Showing papers by "Sandia National Laboratories published in 1985"

Adaptive Signal Processing

Abstract: GENERAL INTRODUCTION. Adaptive Systems. The Adaptive Linear Combiner. THEORY OF ADAPTATION WITH STATIONARY SIGNALS. Properties of the Quadratic Performance Surface. Searching the Performance Surface. Gradient Estimation and Its Effects on Adaptation. ADAPTIVE ALGORITHMS AND STRUCTURES. The LMS Algorithm. The Z-Transform in Adaptive Signal Processing. Other Adaptive Algorithms and Structures. Adaptive Lattice Filters. APPLICATIONS. Adaptive Modeling and System Identification. Inverse Adaptive Modeling, Deconvolution, and Equalization. Adaptive Control Systems. Adaptive Interference Cancelling. Introduction to Adaptive Arrays and Adaptive Beamforming. Analysis of Adaptive Beamformers.

Anisotropic scattering of electrons by N2 and its effect on electron transport.

Abstract: As part of a systematic study of approximations commonly made in solutions of the Boltzmann equation for electrons in molecular gases, we have investigated the effects of anisotropic scattering on electron transport coefficients in ${\mathrm{N}}_{2}$ and have extended our study of the multiterm expansion technique to higher E/n. A critical survey of published data yields a set of differential and integral cross sections for electron energies from 0.003 to ${10}^{4}$ eV. For electric-field--to--gas-density ratios E/n between 10 and 500 Td (1 Td${=10}^{\mathrm{\ensuremath{-}}21}$ V ${\mathrm{m}}^{2}$), the changes in the commonly measured transport and reaction coefficients resulting from the introduction of anisotropic elastic and inelastic scattering, while keeping the elastic momentum-transfer cross section constant, are less than 1%.These calculations were made with use of the multiterm spherical-harmonic expansion solution of the Boltzmann equation. For 500lE/nl1500 Td the changes in scattering anisotropy cause changes in transport and reaction coefficients which increase with E/n to about 10%. The errors in drift velocity, mean energy, and the reaction coefficients resulting from the use of the two-term spherical-harmonic expansion rather than a six-term expansion are less than 3% at 1500 Td.However, the errors in the diffusion coefficients become large (g25%) at our highest E/n. The calculated transport coefficients are in generally good agreement with experiment for E/n less than 300 Td, but the differences increase at higher E/n. The importance of proper interpretation of ionization and excitation experiments at high E/n is illustrated by calculations which model either an exponential growth of density in time or an exponential growth with position. The calculated ionization coefficients are low compared to most experiments for E/n less than 200 Td. At E/ng600 Td the agreement is good for the spatial growth experiments, but the calculated values are below experiment from the temporal growth experiments. The calculated excitation coefficients are generally higher than experiment at low and high E/n but in agreement with experiment at E/n near 150 Td.

Sol → gel → glass: I. Gelation and gel structure

Abstract: The mechanisms of gel formation in silicate systems derived from metal alkoxides were reviewed. There is compelling experimental evidence proving, that under many conditions employed in silica gel preparation, the resulting polysilicate species formed prior to gelation is not a dense colloidal particle of anhydrous silica but instead a solvated polymeric chain or cluster. The skeletal gel phase which results during desiccation is, therefore, expected to be less highly crosslinked than the corresponding melted glass, and perhaps to contain additional excess free volume. It is proposed that, during gel densification, the desiccated gel will change to become more highly crosslinked while reducing its surface area and free volume. Thus, it is necessary to consider both the macroscopic physical structure and the local chemical structure of gels in order to explain the gel to glass conversion.

Calculation of the surface segregation of Ni-Cu alloys with the use of the embedded-atom method

Abstract: The surface composition of Ni-Cu alloys has been calculated as a function of atomic layer, crystal face, and bulk composition at a temperature of 800 K. The results show that the composition varies nonmonotonically near the surface with the surface layer strongly enriched in Cu while the near-surface layers are enriched in Ni. The calculations use the embedded-atom method [M. S. Daw and M. I. Baskes, Phys. Rev. B 29, 6443 (1984)] in conjunction with Monte Carlo computer simulations. The embedding functions and pair interactions needed to describe Ni-Cu alloys are developed and applied to the calculation of bulk energies, lattice constants, and short-range order. The heats of segregation are computed for the dilute limit, and the composition profile is obtained for the (100), (110), and (111) surfaces for a variety of bulk compositions. The results are found to be in accord with experimental data.

