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Reverse time migration in tilted transversely isotropic media

Description: This paper presents a reverse time migration (RTM) method for the migration of shot records in tilted transversely isotropic (TTI) media. It is based on the tilted TI acoustic wave equation that was derived from the dispersion relation. The RTM is a full depth migration allowing for velocity to vary laterally as well as vertically and has no dip limitations. The wave equation is solved by a tenth-order finite difference scheme. Using 2D numerical models, we demonstrate that ignoring the tilt angle will introduce both lateral and vertical shifts in imaging. The shifts can be larger than 0.5 wavelength in the vertical direction and 1.5 wavelength in the lateral direction.
Date: July 2004
Creator: Zhang, Linbing; Rector, James W., III & Hoversten, G. Michael
Partner: UNT Libraries Government Documents Department

Pseudo-MHD ballooning modes in tokamak plasmas

Description: The MHD description of a plasma is extended to allow electrons to have both fluid-like and adiabatic-regime responses within an instability eigenmode. In the resultant {open_quotes}pseudo-MHD{close_quotes} model, magnetic field line bending is reduced in the adiabatic electron regime. This makes possible a new class of ballooning-type, long parallel extent, MHD-like instabilities in tokamak plasmas for {alpha} > s{sup 2}(2 {sup 7/3}/9) (r{sub p}/R{sub 0}) or-d{radical}{Beta}/dr > (2{sup 1/6} /3)(s/ R{sub 0q}), which is well below the ideal-MHD stability boundary. The marginally stable pressure profile is similar in both magnitude and shape to that observed in ohmically heated tokamak plasmas.
Date: August 1, 1996
Creator: Callen, J.D. & Hegna, C.C.
Partner: UNT Libraries Government Documents Department

The planar Green`s function in an infinite multiplying medium

Description: Throughout the history of neutron transport theory, the study of simplified problems that have analytical or semi-analytical solutions has been a foundation for more complicated analyses. Analytical transport results are often used as benchmarks or in pedagogical settings. Benchmark problems in infinite homogeneous media have been studied continually, beginning with the monograph by Case, DeHoffmann, and Placzek. A fundamental problem considered in this work is the Green`s function in an infinite medium. The Green`s function problem considers an infinite planar source emitting neutral particles in the single directions`. Recently, this Green`s function has been used to obtain solutions for finite media. These solutions, which hinge on accurate and fast evaluation of the infinite medium Green`s function, use Fourier and Laplace transform inversion techniques for the evaluation. Until now, only absorbing media have been considered, and applications were therefore limited to non-multiplying media. In an effort to relax this limitation, the infinite medium Green`s function is numerically evaluated for an infinite multiplying medium using the double-sided Laplace transform inversion. Of course, no steady-state mathematical solution exists for an infinite multiplying medium with a source present; however, the non-physical solution in an infinite medium can be used in finite media problems where the solution is physically realizable.
Date: October 1, 1996
Creator: Kornreich, D.E. & Ganapol, B.D.
Partner: UNT Libraries Government Documents Department

Dispersion relationship for solid state instability growth and sensitivity to equation of state

Description: We have derived an approximate analytical dispersion relation for solid state instability growth following the method of Mikaelian. I He starts with the general eigenvalue equation for the velocity of a perturbation on a finite-thickness fluid layer with surface tension and viscosity, and derives an exact solution numerically from det(M)=0, where M is an 8x8 matrix. He then derives an approximate solution analytically by substituting the inviscid eigenfunctions into the exact eigenvalue equation. The integrations yield a dispersion relation which is a polynomial in the growth rate.
Date: June 1, 1997
Creator: Colvin, J.D.; Wiley, L.G.; Chandler, E.A.; Remington, B.A. & Kalantar, D.H.
Partner: UNT Libraries Government Documents Department

The Green`s function method for critical heterogeneous slabs

Description: Recently, the Green`s Function Method (GFM) has been employed to obtain benchmark-quality results for nuclear engineering and radiative transfer calculations. This was possible because of fast and accurate calculations of the Green`s function and the associated Fourier and Laplace transform inversions. Calculations have been provided in one-dimensional slab geometries for both homogeneous and heterogeneous media. A heterogeneous medium is analyzed as a series of homogeneous slabs, and Placzek`s lemma is used to extend each slab to infinity. This allows use of the infinite medium Green`s function (the anisotropic plane source in an infinite homogeneous medium) in the solution. To this point, a drawback of the GFM has been the limitation to media with c < 1, where c is the number of secondary particles produced in a collision. Clearly, no physical steady-state solution exists for an infinite medium that contains an infinite source and is described by c >1; however, mathematical solutions exist which result in oscillating Green`s functions. Such calculations are briefly discussing. The limitation to media with c < 1 has been relaxed so that the Green`s function may also be calculated for media with c {ge} 1. Thus, materials that contain fissionable isotopes may be modeled.
Date: October 1, 1996
Creator: Kornreich, D.E.
Partner: UNT Libraries Government Documents Department

