Publications

HICUP models the angular dependent heavy ion upset cross section. It pulls together many of the parameters and concepts used to characterize the Single Event Upset (SEU) phenomena, unifying them in a single cohesive model. HICUP is based on a Rectangular Parallelepiped (RPP) geometry for the sensitive volume and the Weibull density function for the upset threshold energy. Excellent agreement is obtained between the model and heavy ion test data. HICUP is used to derive the correct scaling laws for transforming angular cross section data to normal incidence, reconciling two previously proposed inverse cosine scaling corrections. The angle-integrated HICUP model, I-HICUP, is used in Galactic Cosmic Ray (GCR) upset rate calculations with results nearly identical to the Space Radiation• code. Letaw [12] has procuced an automated SEU parameter fitting routine based on HICUP and the cH2 method. It ferrets out the best-fit critical SEU parameters embedded within the raw angular test data, including charge collection depth and funnel length. His method couples directly to the upset rate calculation in a self-consistent manner eliminating the need to arbitrarily assume a device depth. Results of this new procedure are presented. © 1995 IEEE

A sinkhole was observed over the edge of the two-level former salt mine that was converted for oil storage. Diagnostic studies suggest a direct connection exists between the surface collapse area and the underground mine as shown by correlative measurements of sediment slump rates and probable brine influx into the mine. The dissolution of salt below the sinkhole that initiated the leak into the mine was likely caused by several confluent geologic processes, and exacerbated by mining-induced stresses that created fractures which served as hydrologic flowpaths. Modelling studies of mine stresses show that years may be required before tensional cracking begins to occur, but once begun can continue to develop, and relieve the stress in that specific regime. The crack regime creates the avenue for incursion of groundwater. Mitigation measures include increasing the mine pressure, slowing the dissolution by injecting brine into the sinkhole throat, and construction of a freeze curtain to restrict hydrologic flowpaths. -from Authors

A general method for applying command shaping to various multiple degree of freedom cranes is described such that the payload moves to a specified point without residual oscillation. A dynamic programming is used for general command shaping for optimal maneuvers. The results taken are compared to near-optimal solutions where the commands are linear combinations of accelerations pulse basis functions. Simulation results and experimental verification for a variable load-line length rotary crane are also presented using design procedures.

The issues of verification, calibration, and validation of computational fluid dynamics (CFD) codes has been receiving increasing levels of attention in the research literature and in engineering technology. Both CFD researchers and users of CFD codes are asking more critical and detailed questions concerning the accuracy, range of applicability, reliability and robustness of CFD codes and their predictions. This is a welcomed trend because it demonstrates that CFD is maturing from a research tool to the world of impacting engineering hardware and system design. In this environment, the broad issue of code quality assurance becomes paramount, However, the philosophy and methodology of building confidence in CFD code predictions has proven to be more difficult than many expected. A wide variety of physical modeling errors and discretization errors are discussed. Here, discretization errors refer to all errors caused by conversion of the original partial differential equations to algebraic equations, and their solution. Boundary conditions for both the partial differential equations and the discretized equations will be discussed. Contrasts are drawn between the assumptions and actual use numerical method consistency and stability. Commen are also made concerning the existence and uniqueness solutions for both the partial differential equations and the discrete equations. Various techniques are suggested for the detection and estimation of errors caused by physical modeling and discretization of the partial differential equations.

Carbon Nitride (CNx) films have been grown by ion-assisted pulsed-laser deposition (IAPLD). Graphite targets were laser ablated while bombarding the substrate with ions from a broad-beam Kaufman-type ion source. The ion voltage, current density, substrate temperature, and feed gas composition (N2 in Ar) have been varied. The resultant films were characterized by Raman, Fourier transform infrared (FTIR), and Rutherford back scattering (RBS) spectroscopy. Samples with ≈30% N/C ratio have been fabricated. The corresponding Raman and FTIR spectra indicate that nitrogen is incorporated into the samples by insertion into sp2- bonded structure. A low level of C≡N triple bonds is also found. As the ion current and voltage are increased with a pure Ar ion beam, Raman peaks associated with nanocrystalline graphite appear in the spectra. Adding low levels of nitrogen to the ion beam first reduces the Raman intensity in the vicinity of the graphite disorder peak without adding detectable amounts of nitrogen to the films (as measured by RBS). At higher nitrogen levels in the ion beam, significant amounts of nitrogen are incorporated into the samples, and the magnitude of the ″disorder″ peak increases. By increasing the temperature of the substrate during deposition, the broad peak due mainly to sp2-bonded C-N in the FTIR spectra is shifted to lower wavenumber. This could be interpreted as evidence of single-bonded C-N; however, it is more likely that the character of the sp2 bonding is changing.