Authentication theory/coding theory

Abstract: We consider a communications scenario in which a transmitter attempts to inform a remote receiver of the state of a source by sending messages through an imperfect communications channel. There are two fundamentally different ways in which the receiver can end up being misinformed. The channel may be noisy so that symbols in the transmitted message can be received in error, or the channel may be under the control of an opponent who can either deliberately modify legitimate messages or else introduce fraudulent ones to deceive the receiver, i.e., what Wyner has called an "active wiretapper" [1]. The device by which the receiver improves his chances of detecting error (deception) is the same in either case: the deliberate introduction of redundant information into the transmitted message. The way in which this redundant information is introduced and used, though, is diametrically opposite in the two cases.For a statistically described noisy channel, coding theory is concerned with schemes (codes) that introduce redundancy in such a way that the most likely alterations to the encoded messages are in some sense close to the code they derive from. The receiver can then use a maximum likelihood detector to decide which (acceptable) message he should infer as having been transmitted from the (possibly altered) code that was received. In other words, the object in coding theory is to cluster the most likely alterations of an acceptable code as closely as possible (in an appropriate metric) to the code itself, and disjoint from the corresponding clusters about other acceptable codes.

Volatile budget of Kilauea volcano

Abstract: The volatile content of magma in the reservoirs of active volcanoes has an important bearing on problems of petrogenesis, magma degassing, eruption mechanisms, eruption forecasting and monitoring and the environmental impact of eruptions. Volcanic gas compositions and the S content of glasses are used to infer the volatile content and the degassing systematics of magma from the Kilauea volcano. Our results are combined with data for Kilauea's magma supply and distribution over the past 27 yr to calculate the volatile budget of the volcano.

Application of the embedded-atom method to liquid transition metals

Abstract: The recently developed embedded-atom method (EAM) of Daw and Baskes [Phys. Rev. B 29, 6443 (1984); Phys. Rev. Lett. 50, 1285 (1983)] is applied to the description of liquid transition metals. A particular set of EAM functions fitted to bulk solid properties is then used to compute the static structure factor and theoretical pressure at the experimental zero-pressure density of various liquid transition metals. The results are in good agreement with experimental data, thus supporting the overall validity of the approach. Further, a systematic prescription for the determination of approximate pair potentials, as well as three- or more-body interactions, from the EAM formalism is presented and shown to give results for the pair correlations in good agreement with the full theory. Finally, the numerical values of the EAM functions used in the calculations for Ni, Pd, Pt, Cu, Ag, and Au are given.

Backward wave oscillators with rippled wall resonators: analytic theory and numerical simulation

Abstract: In this paper, a comprehensive theoretical treatment is developed for backward wave oscillators composed of a relativistic electron beam guided by a strong magnetic field through a slow wave structure consisting of a cylindrical waveguide with a sinusoidally varying wall radius. This analysis, equally applicable to traveling wave tube operation, includes both a linearized theory of small‐amplitude perturbations and numerical simulations of the saturated, large‐amplitude operating regime. The variation of device operating characteristics with system parameters is examined in detail. Comparisons of the analytic and numerical results with experiments and additional calculations show excellent agreement and justify a high degree of confidence in the validity of the theory.

The electrostatic interaction between interfacial colloidal particles

Abstract: The electrostatic interaction between charged, colloidal particles trapped at an air-water interface is considered using linearised Poisson-Boltzmann results for point particles. In addition to the expected screened-Coulomb contribution, which decays exponentially, an algebraic dipole-dipole interaction occurs that may account for long-range interactions in interfacial colloidal systems.