Dispersion in the presence of strong transverse wakefields

Description: To minimize emittance growth in a long linac, it is necessary to control the wakefields by correcting the beam orbit excursions. In addition, the particle energy is made to vary along the length of the bunch to introduce a damping, known as the BNS damping, to the beam break-up effect. In this paper, the authors use a two-particle model to examine the relative magnitudes of the various orbit and dispersion functions involved. The results are applied to calculate the effect of a closed orbit bump and a misaligned structure. It is shown that wake-induced dispersion is an important contribution to the beam dynamics in long linacs with strong wakefields like SLC.
Date: July 1, 1997
Creator: Assmann, R. & Chao, A.
Partner: UNT Libraries Government Documents Department

Rippled-beam free-electron laser

Description: The authors describe a new microwave generation mechanism involving a scalloping annular electron beam. The beam interacts with the axial electric field of a TM{sub 0n} mode in a smooth circular waveguide through the axial free-electron laser interaction, in which the beam ripple period is synchronous with the phase slippage of the rf mode relative to the electron beam. Due to nonlinearities in the orbit equation, the interaction can be made autoresonant, where the phase and amplitude of the gain is independent of the beam energy.
Date: October 1, 1997
Creator: Carlsten, B.E.
Partner: UNT Libraries Government Documents Department

Existence and consequences of Coulomb pairing of electrons in a solid

Description: It is shown from first principles that, in the periodic potential of a crystalline solid, short-range (i.e., screened) binary Coulomb interactions can lead to a two-electron bound state. It is further suggested that these composite bosonic states (charge -2e, and typically spin zero) could mediate an effectively attractive interaction between pairs of conduction electrons close to the Fermi level. This necessarily short range attractive interaction, which is crucially dependent on the band structure of the solid, and is complementary to the phonon-mediated one, may provide a source for the existence and properties of short correlation-length electron pairs (analogous to but distinct from Cooper pairs) needed to understand high temperature superconductivity. Several distinctive and observable characteristics of the proposed pairing scheme are discussed.
Date: November 1, 1996
Creator: Mahajan, S.M. & Thyagaraja, A.
Partner: UNT Libraries Government Documents Department

How to extract the P{sub 33}(1232) resonance contributions from the amplitudes M{sub 1{sup +}}{sup 3/2},E{sub 1{sup +}}{sup 3/2},S{sub 1{sup +}}{sup 3/2} of pion electroproduction on nucleons

Description: Within the dispersion relation approach, solutions of integral equations for the multipoles M{sub 1{sup +}}{sup 3/2},E{sub 1{sup +}}{sup 3/2},S{sub 1{sup +}}{sup 3/2} are found at 0 &lt; Q{sup 2} &lt; 3 GeV{sup 2}. These solutions should be used as input for the resonance and nonresonance contributions in the analyses of pion electroproduction data in the P{sub 33}(1232) resonance region. It is shown that the traditional identification of the amplitude M{sub 1{sup +}}{sup 3/2} (as well of the amplitudes E{sub 1{sup +}}{sup 3/2},S{sub 1{sup +}}{sup 3/2}) with the P{sub 33}(1232) resonance contribution is not right; there is a contribution in these amplitudes which has a nonresonance nature and is produced by rescattering effects in the diagrams corresponding to the nucleon and pion poles. This contribution is reproduced by the dispersion relations. Taking into account nonresonance contributions in the amplitudes M{sub 1{sup +}}{sup 3/2},E{sub 1{sup +}}{sup 3/2}, the helicity amplitudes A{sub p}{sup 1/2}, A{sub p}{sup 3/2} and the ratio E2/M1 for the {gamma}N {r_arrow} P{sub 33}(1232) transition are extracted from experiment at Q{sup 2}=0. They are in good agreement with quark model predictions.
Date: August 1997
Creator: Aznauryan, I. G.
Partner: UNT Libraries Government Documents Department

Spin waves in CsVBr{sub 3}

Description: Inelastic neutron scattering has been used to measure spin wave excitations in the quasi-one dimensional S = 3/2 magnetic material CsVBr{sub 3}. Dispersion relations were determined using standard triple-axis methods. Fits to linear spin wave theory yield model Hamiltonian parameters describing magnetic interactions in the system.
Date: July 10, 1997
Creator: Nagler, S.E.; Mandrus, D.G. & Tennant, D.A.
Partner: UNT Libraries Government Documents Department