Extensive surface pressure measurements were obtained on a hypersonic vehicle configuration at Mach 8. All of the experimental results were obtained in the Sandia National Laboratories Mach 8 hypersonic wind tunnel for lamipar boundary layer conditions. The basic vehicle configuration IS a spherically blunted 100 half-angle cone with a slice parallel with the axis of the vehicle. The bluntness ratio of the geometly IS 10% and the slice begins at 70% of the length of the vehicle. Surface pressure measurements were obtained for angles of attack from -10 to +180. for various roll angles, at 96 locations on the body surface. A new and innovative uncertainty analysis was devised to estimate the contributors to surface pressure meaSment uncenainty. Quantitative estimates were computed for the uncertainty contributions due to the complete insmmentation system, nonunifoxmity of flow in the test section of the wind Nnnel. and variations in the wind tunnel model. This extensive set of high-quality surface pressure measurements is recommended for use in the calibration and validation of computational fluid dynamics codes for hyuersonic flow conditions.

Short-pulse accelerator technology developed during the early 1960's through the late 1980's is being extended to high average power systems capable of use in industrial and environmental applications. Processes requiring high dose levels and/or high volume throughput will require systems with beam power levels from several hundreds of kilowatts to megawatts. Beam accelerating potentials can range from less than 1 MeV to as much as 10 MeV depending on the type of beam, depth of penetration required, and the density of the product being treated. This paper addresses the present status of a family of high average power systems, with output beam power levels up to 200 kW, now in operation that use saturable core switches to achieve output pulse widths of 50 to 80 nanoseconds. Inductive adders and field emission cathodes are used to generate beams of electrons or x-rays at up to 2.5 MeV over areas of 1000 cm2. Similar high average power technology is being used at ≤ 1 MeV to drive repetitive ion beam sources for treatment of material surfaces over 100's of cm2. © 1995.

Performance of an electromagnetic induction launcher is considered for three types of armatures. These are: solid, 1-element wound and 16-element wound aluminum armatures. The one element wound armature has uniform current density throughout and thus can withstand field reversal (working against embedded armature flux) and still maintain low temperature. Slingshot simulations were performed for several configurations. Best performance was obtained for a single element wound armature with two field reversals. For a 60 kg projectile, 10.5 cm coil inner radius and 5.5 cm coil build, the velocity after 50 meters of launcher length (670 stages) exceeded 3.5 km/sec with an overall efficiency of about 45%. For the same parameters the solid and 16-element wound armatures reach a velocity of about 3.3 km/sec after 800 stages (60 meters of launcher length) but without field reversal. A velocity of 3.5 km/sec is possible after 60 meters of launcher length with the 16-element wound armature with one field reversal, but the temperature is close to the melting temperature of aluminum. In all simulations with a solid armature, melting of some of the surface material occurs. However, it is shown that most of the melting occurs after contribution has been made to the forward going pressure, that is, melting does not affect the electrical performance of the launcher. The effect of coil firing time jitter on launcher performance is also considered and is found to be very small for realistic perturbations. For ± 2 μ–secs random jitter, the reduction in the final velocity for a 60 meter launcher with a solid armature is less than 0.1% and the increase in temperature is only 2%. This holds for all types of armatures. © 1995 IEEE

Cold filling refers to flowing a fluid through piping or tubes that are at temperatures below the fluid’s freezing point. Since the piping and areas of the receiver in a molten-nitrate salt central-receiver solar power plant must be electrically heated to maintain their temperatures above the nitrate salt freezing point (430°F, 221 °C), considerable energy could be used to maintain such temperatures during nightly shutdown and bad weather. Experiments and analyses have been conducted to investigate cold filling receiver panels and piping as a way of reducing parasitic electrical power consumption and increasing the availability of the plant. The two major concerns with cold filling are (1) how far can the molten salt penetrate cold piping before freezing closed, and (2) what thermal stresses develop during the associated thermal shock. Cold fill experiments were conducted by flowing molten salt at 550°F (288°C) through cold panels, manifolds, and piping to determine the feasibility of cold filling the receiver and piping. The transient thermal responses were measured and heat transfer coefficients were calculated from the data. Nondimensional analysis is presented which quantifies the thermal stresses in a pipe or tube undergoing thermal shock. In addition, penetration distances were calculated to determine the distance salt could flow in cold pipes prior to freezing closed. © 1995 by ASME.

