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Local field and quantum effects for current perpendicular to planes in multilayers

Description: The calculation of giant-magnetoresistance and in general, of electron transport for multilayers in the case of current perpendicular to the planes (CPP) requires both the two-point conductivity and the solution to the local field problem. In this paper we present a solution to the local field problem at an interface using two approaches. In the first approach we find the semiclassical solution for the local field when there is a band mismatch between two sides of an interface, and examine the deviation of the total resistance from the result of ``self-averaging``, in the lowest order of the value of the potential step. In the second approach, we solve for the quantum correction to the local field through a numerical iterative scheme. The oscillations due to the quantum correction are surprisingly large, but their correction to the total resistance is remarkably small. Our results imply that the ``self-averaging`` of the resistance, which is usually assumed in analysis of CPP, is only approximate. 8 refs., 2 figs., 2 tabs.
Date: December 31, 1996
Creator: Zhang, X.G. & Butler, W.H.
Partner: UNT Libraries Government Documents Department

High flux isotope reactor redesigned beryllium reflector thermal stress calculations

Description: The Beryllium reflector of the High Flux Isotope Reactor is currently redesigned for upgrading the capability of the reactor. The original design criteria are adopted in the redesign analysis. Both nuclear heating and thermal stress calculations are revised. The results show that more margin of safety have been achieved and the updated design assures more precise design estimates for the reflector thermal stress conditions. 1 ref., 2 figs., 2 tabs.
Date: June 1, 1996
Creator: Chang, S.J.
Partner: UNT Libraries Government Documents Department

Dislocation emission from a three-dimensional crack -- A large-scale molecular dynamics study

Description: A series of massively parallel molecular dynamics simulations with up to 35 million atoms is performed to investigate dislocation emission from a three-dimensional crack. The authors observe dislocation loops emitted from the crack front--the first time this has been seen in computer simulations. The sequence of dislocation emission in the process of crack blunting process strongly depends on the crystallographic orientation of the crack front and differs strikingly from anything previously conjectured. This finding is essential to establish a precise dislocation emission criterion (i.e., intrinsic ductility criterion). They also find that boundary conditions and interatomic force laws have a significant effect on jogging or blunting dislocation emission modes.
Date: December 31, 1996
Creator: Zhou, S.J.; Beazley, D.M.; Lomdahl, P.S.; Voter, A.F. & Holian, B.L.
Partner: UNT Libraries Government Documents Department

Escaping radio emission from pulsars: Possible role of velocity shear

Description: It is demonstrated that the velocity shear, intrinsic to the e{sup +}e{sup {minus}} plasma present in the pulsar magnetosphere, can efficiently convert the nonescaping longitudinal Langmuir waves (produced by some kind of a beam or stream instability) into propagating (escaping) electromagnetic waves. It is suggested that this shear induced transformation may be the basic mechanism needed for the eventual generation of the observed pulsar radio emission.
Date: January 1, 1997
Creator: Mahajan, S.M.; Machabeli, G.Z. & Rogava, A.D.
Partner: UNT Libraries Government Documents Department

Study of microwave instability

Description: The microwave instability is usually described by linearizing Vlasov equation in the angle-action variables I, {phi} and assuming that the interaction of azimuthal harmonics {rho}{sub n}(I) of the distribution function {rho} is weak. The argument implied here is that the Hamiltonian flow smears out particles over invariant tori characterized by the action variables, and the remaining azimuthal dependence of the distribution function is small. Indeed, such an approach successfully describes bunch spectrum and the threshold of the microwave instability. However, recently there have been interesting observations of bunch centroid and bunch shape oscillations above instability threshold at LEP and the damping ring at SLAC. There are also indicates that the oscillations sometimes occur in localized region in the longitudinal coordinate instead of affecting the entire longitudinal distribution as one expects by an action-angle analysis. In this paper the authors describe an alternative approach to the problem of bunch stability using decomposition of the Fokker-Plank equation in the system of nonlinear equations for the moments of the distribution function. In particular, this approach allows them to avoid the conventional action-angle decomposition. The physical quantities they are interested in, the moments, are expressed in the Cartesian z {minus} {delta} phase space. To close the infinite hierarchy of moments equations, the authors assume that higher order correlations are small. Although both the action-angle and the Cartesian languages must be equivalent before truncation, they may have different speed of convergence depending on the problem being studied. It is hoped that Cartesian expansion approach would converge faster for the cases corresponding to those observed recently above threshold. The recent experimental observations made them interested in it again. This note is a progress report of their work.
Date: December 1, 1996
Creator: Heifets, S. & Chao, A.
Partner: UNT Libraries Government Documents Department

