Showing papers on "Field (physics) published in 1982"

A Novel Method to Analyze Electromagnetic Scattering of Complex Objects

Abstract: The finite-difference time-domain (FD-TD) method is proposed as a means of accurately computing electromagnetic scattering by arbitrary-shaped extremely complex metal or dielectric objects excited by an external plane wave. In the proposed method, one first uses the FD-TD method to compute the near total fields within a rectangular volume which fully encloses the object. Then, an electromagnetic-field equivalence principle is invoked at a virtual surface of this rectangular volume to transform the tangential near scattered fields to the far field. To verify the feasibility of this method, the surface currents, near scattered fields, far scattered fields, and radar cross section of two canonical two-dimensional objects are presented. For these cases, it is shown that the FD-TD method provides magnitude of current and field predictions which are within ± 2.5 percent and further phase values within ± 30 of values predicted by the method of moments ( MOM) at virtually every point including in shadow regions.

Sound absorption based on ocean measurements. Part II: Boric acid contribution and equation for total absorption

Abstract: The absorption of sound in seawater is considered as the sum of three contributions: those from pure water, magnesium sulfate, and boric acid. Contributions from other reactions are small and are not included. The pure water and magnesium sulfate contributions obtained from analyses of extensive oceanic measurements, including many in the Arctic, were discussed in Part I. In Part II, an analysis is made of all reported measurements in the low‐frequency region (0.2–10 kHz) to evaluate the contribution of boric acid. This is done by subtracting the pure water and magnesium sulfate contributions determined in Part I from the total absorption to give a more accurate estimate of the boric acid contribution than previously obtained. The three contributions are then combined to form an equation with both a theoretical basis and a satisfactory empirical fit that will be useful to researchers and engineers in the field of underwater sound. The equation applies to all oceanic conditions and frequencies from 200 Hz to 1 MHz.

The electron-phonon interaction in quasi-two-dimensional semiconductor quantum-well structures

Abstract: Approximate analytic expressions are obtained for the scattering rates and momentum-relaxation rates of an electron in a quasi-two-dimensional quantum well interacting with acoustic, optical and intervalley modes via the deformation potential and with longitudinal optical modes via the polar interaction. These analytic expressions are obtained using a momentum-conservation approximation. The threshold for optical phonon emission, unlike the case in the bulk, is abrupt. All scattering rates are energy-independent and are inversely proportional to L, the thickness of the well. The momentum-relaxation rate associated with the absorption of polar optical phonons, on the other hand, proves to be proportional to L. These properties are shown to lead to a negative differential resistance for pure polar mode scattering, and to the existence of a runaway field for deformation-potential scattering. The self-energy associated with the emission of polar optical phonons at absolute zero is shown to be divergent unless the polar interaction is screened, and some consequences of this for laser and other optical processes are pointed out. The description of scattering by perturbation theory breaks down in very narrow wells.

Why, how, and when, MHD turbulence becomes two-dimensional

Abstract: A description of MHD turbulence at low magnetic Reynolds number and large interaction parameter is proposed, in which attention is focussed on the role of insulating walls perpendicular to a uniform applied magnetic field. The flow is divided in two regions: the thin Hartmann layers near the walls, and the bulk of the flow. In the latter region, a kind of electromagnetic diffusion along the magnetic field lines (a degenerate form of Alfv6n waves) is displayed, which elongates the turbulent eddies in the field direction, but is not sufficient to generate a two-dimensional dynamics. However the normal derivative of velocity must be zero (to leading order) at the boundaries of the bulk region (as at a free surface), so that when the length scale 1, perpendicular to the magnetic field is large enough, the corresponding eddies are necessarily two-dimensional. Furthermore, if I, is not larger than a second limit, the Hartmann braking effect is negligible and the dynamics of these eddies is described by the ordinary Navier-Stokes equations without electromagnetic forces. MHD then appears to offer a means of achieving experiments on two-dimensional turbulence, and of deducing velocity and vorticity from measurements of electric field.

Removable singularities in Yang-Mills fields

Abstract: We show that a field satisfying the Yang-Mills equations in dimension 4 with a point singularity is gauge equivalent to a smooth field if the functional is finite. We obtain the result that every Yang-Mills field overR 4 with bounded functional (L 2 norm) may be obtained from a field onS 4=R 4∪{∞}. Hodge (or Coulomb) gauges are constructed for general small fields in arbitrary dimensions including 4.

A Monte Carlo analysis of an electron swarm in a nonuniform field: the cathode region of a glow discharge in helium

Abstract: A Monte Carlo method for studying the steady-state behaviour of charged species under the influence of a non-uniform electric field is described. This method, using a null-collision technique based on determination of the increase in kinetic energy between two collisions, avoids calculation of the time variations of the phase space coordinates of the charged species. The electron behaviour in the cathode region of a glow discharge has been analysed using this method; spatial variations of the energy and angular electron distribution functions, and of the macroscopic electron parameters are discussed.