Multivariate Least-Squares Methods Applied to the Quantitative Spectral Analysis of Multicomponent Samples

Abstract: In an extension of earlier work, weighted multivariate least-squares methods of quantitative FT-IR analysis have been developed. A linear least-squares approximation to nonlinearities in the Beer-Lambert law is made by allowing the reference spectra to be a set of known mixtures. The incorporation of nonzero intercepts in the relation between absorbance and concentration further improves the approximation of nonlinearities while simultaneously accounting for nonzero spectral baselines. Pathlength variations are also accommodated in the analysis, and under certain conditions, unknown sample pathlengths can be determined. All spectral data are used to improve the precision and accuracy of the estimated concentrations. During the calibration phase of the analysis, pure component spectra are estimated from the standard mixture spectra. These can be compared with the measured pure component spectra to determine which vibrations experience nonlinear behavior. In the predictive phase of the analysis, the calculated spectra are used in our previous least-squares analysis to estimate sample component concentrations. These methods were applied to the analysis of the IR spectra of binary mixtures of esters. Even with severely overlapping spectral bands and nonlinearities in the Beer-Lambert law, the average relative error in the estimated concentrations was <1%.

Adaptive filters for digital image noise smoothing: An evaluation*

Abstract: Six adaptive noise filtering algorithms were implemented and evaluated. There are (1) median filtering, (2) K-nearest neighbor averaging, (3) gradient inverse weighted smoothing, (4) sigma filtering, (5) Lee additive and multiplicative filtering, and (6) modified Wallis filtering. For the sake of comparison, the mean filter was also included. All algorithms were tested on noise corrupted copies of a composite image consisting of a uniform field, a bar pattern of periods increasing from 2 to 20 pixels, printed text, and a military tank sitting on desert terrain. In one test, uniformly distributed noise between gray levels of −32 and 32 was added to the composite image and filtered. In a second test, multiplicative Gaussian noise with mean 1.0 and standard deviation 0.25 was introduced, then filtered. A 7×7 pixel processing window was used in all six adaptive algorithms and the mean filter for both tests. An adaptive filter was used iteratively with varying window sizes to demonstrate the success of iterative adaptive smoothing. Filtering results were evaluated from statistics, examination of transects plotted from each filtered bar pattern, and from visual ranking by a group of observers.

Recombinative desorption dynamics: Molecular hydrogen from Cu(110) and Cu(111)

Abstract: The rotational and vibrational distributions of H2 and D2 recombinatively desorbing from clean Cu(110) and Cu(111) surfaces following atomic permeation are studied using multiphoton ionization combined with time‐of‐flight mass spectrometry. Rotational distributions are found to be non‐Boltzmann and to possess mean rotational energies which are 80%–90% of the surface temperature, Ts. These distributions are identical to within the experimental accuracy for H2 and D2 and also for desorption from the (110) and (111) faces. Moreover, the ortho and para nuclear spin modifications of both isotopes are statistically populated. In contrast, the vibrational population ratio, Pv‘=1/Pv‘=0, is found to be as much as 100 times greater than the ratio corresponding to a Boltzmann vibrational population at Ts. Specifically, the Pv‘=1/Pv‘=0 ratio for H2 (D2) is 0.052±0.014 (0.24±0.20) desorbing from Cu(110), and 0.084±0.030 (0.35±0.20) desorbing from Cu(111). For comparison the Boltzmann‐at‐Ts ratios would be 0.0009 for H...

Closure and Repropagation of Healed Cracks in Silicate Glass

Abstract: Cracks in soda-lime-silica and vitreous silica glass close against a finite load at humidities between 0.01% and 100%. The force associated with crack closure (0.15 J/m/sup 2/) can be predicted by a model which involves hydrogen-bonded linkages of surfaceadsorbed water molecules. The fracture energy to reopen healed cracks in vitreous silica is also in the range of hydrogenbonding interactions. In the driest environments used, healed cracks in soda-lime-silica glass required 1.7 + or - 0.2 J/m/sup 2/ to reopen. This bonding energy can be attributed to the formation of either cationic bridges or siloxane bonds between fracture surfaces. Since crack healing was observed to be independent of the number of cycles, a cationic bridging model is favored to explain healing effects at room temperature.

