Digital Commons Network^™

Advection Of A Passive Scalar By A Vortex Couple In The Small-Diffusion Limit, Joseph F. Lingevitch, Andrew J. Bernoff

All HMC Faculty Publications and Research

We study the advection of a passive scalar by a vortex couple in the small-diffusion (i.e. large Péclet number, Pe) limit. The presence of weak diffusion enhances mixing within the couple and allows the gradual escape of the scalar from the couple into the surrounding flow. An averaging technique is applied to obtain an averaged diffusion equation for the concentration inside the dipole which agrees with earlier results of Rhines & Young for large times. At the outer edge of the dipole, a diffusive boundary layer of width O(Pe^−½) forms; asymptotic matching to the interior …

Optical Properties Of Human Uterus At 630 Nm, Steen J. Madsen, Bruce J. Tromberg, Yona Tadir, Pius Wyss, Lars O. Svaasand, Richard C. Haskell

All HMC Faculty Publications and Research

The optical properties of normal and fibriotic human uteri were determined using frequency-domain and steady-state techniques .

Effect Of High Latitude Ionospheric Convection On Sun-Aligned Polar Caps, Jan Josef Sojka, Lie Zhu, D. J. Crain, Robert W. Schunk

All Physics Faculty Publications

A coupled magnetospheric-ionospheric (M-I) MHD model has been used to simulate the formation of Sun-aligned polar cap arcs for a variety of interplanetary magnetic field (IMF) dependent polar cap convection fields. The formation process involves launching an Alfvén shear wave from the magnetosphere to the ionosphere where the ionospheric conductance can react self-consistently to changes in the upward currents. We assume that the initial Alfvén shear wave is the result of solar wind-magnetosphere interactions. The simulations show how the E region density is affected by the changes in the electron precipitation that are associated with the upward currents. These changes …

Cartesian Lumière And Newtonian Light, Brian Canfield

Honors Theses, 1963-2015

My thesis examines the scientific methods of René Descartes and Sir Isaac Newton. I begin by looking briefly at Aristotle's concept of light. Aristotle believed that light was a property that the medium acquired instantaneously from a luminous source. Descartes later separated light from the medium and the luminous object as in instantaneous pression in the ether. He introduced a more mechanical way of examining light with this separation. Descartes' methods consist mainly of analogies between the behavior of light and observations of processes unrelated to optics. Descartes appears to have derived the law of refraction. This law is a …

Modelling Sun-Aligned Polar Cap Arcs, D. J. Crain, Jan Josef Sojka, Robert W. Schunk, Lie Zhu

All Physics Faculty Publications

We present results of a new model of the time-dependent ionospheric response to a generalized steady state Sun-aligned (SA) arc structure. The thermal and plasma structure of a “prototype” arc is compared to the general features of observed SA arcs. We find that the general features of electron density, N_e , electron temperature, T_e , and ion temperature, T_i , are determined by the distribution of the particle precipitation and E × B convection associated with the SA arc. The model results are extended to predict the possible variation of N_e, T_e , and …

Magnetospheric Shortcomings In Ionospheric-Magnetospheric Coupling: An Ionospheric Perspective, Jan Josef Sojka

All Physics Faculty Publications

The ionosphere, on a global scale, is reasonably well understood from a climatology perspective. However, the storm dynamics of the ionosphere are not fully understood. This partly arises from the complex response function of the Thermosphere-Ionosphere(T-I) system but also from the uncertainty in the space and time dynamics of the magnetospheric inputs to the ionosphere. In the context of M-I coupling, the ionosphere responds to magnetospheric electrodynamic forcing by altering the conductivity in the ionosphere and by plasma transport. Phenomenologically,we understand how to let the ionospheric conductivity evolve in response to local precipitation and how to transport plasma in the …

Theoretical Study Of Polar Cap Arcs: Time-Dependent Model And Its Applications, Lie Zhu, Jan Josef Sojka, Robert W. Schunk, D. J. Crain

All Physics Faculty Publications

A time-dependent theoretical model of polar cap arcs developed during the Coupling, Energetics, and Dynamics of Atmospheric Regions/High-Latitude Plasma Structures (CEDAR/HLPS) campaigns in the past two years is briefly described. In the model the electrodynamics of the polar cap arcs are treated self-consistently in the frame of the coupled magnetosphere-ionosphere system. The preliminary simulation results of the temporal evolution and spatial structure of the polar cap arcs for both winter and summer conditions are presented. The model can be used to conduct both the model-observation study of specific features of the polar cap arcs and the quantitative theoretical study of …

Ionospheric Response To The Sustained High Geomagnetic Activity During The March '89 Great Storm, Jan Josef Sojka, Robert W. Schunk, W. F. Denig