Thermal and flow analyses of the Nuclear Materials Storage Facility Renovation Title I 60% design

Description: The authors are continuing to use the computational fluid dynamics code CFX-4.2 to evaluate the steady-state thermal-hydraulic conditions in the Nuclear Material Storage Facility Renovation Title 1 60% Design. The analyses build on those performed for the 30% design. They have run an additional 9 cases to investigate both the performance of the passive vault and of an individual drywell. These cases investigated the effect of wind on the inlet tower, the importance of resolving boundary layers in the analyses, and modifications to the porous-medium approach used in the earlier analyses to represent better the temperature fields resulting from the detailed modeling of the boundary layers. The difference between maximum temperatures of the bulk air inside the vault for the two approaches is small. They continued the analyses of the wind effects around the inflector fixture, a canopy and cruciform device, on the inlet tower by running a case with the wind blowing diagonally across the inflector. The earlier analyses had investigated a wind that was blowing parallel to one set of vanes on the inflector. Several subcases for these analyses investigated coupling the analysis to the facility analysis and design changes for the inflector.
Date: August 1, 1998
Creator: Knight, T.D.; Steinke, R.G. & Mueller, C.
Partner: UNT Libraries Government Documents Department

Application of soft x-ray laser interferometry to study large-scale-length, high-density plasmas

Description: We have employed a Mach-Zehnder interferometer, using a Ne-like Y x- ray laser at 155 {Angstrom} as the probe source, to study large-scale- length, high-density colliding plasmas and exploding foils. The measured density profile of counter-streaming high-density colliding plasmas falls in between the calculated profiles using collisionless and fluid approximations with the radiation hydrodynamic code LASNEX. We have also performed simultaneous measured the local gain and electron density of Y x-ray laser amplifier. Measured gains in the amplifier were found to be between 10 and 20 cm{sup {minus}1}, similar to predictions and indicating that refraction is the major cause of signal loss in long line focus lasers. Images showed that high gain was produced in spots with dimensions of {approximately} 10 {mu}m, which we believe is caused by intensity variations in the optical drive laser. Measured density variations were smooth on the 10-{mu}m scale so that temperature variations were likely the cause of the localized gain regions. We are now using the interferometry technique as a mechanism to validate and benchmark our numerical codes used for the design and analysis of high-energy-density physics experiments. 11 refs., 6 figs.
Date: May 1, 1996
Creator: Wan, A.S.; Barbee, T.W., Jr. & Cauble, R.
Partner: UNT Libraries Government Documents Department

Cellular automata modeling of weld solidification structure

Description: The authors explore the use of cellular automata in modeling arc-welding processes. A brief discussion of cellular automata and their previous use in micro-scale solidification simulations is presented. Macro-scale thermal calculations for arc-welding at a thin plate are shown to give good quantitative and qualitative results. Combining the two calculations in a single cellular array provides a realistic simulation of grain growth in a welding process. Results of simulating solidification in a moving melt pool in a poly-crystalline alloy sheet are presented.
Date: December 31, 1993
Creator: Dress, W.B.; Zacharia, T. & Radhakrishnan, B.
Partner: UNT Libraries Government Documents Department

Population dynamics of minimally cognitive individuals. Part 2: Dynamics of time-dependent knowledge

Description: The dynamical principle for a population of interacting individuals with mutual pairwise knowledge, presented by the author in a previous paper for the case of constant knowledge, is extended to include the possibility that the knowledge is time-dependent. Several mechanisms are presented by which the mutual knowledge, represented by a matrix K, can be altered, leading to dynamical equations for K(t). The author presents various examples of the transient and long time asymptotic behavior of K(t) for populations of relatively isolated individuals interacting infrequently in local binary collisions. Among the effects observed in the numerical experiments are knowledge diffusion, learning transients, and fluctuating equilibria. This approach will be most appropriate to small populations of complex individuals such as simple animals, robots, computer networks, agent-mediated traffic, simple ecosystems, and games. Evidence of metastable states and intermittent switching leads them to envision a spectroscopy associated with such transitions that is independent of the specific physical individuals and the population. Such spectra may serve as good lumped descriptors of the collective emergent behavior of large classes of populations in which mutual knowledge is an important part of the dynamics.
Date: July 1, 1995
Creator: Schmieder, R.W.
Partner: UNT Libraries Government Documents Department