New constraints on dispersive form factor parameterizations from the timelike region

Description: The authors generalize a recent model-independent form factor parameterization derived from rigorous dispersion relations to include constraints from data in the timelike region. These constraints dictate the convergence properties of the parameterization and appear as sum rules on the parameters. They further develop a new parameterization that takes into account finiteness and asymptotic conditions on the form factor, and use it to fit to the elastic {pi} electromagnetic form factor. They find that the existing world sample of timelike data gives only loose bounds on the form factor in the spacelike region, but explain how the acquisition of additional timelike data or fits to other form factors are expected to give much better results. The same parameterization is seen to fit spacelike data extremely well.
Date: February 1, 1998
Creator: Buck, W. W. & Lebed, Richard F.
Partner: UNT Libraries Government Documents Department

Is the momentum space optimally used with the FODO lattices?

Description: The available momentum space of a FODO lattice is determined by the maximum value of the dispersion function ({delta}x = D{sub x} {partial_derivative}p/p). In a regular FODO lattice the dispersion function oscillates between its maximum and minimum values, which are always positive. The maximum value of the dispersion function in a FODO cell of a fixed length depends on the cell phase difference. An example of a new lattice, in which the dispersion function is lowered to half its value in the same FODO cell, is presented. ne available momentum space in the new lattice is raised to twice that in the FODO lattice by allowing the dispersion function to oscillate between the same positive and negative values. The maxima of the dispersion function in the new lattice have half the value of those within the regular 900 cells.
Date: July 1, 1993
Creator: Trbojevic, D.; Ng, K.Y. & Lee, S.Y.
Partner: UNT Libraries Government Documents Department

Phase-stable, microwave FEL amplifier

Description: Free-electron laser (FEL) amplifiers have demonstrated high efficiency and high output power for microwave wavelengths. However, using present technology, microwave FEL amplifiers are not phase stable enough to be suitable for driving linear accelerators, where several much amplifiers need to be phase locked. The growing wave`s phase sensitivity to the beam voltage in the small-signal gain regime is responsible for the largest contribution to this phase instability. We discuss a scheme that reduces the phase sensitivity to the beam voltage by operating off synchronism and matching the phase variation resulting from the desynchronism to the phase variation from the reduced plasma wavenumber as the beam voltage changes.
Date: July 1, 1995
Creator: Carlsten, B.E.; Fazio, M.V.; Haynes, W.B.; May, L. & Potter, M.
Partner: UNT Libraries Government Documents Department

Band anticrossing in highly mismatched semiconductor alloys

Description: The basic theoretical aspects of the band anticrossing effects in highly electronegativity-mismatched semiconductor alloys are reviewed. The many-impurity Anderson model treated in the coherent potential approximation is applied to the semiconductor alloys, in which metallic anion atoms are partially substituted by atoms of a highly electronegative element. Analytical solutions for the Green's function describe dispersion relations and state broadening effects for the restructured conduction band. The solutions are identical to those obtained from the physically intuitive and widely used two-level band anticrossing model. It is shown that the model explains key experimental observations including the unusual composition and pressure dependence of the interband optical transitions and the large enhancement of the electron effective mass.
Date: July 26, 2002
Creator: Walukiewicz, W.
Partner: UNT Libraries Government Documents Department


Description: Results from studies of irradiation induced transformations in U--Mo and U--Nb alloys are reported. Progress in the development of a satisfactory brazing technique using Al alloy for use with U/sub 3/Si mods in Zircaloy-2 tubes is reviewed. Additional data are given on the corrosion of bare of U--q systems in water. Work directed toward the development of satisfactory UO/sub 2/ for fuel elements is reviewed. (D.E.B.)
Date: February 1, 1956
Creator: Losco, E.F. & Belle, J. eds.
Partner: UNT Libraries Government Documents Department

On the Possibility of the Sheath-Driven, Finite-Beta Modes Localized Near the Divertor Plate

Description: It is shown that, in a finite beta plasma, there may exist sheath driven modes whose amplitude decreases exponentially with the distance from the divertor plate. The modes are sensitive to the radial tilt of the divertor plate. The short-wavelength branch of the instability, with the cross-field wavelength Dof order of a few ion gyroradii, is present in the case of a ''positive'' tilt of the divertor plate, whereas the long-wavelength branch, with D of order of 10 or so gyroradii is unstable for the opposite sign of the tilt. The parallel e-folding length becomes less than the distance from the plate to the X point (thereby making the mode insensitive to the processes near the X-point and the upper scrape-off layer) at the plasma betas exceeding (2-3) {center_dot} 10{sup -4}. A detailed analysis of the dispersion relations is provided. The features of the modes that can be used for their experimental identification are discussed. It is pointed out that the analog of these modes may also exist in linear plasma devices with shaped end electrodes.
Date: February 10, 2005
Creator: Cohen, R & Ryutov, D
Partner: UNT Libraries Government Documents Department