Correlation of Radiation Effects in Transistors and Integrated Circuits

Abstract: The effects of ionizing radiation on discrete MOS n- and p-channel transistors are correlated with performance degradation of CMOS integrated circuits. The individual components of radiation induced charge, oxide-trapped charge and interface-state charge, are separated using a subthreshold current technique. Processing splits and post-irradiation biased anneals are used to vary the ratio of oxide-trapped charge to interface-state charge. It is shown that the effective channel mobility depends to first order on the interface-state charge density. Static power supply current is correlated with the n-channel leakage at zero gate voltage while output drive currents are a function of both threshold voltage and channel mobility. Changes in propagation delay of signals through integrated circuits can be understood when both mobility and threshold voltage are considered as a function of the bias dependent charge buildup. A new transistor switching time figure of merit, t/C, which measures the drain to source drive over a full logic level voltage swing at the drain node, is introduced. This index is then shown to correlate with propagation delay in an IC. Finally, performance changes in an IC are modeled using only the measured buildup of oxide-trapped and interface-state charges from transistors as a function of radiation.

Sol → gel → glass: II. Physical and structural evolution during constant heating rate experiments

Abstract: Porous, multicomponent gels were converted to dense glasses at temperatures less than 700°C and at heating rates ranging from 0.5 to 40°C/min. The results of shrinkage, weight loss and differential scanning calorimetry experiments were used to elucidate mechanisms responsible for gel densification. We propose that: (1) capillary contraction, (2) condensation polymerization, (3) structural relaxation, and (4) viscous sintering are the principal gel densification mechanisms. Condensation-polymerization and structural relaxation result in skeletal densification, the magnitude of which closely accounts for all the observed shrinkage between 150 and 525°C. Viscous sintering is the predominant shrinkage mechanism above 525°C. Due to the complex interdependency of the densification mechanisms, the kinetics of gel densification depend strongly on thermal history and, therefore, general constant heating rate analyses are inappropriate for deriving meaningful kinetic information regarding the gel → glass conversion.

The determination of impact ionization coefficients in (100) gallium arsenide using avalanche noise and photocurrent multiplication measurements

Abstract: The electron and hole impact ionization coefficients in (100) GaAs have been determined using photomultiplication measurements performed on specially fabricated p+-n diode structures having active region carrier concentrations from 1.1 X 1017 to 2.2 X 1015 cm-3. These structures are designed to permit pure electron and hole injection in the same diodes. In diodes having heavy p+ doping, the effects of electron dead space have been observed. This dead-space region corresponds to the distance required for an injected carrier to accelerate ballistically to the impact ionization threshold energy, and a method to include this phenomenon in the calculation of α and β from the experimental multiplication data is presented. Agreement among the results from all these structures is obtained with an electron threshold energy of 1.7 eV, and the corrected data are also in agreement with data obtained from device structures designed to eliminate dead-space effects. The measured ratio of α/β in GaAs is found to decrease from 2.4 at 2.2 X 105 V/cm to 1.0 at 6.25 X 105 V/cm. Avalanche noise measurements performed at 30 MHz on the same devices under both electron and hole injection yield a keff of 0.6 and a k'eff of 1.7, respectively, in agreement with the values of these parameters obtained from the photocurrent results.

Static and dynamic scattering from fractals

Abstract: A scaling form for the pair correlation function of random fractals is combined with the scaling form of the percolation distribution of cluster sizes to obtain expressions for the static structure factor S(q) and first cumulant ${K}_{1}$ of the dynamic structure factor S(q,t) at the gel point. We find S(q)\ensuremath{\sim}${q}^{\mathrm{\ensuremath{-}}\ensuremath{\mu}}$ and ${K}_{1}$\ensuremath{\sim}${q}^{3}$ with \ensuremath{\mu}=D(3-\ensuremath{\tau}), where D is the fractal dimension and \ensuremath{\tau} is the exponent for the distribution of cluster sizes. Special forms are suggested for the scaling functions to obtain results for the nonscaling regime.

The subliminal channel and digital signatures

Abstract: In a paper entitled "The Prisoners' Problem and the Subliminal Channel" [1], the present author showed that a message authentication without secrecy channel providing m bits of overt communication and r bits of message authentication could be perverted to allow an l < r bit covert channel between the transmitter and a designated receiver at the expense of reducing the message authentication capability to r-l bits, without affecting the overt channel. It was also shown that under quite reasonable conditions the detection of even the existence of this covert channel could be made as difficult as the underlying cryptoalgorithm was difficult to "break." In view of this open -- but indetectable -- existence, the covert channel was called the "subliminal" channel. The examples constructed in [1], although adequate to prove the existence of such channels, did not appear to be feasible to extend to interesting communications systems. Fortunately, two digital signature schemes have been proposed since Crypto 83 -- one by Ong-Schnorr-Shamir [2] based on the difficulty of factoring sufficiently large composite numbers and one by Gamal [3] based on the difficulty of taking discrete logarithms with respect to a primitive element in a finite field -- that provide ideal bases for implementing practical subliminal channels. This paper reviews briefly the essential features of the subliminal channel and then discusses implementations in both the Ong-Schnorr-Shamir and Gamal digital signature channels.