Understanding the evolution of porous silicon (PS) layers at the early stages of growth is important for determining the mechanism of PS film growth and controlling the film properties. We have used X-ray reflectivity (XRR) to determine the evolution of layer thickness and interfacial roughness during the growth of thin PS layers (< 200 nm) prepared by electrochemical anodization. The porous layer grows at a constant rate for films as thin as 15 nm indicating a very short incubation period during which the surface may be electropolished before the PS structure begins to form. Interface roughness measurements indicate that the top surface of the film remains relatively smooth during growth while the roughness of the PS/silicon interface increases only slightly with film thickness. The XRR results are compared with results obtained from the same films by cross-sectional transmission electron microscopy (XTEM), atomic force microscopy (AFM) and gravimetry.

Uncertainty and sensitivity analysis techniques based on Latin hypercube sampling, partial correlation analysis and stepwise regression analysis are used in an investigation with the MACCS model of the food pathways associated with a severe accident at a nuclear power station. The primary purpose of this study is to provide guidance on the variables to be considered in future review work to reduce the uncertainty in the important variables used in the calculation of reactor accident consequences. The effects of 87 imprecisely-known input variables on the following reactor accident consequences are studied: crop growing-season dose, crop long-term dose, milk growing-season dose, total food pathways dose, total ingestion pathways dose, total long-term pathways dose, area dependent cost, crop disposal cost, milk disposal cost, condemnation area, crop disposal area and milk disposal area. When the predicted variables are considered collectively, the following input variables were found to be the dominant contributors to uncertainty: fraction of cesium deposition on grain fields that is retained on plant surfaces and transferred directly to grain, maximum allowable ground concentrations of Cs-137 and Sr-90 for production of crops, ground concentrations of Cs-134, Cs-137 and I-131 at which the disposal of milk will be initiated due to accidents that occur during the growing season, ground concentrations of Cs-134, I-131 and Sr-90 at which the disposal of crops will be initiated due to accidents that occur during the growing season, rate of depletion of Cs-137 and Sr-90 from the root zone, transfer of Sr-90 from soil to legumes, transfer of Cs-137 from soil to pasture, transfer of cesium from animal feed to meat, and the transfer of cesium, iodine and strontium from animal feed to milk. © 1995.

Uncertainty and sensitivity analysis techniques based on Latin hypercube sampling, partial correlation analysis and stepwise regression analysis are used in an investigation with the MACCS model of the chronic exposure pathways associated with a severe accident at a nuclear power station. The primary purpose of this study is to provide guidance on the variables to be considered in future review work to reduce the uncertainty in the important variables used in the calculation of reactor accident consequences. The effects of 75 imprecisely known input variables on the following reactor accident consequences are studied: crop growing-season dose, crop long-term dose, water ingestion dose, milk growing-season dose, long-term groundshine dose, long-term inhalation dose, total food pathways dose, total ingestion pathways dose, total long-term pathways dose, total latent cancer fatalities, area-dependent cost, crop disposal cost, milk disposal cost, population-dependent cost, total economic cost, condemnation area, condemnation population, crop disposal area and milk disposal area. When the predicted variables are considered collectively, the following input variables were found to be the dominant contributors to uncertainty: dry deposition velocity, transfer of cesium from animal feed to milk, transfer of cesium from animal feed to meet, ground concentration of Cs-134 at which the disposal of milk products will be initiated, transfer of Sr-90 from soil to legumes, maximum allowable ground concentration of Sr-90 for production of crops, fraction of cesium entering surface water that is consumed in drinking water, groundshine shielding factor, scale factor defining resuspension, dose reduction associated with decontamination, and ground concentration of I-131 at which disposal of crops will be initiated due to accidents that occur during the growing season. Reducing the uncertainty in the preceding variables was found to substantially reduce the uncertainty in the predicted variables under consideration. For total number of latent cancer fatalities, the dominant variable was dry deposition velocity, with small effects indicated for a large number of additional variables. © 1995.

The effect of the pump, signal, and idler wave phases on three-wave nonlinear parametric mixing is investigated in a series of single-pass- gain experiments. Measurements are made with two angle-tuned KTP crystals in a 532 nm pumped, walkoff-compensated, optical parametric amplifier that is seeded by an 800 nm cw diode laser. In one of the measurements the second crystal is orientated to have its effective nonlinearity deff. of opposite sign to that of the first crystal, so that all mixing that occurred in the first crystal is canceled by the second when the phase mismatch Δkcrystal 1 = Δkcrystal 2 = 0. Efficient two-crystal amplification is subsequently restored by selecting the correct phase relationship for the three waves entering the crystal by inserting a dispersive plate between the crystals. The experimental results are explained in a straightforward manner with diagrams involving the three input wave polarizations. These results demonstrate that walkoff-compensated geometries require phase correction to achieve efficient mixing in the second crystal whenever the nonlinear interaction involves two extraordinary waves (e-waves). One practical application of this work may be lower oscillation thresholds and enhanced performance in walkoff-compensated optical parametric oscillators which use two e-waves.