All Physics Faculty Publications

A simulation was conducted to model the high-latitude ionospheric response to the sustained level of high geomagnetic activity for the great magnetic storm period of March 13-14, 1989. The geomagnetic and solar activity indices and the DMSP F8 and F9 satellite data for particle precipitation and high-latitude convection were used as inputs to a time-dependent ionospheric model (TDIM). The results of the TDIM were compared to both DMSP plasma density data and ground-based total electron content (TEC) measurements for the great storm period as well as with earlier storm observations. The comparisons showed that the overall structure of the high-latitude …

A Fast Numerical Solution Of Scattering By A Cylinder: Spectral Method For The Boundary Integral Equations, Fang Q. Hu

Mathematics & Statistics Faculty Publications

It is known that the exact analytic solutions of wave scattering by a circular cylinder, when they exist, are not in a closed form but in infinite series which converge slowly for high frequency waves. In this paper, a fast numerical solution is presented for the scattering problem in which the boundary integral equations, reformulated from the Helmholtz equation, are solved using a Fourier spectral method. It is shown that the special geometry considered here allows the implementation of the spectral method to be simple and very efficient. The present method differs from previous approaches in that the singularities of …

Frequency-Domain Photon Migration In Turbid Media, Bruce J. Tromberg, Steen J. Madsen, Curtis Chapman, Lars O. Svaasand, Richard C. Haskell

All HMC Faculty Publications and Research

An analytical model is presented for the propagation of diffuse photon density waves in turbid media. The frequency- and wavelength-dependence of photon density waves are measured using Frequency-domain Photon Migration (FDPM). Media optical properties, including absorption, transport, and fluorescence relaxation times are calculated from experimental results.

Direct Simulation Monte Carlo For Thin Film Bearings, Alejandro Garcia, B. Alder, F. J. Alexander

Faculty Publications

The direct simulation Monte Carlo (DSMC) scheme is used to study the gas flow under a read/write head positioned nanometers above a moving disk drive platter (the slider bearing problem). In most cases, impressive agreement is found between the particle-based simulation and numerical solutions of the continuum hydrodynamic Reynolds equation which has been corrected for slip. However, at very high platter speeds the gas is far from equilibrium, and the load capacity for the slider bearing cannot be accurately computed from the hydrodynamic pressure.

Rate Coefficient For The Reaction N + No, Jane L. Fox

Physics Faculty Publications

Evidence has been advanced that the rate coefficient for the reaction N + NO → N₂; + O has a small positive temperature dependence at the high temperatures (900-1500 K) that prevail in the terrestrial middle and upper thermosphere by Siskind and Rusch (1992), and at the low temperatures (100-200 K) of the Martian lower thermosphere by Fox (1993). Assuming that the rate coefficient recommended by the Jet Propulsion Laboratory evaluation (DeMore et al., 1992) is accurate at 300 K, we derive here the low temperature value of the activation energy for this reaction and thus the rate …

Hopping Conduction In Molecular Beam Epitaxial Gaas Grown At Very Low Temperatures, David C. Look, Z-Q. Fang, J. W. Look, J. R. Sizelove

Physics Faculty Publications

Conductivity and Hall effect measurements have been performed on 2 μm thick molecular beam epitaxial layers grown at very low substrate temperatures, 200 to 400°C. For growth temperatures below 300°C, the conduction is dominated by hopping between arsenic antisite defects of concentrations up to 10²⁰ cm⁻³. Below measurement temperatures of about 130 K, the hopping conduction can be quenched by strong IR light illumination, because the antisite then becomes metastable. The antisite has a thermal activation energy of , and thus is not identical to the famous EL2. Both nearest‐neighbor and variable‐range hopping mechanisms are considered in …

Compton Scattering Of Gamma-Rays From Electrons In Advanced Laboratory, Christine A. Byrd, Morgan T. Burks, Lawrence A. Yates, Wilfred J. Braithwaite

Journal of the Arkansas Academy of Science

A kinematically-complete 2-body final state measurement of Compton scattering of 662-keV photons is presened, where both scattered photon energy and electron recoil energy are measured versus photon scattering angle, theta gamma. Passive collimation of the photon beam is avoided; each recoiling electron triggers a photon-scattering event providing active beam collimation. Recoiling electrons have low energies at small theta gamma, impairing electron detection efficiency. Examining the recoiling-electron energy spectra in coincidence with high-resolution gammas indicates a 1"x 1"NaI detector is superior to a 1"x1" NE-102 plastic scintillator as the active scattering material, for efficient recoil-electron detection. Electron efficiencies versus theta gamma …

Fluid Dynamics Model Of Data Acquisition And Date Analysis For High-Energy Physics, Charles M. Byrd, Christine A. Byrd, Wilson H. Howe, W. J. Braithwaite

Journal of the Arkansas Academy of Science

No abstract provided.