Effects of incoherent space charge forces on the fast head-tail instability

Description: The effects of incoherent space charge forces on the fast head-tail instability are studied numerically. It is found that incoherent space charge forces can dramatically increase the threshold current for a fixed wall impedance. The algorithm used for this study is described.
Date: July 1, 1997
Creator: Blaskiewicz, M.
Partner: UNT Libraries Government Documents Department

The column density distribution of the Lyman-alpha forest: A measure of small scale power

Description: Absorption lines in the Ly{alpha} forest can be thought of as peaks in neutral hydrogen density along lines of sight. The column density distribution (the number density of absorption lines as a function of column density) is then a statistic of density peaks, which contains information about the underlying power spectrum. In particular, we show that the slope of the distribution provides a measure of power on scales smaller than those probed by studies of present day large scale structure. 2 refs., 1 fig.
Date: February 19, 1997
Creator: Hui, Lam; Gnedin, Nickolay Y. & Zhang, Yu
Partner: UNT Libraries Government Documents Department

Surface characterization of optics for EUV lithography

Description: The surface topography of optics fabricated for Extreme Ultraviolet Lithography has been measured using a combination of phase-measuring interferometry and atomic force microscopy. Power Spectral Densities were computed over spatial frequencies extending from 2.0{times}10{sup {minus}8} nm{sup {minus}1} to 7.7{times}10{sup {minus}2} nm{sup {minus}1}. Roughness values for frequencies greater than 1.0{times}10{sup {minus}6} were 0.64 nm rms for a spherical optic and 0.95 nm rms for an aspheric optic. These values are significantly larger than 0.088 nm rms, which as obtained using a spherical optic representative of current limits in surface polishing technology. 9 refs., 4 figs.
Date: May 28, 1996
Creator: Gaines, D.P.; Sweeney, D.W.; DeLong, K.W.; Vernon, S.P.; Baker, S.L.; Tichenor, D.A. et al.
Partner: UNT Libraries Government Documents Department

Nonlinear interaction of plane elastic waves

Description: The paper presents basic first order results of nonlinear elastic theory by Murnaghan for elastic wave propagation in isotropic solids. The authors especially address the problem of resonant scattering of two collimated beams and present analytical solutions for amplitudes of all possible types of resonant interactions for elastic plane waves. For estimation of nonlinear scattered waves they use measured elastic parameters for sandstone. The most profound nonlinear effect is expected for interactions of two SH waves generating compressional P wave at sum frequency. Estimations show that nonlinear phenomena is likely to be observed in seismic data. Basic equations of nonlinear five-constant theory by Murnaghan are also presented.
Date: June 1, 1998
Creator: Korneev, V.A.; Nihei, K.T. & Myer, L.R.
Partner: UNT Libraries Government Documents Department

The size of a polymer of string-bits: a numerical investigation

Description: In string bit models, string is described as a polymer of point-like constituents. We attempt to use string-bit ideas to investigate how the size of string is affected by string interactions in a non-perturbative context. Lacking adequate methods to deal with the full complications of bit rearrangement interactions, we study instead a simplified analog model with only ``direct`` potential interactions among the bits. We use the variational principle in an approximate calculation of the mean-square size of a polymer as a function of the number of constituents/bits for various interaction strengths {ital g} in three specific models. 14 refs., 2 figs.
Date: April 1, 1997
Creator: Bergman, O. & Thorn, C.B.
Partner: UNT Libraries Government Documents Department

A collisional-radiative average atom model for hot plasmas

Description: A collisional-radiative `average atom` (AA) model is presented for the calculation of opacities of hot plasmas not in the condition of local thermodynamic equilibrium (LTE). The electron impact and radiative rate constants are calculated using the dipole oscillator strengths of the average atom. A key element of the model is the photon escape probability which at present is calculated for a semi infinite slab. The Fermi statistics renders the rate equation for the AA level occupancies nonlinear, which requires iterations until the steady state. AA level occupancies are found. Detailed electronic configurations are built into the model after the self-consistent non-LTE AA state is found. The model shows a continuous transition from the non-LTE to the LTE state depending on the optical thickness of the plasma. 22 refs., 13 figs., 1 tab.
Date: October 17, 1996
Creator: Rozsnyai, B. F.
Partner: UNT Libraries Government Documents Department