The Critical Dynamics in Microgravity Experiment, DYNAMX, is under development for space flight at Sandia National Laboratories and the University of New Mexico with Dr. Rob Duncan as the Principal Investigator. This experiment will investigate the effects on the superfluid transition in 4He of currents generated by heat flow, measuring the thermal conductivity in the fluid as a function of applfed heat. DYNAMX will also take advantage of the weightless environment to measure the conductivity properties in the region of the interface between the two phases. Thus, DYNAMX represents an experiment that will explore a system driven far from equilibrium. The experiment development is sponsored by the Microgravity Science and Applications Division of NASA, with the Jet Propulsion Laboratory as the managing center. This paper will describe the science objectives, the current design of the experiment apparatus, the steps being taken to prepare this experiment for flight, and the results of ground-based feasibility demonstrations now underway.

We present magnetoluminescence data which provides a quantitative measure of the energy- band dispersion curves of novel compound semiconductor optoelectronic materials. Data for a n-type strained-layer InGaAs/GaAs (quantum-well width approximately 8 nm) and a n-type 4.5 nm-wide GaAs/AlGaAs lattice-matched single-quantum well are presented. We find that the conduction-bands are almost parabolic, with a mass of about 0.068m0 for the GaAs/AlGaAs structure. The valence-bands are nonparabolic with wave vector dependent in- plane valence-band masses varying from about 0.1m0 at zone center to about 0.3m0 for 20 meV energies.

Sol-gel processing methods are frequently used for the fabrication of lead zirconate titanate (PZT) thin films for many electronic applications. Our standard approach for film fabrication utilizes lead acetate and acetic acid modified metal alkoxides of zirconium and titanium in the preparation of our precursor solutions. This report highlights some of our recent results on the effects of the addition of a second chelating ligand, acetylacetone, to this process. We discuss the changes in film drying behavior, densification and ceramic microstructure which accompany acetylacetone additions to the precursor solution and relate the observed variations in processing behavior to differences in chemical precursor structure induced by the acetylacetone ligand. Improvements in thin film microstructure, ferroelectric and optical properties are observed when acetylacetone is added to the precursor solution.

Since both FeS2 and MoS2 are optically opaque and have bandgaps in the near IR, photochemical applications of these materials are non-existent. It is however demonstrated in the study that nanosize FeS2 and MoS2 have bandgaps that can be adjusted to the visible and even UV region of the spectrum by control of the cluster size. Discussed are measurements of size, structure, optical absorbance and photoemission for these new materials. Also, briefly outlines are synthetic procedures and novel methods for chemical processing of these nanoclusters.

We report a real-time, two-dimensional light scattering study of the evolution of structure of a two component nonionic micelle system undergoing phase separation. The micelles act like molecular slug-a-beds whose domain growth is pathetically lathargic (i.e. slower than the cube root of time prediction for simple binary fluids). In fact, the growth kinetics can be empirically described as a stretched exponential approach to a pinned domain size. Although the kinetics are not yet understood, this anomalous behavior may be due to the ability of the spherical micelles to reorganize into more complex structures.

A jib crane consists of a pendulum-like end line attached to a rotatable jib. Within this general category of cranes there exist devices with multiple degrees of freedom including variable load-line length and variable jib length. These cranes are commonly used for construction and transportation applications. Point-to-point payload maneuvers using jib cranes are performed so as not to excite the spherical pendulum modes of their cable and payload assemblies. Typically, these pendulum modes, although time-varying, exhibit low frequencies. The resulting maneuvers are therefore performed slowly, contributing to high construction and transportation costs. The crane considered here consists of a spherical pendulum attached to a rigid jib. The other end of the jib is attached to a direct drive motor of generating rotational motion. A general approach is presented for determining the open-loop trajectories for the jib rotation for accomplishing fixed-time, point-to-point, residual oscillation free, symmetric maneuvers. These residual oscillation free trajectories purposely excite the pendulum modes in such a way that at the end of the maneuver the oscillatory degrees of freedom are quiescent. Simulation results are presented with experimental verification.