Dominance Of The Thomas Mechanism For Electron Capture From Orientated Rydberg Atoms, Jee-Ching Wang, Ronald E. Olson

Chemical and Biochemical Engineering Faculty Research & Creative Works

We investigate electron capture from initially oriented circular Rydberg atoms by proton impact at collision speeds comparable to the electron orbital speed. The Thomas double scattering mechanism is observed even at these low relative speeds. Furthermore, we find evidence for the dominance of a quasi Thomas capture mechanism in the form of not a single peak, but a double-peaked structure in the differential cross section when the plane of the circular orbit is nearly perpendicular to the incident direction of the projectile. © 1994 The American Physical Society.

Anisotropy Effects In Electron Capture By O6+ From Aligned Na*(3p), S. Schippers, A. R. Schlatmann, W. P. Wiersema, R. Hoekstra, R. Morgenstern, Ronald E. Olson, J. Pascale

Physics Faculty Research & Creative Works

We report results of one electron capture by highly charged ions colliding with laser excited aligned Na*(3p). The O vi(10→8) photon emission cross section after electron capture by the O6+ projectile is measured in the collision energy range 2-8 keV/amu. Effects of the Na*(3p) orbital alignment on the) O vi(10→8) emission are observed and found to be "reversed" as compared to the He2+→Na*(3p) collision system. Classical trajectory Monte Carlo calculations agree with the measured data and indicate that capture to states with n>10, contributing to the O vi(10→8) emission via cascades, plays a crucial role in the alignment cross …

Solid State Proton Spin Relaxation And Methyl And T-Butyl Reorientation, Peter A. Beckmann, Hania A. Al-Hallaq, Anne M. Fry, Amy L. Plofker, Brian A. Roe, Jessica A. Weiss

Physics Faculty Research and Scholarship

No abstract provided.

Several Considerations Regarding The Variable Length Blade Rotor, Bogdan Popescu, Victor Giurgiutiu

Faculty Publications

No abstract provided.

Interstellar Gas And Dust In The Young Cluster Ic 348, Theodore P. Snow, Margaret Murray Hanson, C Gregory Seab, Jon M. Saken

Physics Faculty Publications

We have completed a multiband absorption- and emission-line study of a star embedded in the young cluster IC 348, to determine the environmental effect of star formation on the interstellar medium (ISM) local to the region. The hottest and youngest star in IC 348 is BD+31°643, a B5 V star which samples the inner bright nebular region. The nearby star o Per, which lies only 8 to the north and is thought to lie beyond IC 348, samples the gas and dust which has not been processed by very recent star formation. We speculate that the ISM throughout the region …

Electron-Electron Interaction In Projectile Ionization Investigated By High Resolution Recoil Ion Momentum Spectroscopy, R. Dörner, V. Mergel, R. Ali, U. Buck, C. L. Cocke, K. Froschauer, O. Jagutzki, S. Lencinas, W. E. Meyerhof, S. Nüttgens, Ronald E. Olson, H. Schmidt-Böcking, L. Spielberger, K. Tökesi, J. Ullrich

Physics Faculty Research & Creative Works

Contributions of the electron-electron and the nuclear-electron interactions to projectile electron loss in He+ on helium collisions have been clearly separated kinematically for the first time by measuring the longitudinal and transverse momenta of the recoiling target ion with precision of better than ±0.15 a.u. The longitudinal and transverse momentum distributions are in general agreement with two-center n-body classical trajectory Monte Carlo calculations. © 1994 The American Physical Society.

Low-Energy Electrons And Their Dynamical Correlation With Recoil Ions For Single Ionization Of Helium By Fast, Heavy-Ion Impact, R. Moshammer, J. Ullrich, M. Unverzagt, W. Schmidt, P. Jardin, Ronald E. Olson, R. Mann, R. Dörner, V. Mergel, U. Buck, H. Schmidt-Böcking

Physics Faculty Research & Creative Works

Helium single ionization by 3.6 me V/u Ni24+ impact was explored in a kinematically complete experiment by combining a high-resolution recoil-ion momentum spectrometer with a novel 4 low-energy electron analyzer. More than 90% of the "soft electrons" (Ee50 eV) are ejected in the forward direction in agreement with classical-trajectory Monte Carlo predictions. The electron longitudinal momentum is not balanced by the longitudinal momentum change of the projectile but mainly by the backwards recoiling He1+ ion. Energy losses of the 0.2 GeV projectiles as small as EPEP=3.4x10-7 are observable. © 1994 The American Physical Society.