Description: A free-surface centrifugal pump, incorporating a hydraulic bearing running in sodium, was operated at the conditions required for service in the Hallam Power Reactor. After difficulties arising from inadequate shaft clearances were alleviated, the pump performed properly at a flow rate of 7200 gpm of 945 gas-cooled F sodium at 150-ft head. Results Hallam plant. (auth)
Date: June 30, 1960
Creator: Atz, R. W.
Partner: UNT Libraries Government Documents Department

World Sheet Commuting beta-gamma CFT and Non-Relativistic StringTheories

Description: We construct a sigma model in two dimensions with Galilean symmetry in flat target space similar to the sigma model of the critical string theory with Lorentz symmetry in 10 flat spacetime dimensions. This is motivated by the works of Gomis and Ooguri[1] and Danielsson et. al.[2, 3]. Our theory is much simpler than their theory and does not assume a compact coordinate. This non-relativistic string theory has a bosonic matter {beta}{gamma} CFT with the conformal weight of {beta} as 1. It is natural to identify time as a linear combination of {gamma} and {bar {gamma}} through an explicit realization of the Galilean boost symmetry. The angle between {gamma} and {bar {gamma}} parametrizes one parameter family of selection sectors. These selection sectors are responsible for having a non-relativistic dispersion relation without a nontrivial topology in the non-relativistic setup, which is one of the major differences from the previous works[1, 2, 3]. This simple theory is the non-relativistic analogue of the critical string theory, and there are many different avenues ahead to be investigated. We mention a possible consistent generalization of this theory with different conformal weights for the {beta}{gamma} CFT. We also mention supersymmetric generalizations of these theories.
Date: August 30, 2007
Creator: Kim, Bom Soo
Partner: UNT Libraries Government Documents Department

Dispersion Analysis of the Pulseline Accelerator

Description: The authors analyze the sheath helix model of the pulseline accelerator. They find the dispersion relation for a shielded helix with a dielectric material between the shield and the helix and compare it against the results from 3-D electromagnetic simulations. Expressions for the fields near the beam axis are obtained. A scheme to taper the properties of the helix to maintain synchronism with the accelerated ions is described. An approximate circuit model of the system that includes beam loading is derived.
Date: May 10, 2005
Creator: Caporaso, G J; Briggs, R J; Poole, B R & Nelson, S D
Partner: UNT Libraries Government Documents Department

High Resolution Angle-Resolved Photoemission Studies of the M-Point Surface on Cu(001)

Description: High resolution angle-resolved photoemission studies were carried out on the surface state reported by Heimann et al. at the {bar M} point of the two-dimensional Surface Brillouin Zone of Cu(001). The symmetry of the state is shown to be odd with respect to the (100) mirror plane, as hypothesized by Heimann et al. Experimental E({rvec k}{parallel}) dispersion relations in good agreement with the earlier study are reported, and accurate determinations of peak width as a function of k{parallel} are shown to be reasonably well-fitted by a simple model. A novel temperature effect is reported along with a possible explanation.
Date: December 1, 1979
Creator: Kevan, S. D. & Shirley, D. A.
Partner: UNT Libraries Government Documents Department

t-Channel unitarity construction of small-x kernels

Description: In the leading-log approximation, the small-x behavior of parton distributions in QCD is derived from the BFKL evolution equation. The authors describe the ion as a reggeon Bethe-Salpeter equation and discuss the use of reggeon diagrams to obtain 2-2 and 2-4 reggeon interactions at O(g{sup 4}). They then outline the dispersion theory basis of multiparticle j-plane analysis and describe how a gauge theory can be studied by combining Ward identity constraints with the group structure of reggeon interactions. Gluon reggeization, the O(g{sup 2}) BFKL kernel and O(g{sup 4}) corrections to it, are derived within this formalism. They give an explicit expression for the O(g{sup 4}) forward ``parton`` kernel in terms of logarithms and evaluate the eigenvalues. A separately infra-red finite component with a holomorphically factorizable spectrum is shown to be present and conjectured to be a new leading-order partial-wave amplitude. A comparison is made with Kirschner`s discussion of O(g{sup 4}) contributions from the multi-Regge effective action.
Date: 1994-12~
Creator: Coriano, C. & White, A. R.
Partner: UNT Libraries Government Documents Department