First-principles density-functional calculations utilizing ab initio pseudopotentials and plane- wave expansions are used to determine lattice parameters, bulk moduli, and band structures for AlN, GaN, and InN. It is found that large numbers of plane waves are necessary to resolve the nitrogen 2p wave functions and that explicit treatment of the gallium 3d and indium 4d electrons is important for an accurate description of GaN and InN. Several properties of ternary zinc-blende alloys are determined including their bond-length and bond-angle relaxation and their energy-gap bowing parameters. The similarity of the calculated zinc- blende and wurtzite direct gaps also allows estimates to be made of the energy gap versus composition for wurtzite alloys.

To address the need for a fast path planner, we present a learning algorithm that improves path planning by using past experience to enhance future performance. The algorithm relies on an existing path planner to provide solutions to difficult tasks. From these solutions, an evolving sparse network of useful robot configurations is learned to support faster planning. More generally, the algorithm provides a framework in which a slow but effective planner may be improved both cost-wise and capability-wise by a faster but less effective planner coupled with experience. We analyze the algorithm by formalizing the concept of improvability and deriving conditions under which a planner can be improved within the framework. The analysis is based on two stochastic models, one pessimistic (on task complexity), the other randomized (on experience utility). Using these models, we derive quantitative bounds to predict the learning behavior. We use these estimation tools to characterize the situations in which the algorithm is useful and to provide bounds on the training time. In particular, we show how to predict the maximum achievable speedup. Additionally, our analysis techniques are elementary and should be useful for studying other types of probabilistic learning as well.

A simultaneous determination of both the conduction and valence-band dispersion curves (and masses) from a single compound semiconductor quantum-well structure using magnetoluminescence is discussed. Data from InGaAs/GaAs strained-single-quantum wells and GaAs/AlGaAs lattice matched quantum wells are presented. The conduction bands are found to be parabolic for densities approaching 1 × 1012cm-2. However, the valence bands are highly nonparabolic and strongly affected by heavy-hole light-hole mixing. © 1995.

Barrier layers to limit percolation through cover systems are principal features of engineered, multi-component cover designs. Conventional barrier layer components developed for humid climates have limitations in dry climates. One alternative barrier layer is a capillary barrier, which consists of a fine-over-coarse soil arrangement. The capacity of capillary barriers to laterally divert downward moving water is the key to their success. Another alternative is a dry barrier, in which atmospheric air is circulated through a coarse layer within the cover to remove water vapor. Incorporating a coarse layer which stores water for subsequent removal by air flow reduces the requirements for the air flow velocity and increases the applicability of the dry barrier.

In the US, concrete containment buildings for commercial nuclear power plants have steel liners that act as the internal pressure boundary. The liner abuts the concrete, acting as the interior concrete form. The liner is attached to the concrete by either studs or by a continuous structural shape (such as a T-section or channel) that is either continuously or intermittently welded to the liner. Studs are commonly used in reinforced concrete containments, while prestressed containments utilize a structural element as the anchorage. The practice in some countries follows the US practice, while in other countries the containment does not have a steel liner. In this latter case, there is a true double containment, and the annular region between the two containments is vented. This paper will review the practice of design of the liner system prior to the consideration of severe accident loads (overpressurization loads beyond the design conditions). An overpressurization test of a 1:6 scale reinforced concrete containment at Sandia National Laboratories resulted in a failure mechanism in the liner that was not fully anticipated. Post-test analyses and experiments have been conducted to understand the failure better. This work and the activities that followed the test are reviewed. Areas in which additional research should be conducted are given. © 1995.

We have performed computational simulations to determine how energy from a large hypervelocity impact on the Earth's surface would couple to its interior. Because of the first-order axial symmetry of both the impact energy source and the stress-wave velocity structure of the Earth, a disproportionate amount of energy is dissipated along the axis defined by the impact point and its antipode (point opposite the impact). For a symmetric and homogeneous Earth model, all the impact energy that is radiated as seismic waves into the Earth at a given takeoff angle (ray parameter), independent of azimuthal direction, is refocused (minus attenuation) on the axis of symmetry, regardless of the number of reflections and refractions it has experienced. Material on or near the axis of symmetry experiences more strain cycles with much greater amplitude than elsewhere, and therefore experiences more irreversible heating. The focusing is most intense in the upper mantle, within the asthenosphere, where seismic energy is most effectively converted to heat. For a sufficiently energetic impact, this mechanism might generate enough local heating to create an isostatic instability leading to uplift, possibly resulting in rifting, volcanism, or other rearrangement of the interior dynamics of the planet. These simulations demonstrate how hypervelocity impact energy can be transported to the Earth's interior, supporting the possibility of a causal link between large impacts on Earth and major internally-driven geophysical processes. © 1995.