Brittle failure kinetics model for concrete

Description: A new constitutive model is proposed for the modeling of penetration and large stress waves in concrete. Rate effects are incorporated explicitly into the damage evolution law, hence the term brittle failure kinetics. The damage variable parameterizes a family of Mohr-Coulomb strength curves. The model, which has been implemented in the CTH code, has been shown to reproduce some distinctive phenomena that occur in penetration of concrete targets. Among these are the sharp spike in deceleration of a rigid penetrator immediately after impact. Another is the size scale effect, which leads to a nonlinear scaling of penetration depth with penetrator size. This paper discusses the theory of the model and some results of an extensive validation effort.
Date: March 1, 1997
Creator: Silling, S.A.
Partner: UNT Libraries Government Documents Department

Scalar mass relations and flavor violations in supersymmetric theories

Description: Supersymmetry provides the most promising solution to the gauge hierarchy problem. For supersymmetry to stablize the hierarchy, it must be broken at the weak scale. The combination of weak scale supersymmetry and grand unification leads to a successful prediction of the weak mixing angle to within 1{percent} accuracy. If supersymmetry is a symmetry of nature, the mass spectrum and the flavor mixing pattern of the scalar superpartners of all the quarks and leptons will provide important information about a more fundamental theory at higher energies. We studied the scalar mass relations which follow from the assumption that at high energies there is a grand unified theory which leads to a significant prediction of the weak mixing angle; these will serve as important tests of grand unified theories. Two intragenerational mass relations for each of the light generations are derived. A third relation is also found which relates the Higgs masses and the masses of all three generation scalars. In a realistic supersymmetric grand unified theory, nontrivial flavor mixings are expected to exist at all gaugino vertices. This could lead to important contributions to the neutron electric dipole moment, the decay mode p {r_arrow} K{sup 0}{mu}{sup +}, weak scale radiative corrections to the up-type quark masses, and lepton flavor violating signals such as {mu} {r_arrow} e{gamma}. These also provide important probes of physics at high energy scales. Supersymmetric theories involving a spontaneously broken flavor symmetry can provide a solution to the supersymmetric flavor-changing problem and an understanding of the fermion masses and mixings. We studied the possibilities and the general conditions under which some fermion masses and mixings can be obtained radiatively. We also constructed theories of flavor in which the first generation fermion masses arise from radiative corrections while flavor-changing constraints are satisfied. 69 refs., 19 figs., 9 tabs.
Date: May 9, 1996
Creator: Cheng, Hsin-Chia
Partner: UNT Libraries Government Documents Department

Research program on fractured petroleum reservoirs. [Quarterly report], October 1--December 31, 1995

Description: A number of experiments have been performed to study water injection in fractured porous media. These experiments reveal that: (1) the co-current imbibition may be the primary flow process in water-wet fractured media, and (2) the imbibition may result in over 20 percent recovery from very tight rock (Austin Chalk with K{sub ma} of the order of 0.01 md) for an imbibition period of about 2 months. Theoretical consideration reveal that the exponential function of Aronofsky et al. [``A Model for the Mechanism of Oil Recovery from Porous Matrix Due to Water Invasion In Fractured Reservoirs,`` Trans. AIME (1958) 213, 17-19] does not describe the early-time, but may represent the late-time recovery.
Date: January 31, 1996
Creator: Firoozabadi, A.
Partner: UNT Libraries Government Documents Department

Calculated dose rates in halls using CASIM

Description: The first part of this document shows the results of a series of CASIM runs designed to optimize the size of a steel cylinder so as to produce the highest density of stars at any given radius perpendicular to beam direction. For the next set of runs the length of the cylinder is now fixed at three meters and the authors vary the radius. These runs show that a cylinder with a radius of seven centimeters produces the highest dose rate. For the rest of the CASIM runs a cylinder three meters long with a radius of seven centimeters will be used. Using the optimized cylinder they now make runs using geometries more closely representing real experimental hall geometries in an attempt to predict possibly dose rates.
Date: February 1, 1995
Creator: Sondgeroth, A.
Partner: UNT Libraries Government Documents Department

