously demonstrated method for long-range fast neutron imaging is double-scatter imaging. Active coded-aperture neutron imaging should offer a highly efficient alternative for improved detection speed, range, and sensitivity. We will describe our detector including design considerations and present initial results from a lab prototype. ©2009 IEEE.

Nuclear nonproliferation efforts are supported by measurements that are capable of rapidly characterizing special nuclear materials (SNM). Neutron multiplicity counting is frequently used to estimate properties of SNM, including neutron source strength, multiplication, and generation time. Different classes of model have been used to estimate these and other properties from the measured neutron counting distribution and its statistics. This paper describes a technique to compute statistics of the neutron counting distribution using deterministic neutron transport models. This approach can be applied to rapidly and accurately analyze neutron multiplicity counting measurements.

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