Calculation of the structure and extinction limit of a methane-air counterflow diffusion flame in the forward stagnation region of a porous cylinder

Abstract: The paper addresses itself to the numerical representation of the structure of counterflow methane-air diffusion flames, with the use of complex chemistry and detailed formulation of the transport fluxes. A similarity solution is briefly outlined which allows the problem to be treated as one-dimensional in space. Following this, discretization and solution of the resulting algebraic equations has been achieved by five different methods. Although there are some differences between the results of the separate computations, these results agree within the probable experimental error with the observations of Tsuji and Yamaoka as regards the temperature and species mole fraction profiles in a particular flame. The more detailed chemical models are able to predict the profiles of C 2 hydrocarbons with reasonable precision; but even somewhat less detailed models are able satisfactorily to predict the major structural features. Despite this agreement, the system overall does not behave as a straightforward boundary layer flow, and in order fully to match the solution to the experimental results it was necessary to modify the measured velocity gradient of 100 s −1 for the cold flow to near 130 s −1 in the flame region. The discrepancy is probably caused by modification of the pressure field due to apparatus effects or to the flame itself, and a full solution requires a two-dimensional treatment to include this effect. As the velocity gradient which characterises these flames is increased, the increased strain in the flame reaction zone (or flame stretch) may lead to extinction. The effect of increasing strain on the flame structure and position near the porous cylinder is examined. There are differences between the two available predictions of the quenching velocity gradient, but both predictions are consistent with the measured, nominal gradient. It is shown that thermal quenching effects at the cylinder do not contribute appreciably to the observed limits. The significance of the extinction limits in the context of non-premixed turbulent combustion is briefly discussed.

Direct observations of the (1×2) surface reconstruction on the Pt(110) plane

Abstract: The reconstruction of the Pt(110) plane from an ordered (1 \ifmmode\times\else\texttimes\fi{} 1) surface to an ordered (1 \ifmmode\times\else\texttimes\fi{} 2) surface has been directly observed for the first time. The (1 \ifmmode\times\else\texttimes\fi{} 1) surface, produced by field evaporation at 78 K, reconstructed at temperatures above 310 K. Field-ion images show clearly that the reconstructed surface consists of alternate missing rows of atoms. The transition was observed for clusters as small as five atoms, implying that the reconstructed surface is stabilized by shortranged, atomic interactions.

Dynamic fracture growth and interaction in one dimension

Abstract: The present study considers concepts relating fracture energy and fracture dynamics. Analyses of Mott are expanded to explicitly account for energy dissipation during the fracture process. Expressions are obtained for the nominal fragment size, fracture time, and dynamic fracture strain. It is shown that communication between growing fractures is relatively slow. Numerical analysis is used to examine the interaction of ductile fractures and the conditions that may lead to arrest or completion of growing fractures.

Calculation of collisionally narrowed coherent anti-Stokes Raman spectroscopy spectra.

Abstract: High-resolution coherent anti-Stokes Raman spectroscopy spectra of the N(2) Q branch at 294 K have been obtained at 1, 5, and 10 atm. Even at 1-atm pressure, disagreements with spectra calculated using the isolated line approximation were observed, indicating the importance of collisional narrowing effects in describing these spectra. A method of using the full G-matrix approach for the calculation of these spectra that is both exact and computationally efficient (requiring only one matrix diagonalization and inversion per spectrum) is discussed. Excellent agreement with experimental data is obtained using this method and a simple exponential gap model for the off-diagonal G-matrix elements.

Ignition delay characteristics of alternative diesel fuels: implications on cetane number

Abstract: Caracteristiques de retard a l'allumage des carburants de remplacement pour moteurs diesel. Consequence pour l'indice de cetane