Collisionless damping of perpendicular magnetosonic waves in a two-ion-species plasma

Description: Propagation of finite-amplitude magnetosonic waves in a collisionless plasma containing two ion species is studied with a one-dimensional, fully electromagnetic code based on a three-fluid model. It is found that perpendicular magnetosonic waves are damped in a two-ion-species plasma; a magnetosonic pulse accelerates heavy ions in the direction parallel to the wave front, which results in the excitation of a longer wavelength perturbation behind the pulse. The damping due to the energy transfer from the original pulse to the longer wavelength perturbation occurs even if the plasma is collisionless and the pulse amplitude is small. The theoretically obtained damping rate is in agreement with the simulation result.
Date: August 1, 1996
Creator: Dogen, Daiju; Toida, Mieko & Ohsawa, Yukiharu
Partner: UNT Libraries Government Documents Department

Specification of multipole tolerances for the APS quadrupole magnet

Description: This note will address a proposed method for specifying the multipole tolerance for the design and production of APS quadrupole magnets. The tolerances for the multipole components for the quadrupole magnets will be set to that level which reduces the dynamic aperture by about 10--15% from the ideal machine dynamic aperture (as specified in CDR-87). This level may appear rather stringent, especially compared to the 50--60% reduction resulting from quad placement errors. However, when all tolerances are taken together, the residual dynamic aperture would be prohibitively small and commissioning would be difficult if these tolerances were at twice this level. The dynamic aperture was determined using the numerical tracking program RACETRACK.
Date: August 1, 1988
Creator: Kramer, S. L.
Partner: UNT Libraries Government Documents Department

The infrared behaviour of {alpha}{sub s} from Mandelstam`s approximation to the gluon Dyson-Schwinger equation

Description: Using Mandelstam`s approximation to the gluon Dyson-Schwinger equation we calculate the gluon self-energy in a renormalisation group invariant fashion. We obtain a non-perturbative {beta} function. The scaling behaviour near the ultraviolet stable fixed point is in good agreement with perturbative QCD. No further fixed point for positive values of the coupling is found, i.e., {alpha}{sub s} increases without bound in the infrared. The non-perturbative result for an infrared singular quark interaction can be used to relate the string tension {sigma} to the QCD scale {Lambda}. 3 refs., 1 fig.
Date: October 1, 1996
Creator: Hauck, A.; Alkofer, R. & Von Smekal, L.
Partner: UNT Libraries Government Documents Department

Comparison of elastic-viscous-plastic and viscous-plastic dynamics models using a high resolution Arctic sea ice model

Description: A nonlinear viscous-plastic (VP) rheology proposed by Hibler (1979) has been demonstrated to be the most suitable of the rheologies commonly used for modeling sea ice dynamics. However, the presence of a huge range of effective viscosities hinders numerical implementations of this model, particularly on high resolution grids or when the ice model is coupled to an ocean or atmosphere model. Hunke and Dukowicz (1997) have modified the VP model by including elastic waves as a numerical regularization in the case of zero strain rate. This modification (EVP) allows an efficient, fully explicit discretization that adapts well to parallel architectures. The authors present a comparison of EVP and VP dynamics model results from two 5-year simulations of Arctic sea ice, obtained with a high resolution sea ice model. The purpose of the comparison is to determine how differently the two dynamics models behave, and to decide whether the elastic-viscous-plastic model is preferable for high resolution climate simulations, considering its high efficiency in parallel computation. Results from the first year of this experiment (1990) are discussed in detail in Hunke and Zhang (1997).
Date: December 31, 1997
Creator: Hunke, E.C. & Zhang, Y.
Partner: UNT Libraries Government Documents Department

Thickness dependencies in the calculated properties of metallic ultra-thin films

Description: Ultra-thin film (UTF) electronic structure calculations are a common tool for investigating surface properties. For this approximation to be useful, the UTF must be thick enough that the surfaces are decoupled and the interior is bulk-like, yet thin enough that a high precision electronic structure calculation is affordable. These conditions can only be satisfied simultaneously if the properties of interest converge rapidly as the UTF thickness is increased. In this work, electronic structure calculations for Al(111) films ranging from one to twelve atoms thick are used to illustrate some of the difficulties that can arise when one attempts to determine surface properties of metals with UTF calculations.
Date: December 1, 1997
Creator: Boettger, J.C.
Partner: UNT Libraries Government Documents Department