Prediction Of The Stochastic Behavior Of Nonlinear Systems By Deterministic Models As A Classical Time-Passage Probabilistic Problem, L. M. Ivanov, A. D. Kirwan Jr., O. V. Melnichenko

CCPO Publications

Assuming that the behaviour of a nonlinear stochastic system can be described by a Markovian diffusion approximation and that the evolution equations can be reduced to a system of ordinary differential equations, a method for the calculation of prediction time is developed. In this approach, the prediction time depends upon the accuracy of prediction, the intensity of turbulence, the accuracy of the initial conditions, the physics contained in the mathematical model, the measurement errors, and the number of prediction variables. A numerical application to zonal channel flow illustrates the theory. Some possible generalizations of the theory are also discussed.

Excitation Of The Classical-Limit State Of An Atom, Z. D. Gaeta, Michael W. Noel, C. R. Stroud Jr.

Physics Faculty Research and Scholarship

We describe a technique designed to excite a classical-limit state of an atom. A picosecond electric field pulse converts a circular state into a Rydberg wave packet which is localized in all three dimensions and travels along a classical Kepler orbit with arbitrary ellipticity.

A Fringe Center Detection Technique Based On A Sub-Pixel Resolution, And Its Applications Using Sinusoidal Gratings, Ming Chang, Paul P. Lin, Wen Chih Tai

Mechanical Engineering Faculty Publications

A common problem in optical profilometry is the accuracy in locating fringe centers. This paper presents an accurate fringe center detection technique based on sub-pixel resolution using the fringe projection method. An optimum reconstruction filter is developed which has low sensitivity to noise. In fringe center detection, computer simulation results of using one-pixel and sub-pixel resolutions are compared. The detection technique is then applied to radius measurement of cylindrical objects and surface profile measurement of diffuse objects. The experimental results thus obtained through the proposed optimum reconstruction filter show significant improvement in measurement accuracy.

Molecular-Dynamics Simulations Of Some Baxf4 Compounds, John Flocken, Z. Mo, Wai-Ning Mei, John R. Hardy, Dorian Hatch

Physics Faculty Publications

We have carried out molecular-dynamics simulations on BaXF₄ compounds, where X is Mg, Mn, or Zn. Ab initio potentials, with no adjustable parameters, were used to obtain short-range interactions between ion pairs. We found a polar ground-state structure which is in agreement with the A2₁am space group reported experimentally. We were able to reverse polarization in BaMgF₄ at high temperatures, using large fields, but were unable to reverse polarization in the other compounds. The second-order phase transition in the Mn compound at 250 K was reproduced. We believe this to be the first extension of …

A Thermoelastic Model Applied To Stress Control In Laser Heating Of Ceramics, Jeff Alexander Wagner

Theses

Localized laser heating is widely used in materials processing. In extending these techniques to materials with relatively low thermal conductivities and ductilities such as ceramics and glasses, existing methods must be modified to control the high thermal stresses which are associated with the localized heating of these materials. Thermal profiles must be designed to minimize damage to regions adjacent to the processed area. To achieve this with single beam sources the power and radius can be varied in time, or the beam can be moved across the surface in a programmed pattern to achieve the desired thermal profile.

In this …

The Physics Of Grain-Grain Collisions And Gas-Grain Sputtering In Interstellar Shocks, A G. Tielens, C F. Mckee, C G. Seab, D J. Hollenbach

Physics Faculty Publications

Grain-grain collisions and ion sputtering destroy dust grains in interstellar shocks. An analytical theory is developed for the propagation of shock waves in solids driven by grain-grain collisions, which compares very favorably with detailed numerical calculations. This theory is used to determine the fraction of a grain vaporized by a grain-grain collision. Our results predict much less vaporization of colliding grains in interstellar shocks than previous estimates. This theory can also be used to determine the fraction of a colliding grain that melts, shatters, or undergoes a phase transformation to a higher density phase. In particular, the latter two processes …

Einstein's Equations In Vacuum Spacetimes With Two Spacelinke Killing Vectors Using Affine Projection Tensor Geometry, Miles D. Lawrence

Theses and Dissertations

Einstein's equations in vacuum spacetimes with two spacelike killing vectors are explored using affine projection tensor geometry. By doing a semi-conformal transformation on the metric, a new "fiducial" geometry is constructed using a projection tensor fields. This fiducial geometry provides coordinate independent information about the underlying structure of the spacetime without the use of an explicit form of the metric tensor.

Reflection High-Energy Electron-Diffraction Study Of Surface Disorder And Anomalous Expansion Of Pb(100), D. Tang, H. E. Elsayed-Ali

Electrical & Computer Engineering Faculty Publications

The temperature-dependent surface structural behavior of Pb(100) is studied using reflection high-energy electron diffraction. Anomalous surface expansion for temperatures between room temperature to about 500 K is observed. A high density of surface vacancies appears at temperatures above ~ 500 K. © 1994 The American Physical Society.