A geomagnetic field spectrum

Abstract: A spherical harmonic model of the earth's internal magnetic field of degree and order 23 is derived from selected Magsat data, and its power spectrum, computed with terms developed by Mauersberger (1956) and Lowes (1974), is found to exhibit a change of a slope at n = 14 which is interpreted as an indication that the core field dominates at values lower than 13 while the crust field dominates above a value of 15. The representations of the two portions of the spectrum obtained can be used to establish order-of-magnitude inaccuracies due to both crustal fields and the inability to observe core field wavelengths beyond n = 13, at which point they are obscured by the crustal field, in core field models.

Effect of an electric field on the luminescence of GaAs quantum wells

Abstract: Low-temperature photoluminescence (PL) measurements have been performed in narrow GaAs-${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ quantum wells subject to an electric field perpendicular to the well plane. At low fields the PL spectra show two peaks associated, respectively, with exciton and free-electron-to-impurity recombination. With increasing field the PL intensity decreases, with the excitonic structure decreasing at a much faster rate, and becomes completely quenched at a field of a few tens of kV/cm. This is accompanied by a shift in the peak position to lower energies. The results are interpreted as caused by the field-induced separation of carriers and modification of the quantum energies. Variational calculations performed for isolated, finite quantum wells explain qualitatively the experimental observations.

Nuclear magnetic resonance whole-body imager operating at 3. 5 Kgauss

Abstract: The theoretical advantages of nuclear magnetic resonance imaging at higher field strengths are discussed. Examples of images created at 3.5 KGauss (0.35 T) are demonstrated. The authors present a method of collecting several tomographic images sequentially during the time required for a single image.

Rotational viscosity γ1 of nematic liquid crystals

Abstract: The rotational viscosity coefficients γ1 are measured with the rotating field method as a function of temperature in the nematic phases of 4‐methoxybenzylidene‐4′‐n‐butylaniline, 4‐n‐pentyl‐, and 4‐n‐octyl‐4′‐cyanobiphenyl, of the eutectic mixture of two 4‐n‐pentylphenyl 4‐alkoxybenzoates and of the eutectic mixture of the two isomers of 4‐methoxy‐4′‐n‐butylazoxybenzene. Equations are proposed which describe the temperature dependence very well. The Leslie coefficients αi are calculated from the γ1 values and recently measured shear viscosity coefficients.

Control of laminar-instability waves using a new technique

Abstract: A new technique using surface-film activators has been developed to induce and control laminar-instability waves by periodic heating. A flat plate was instrumented and installed in the GALCIT High-speed Water Tunnel with flush-mounted surface heaters and probes. Extremely two-dimensional naturally occurring Tolmien-Schlichting (TS) waves were observed along with the subsequent formation of turbulent spots. Laminar-instability waves were then excited in a controlled fashion using the surface-mounted heaters. A preliminary experiment on cancellation of excited laminar-instability waves was carried out. Finally, turbulent spots were produced using amplitude-modulated bursts to form Gaussian TS wave packets. Flow visualization, along with wall shear measurements, was used to infer the velocity and vorticity field near the wall.

Model for modulation doped field effect transistor

Abstract: A model describing C-V and I-V characteristics of modulation doped FET's is proposed. The model takes into account the change in the Fermi energy with the gate voltage. At high two dimensional electron concentrations, the equations of the model for the charge control by the gate voltage become similar to the equations of the charge control model where the thickness d of AlGaAs layer should be substituted by d + Δ d and Δ d is the effective width of the potential well (≃ 80 A). Another important prediction of the model is the existence of the "subthreshold" current. A very good quantitative agreement is obtained with our experimental I-V curves using the measured values of the low field mobility and the source resistance.

Agglomerate formation in a magnetic fluid

Abstract: Monte Carlo techniques have been used to investigate the effects of magnetostatic and repulsive particle interactions on the formation of agglomerates in a magnetic fluid The dependence on particle size and applied field of the form of the agglomerates was studied using a spatial distribution function which allows a quantitative distinction to be made between clusters and anisotropic chain structures Magnetization curves have been calculated for magnetic particle sizes varying from 5 to 15 nm with and without magnetostatic interactions For the larger particle sizes, it was found that the initial susceptibility is reduced in the presence of interactions This is associated with the presence of pronounced agglomeration in zero field, where open clusters are formed As the applied field is increased the clusters break up to form long chains aligned in the field direction At intermediate particle sizes, there is evidence of magnetic field induced agglomeration leading to the formation of dimers and trimers preferentially aligned in the field direction The smallest particle size showed little evidence of ordering even in strong applied fields, since thermal disordering dominates the situation

Electric field gradients and charge density in corundum, α-Al2O3

Abstract: The charge density in α-Al2O3 has been refined with respect to X-ray structure factors [complete spheres for both Mo Kα and Ag Kα radiations with (sin θ/λ)max = 1.19 and 1.495 A-1 respectively] and electric-field gradient tensors at both atomic sites, using Hirshfeld- type deformation functions. Atomic charges from a κ refinement are + 1.32 (5) for Al and -0.88 (8) e for O. The charge distribution of O is polarized towards the four Al neighbours, and metal-metal bonds are clearly absent. Quadrupole coupling constants and asymmetry parameters of the field-gradient tensors cannot be determined from the structure factors. They define the quadrupolar deformations near the atomic centers, and the X-ray data define the charge density between the atoms. The orientational parameters of the tensors and the signs of their largest eigenvalues can be qualitatively retrieved from the X-ray data. The refinement of anisotropic secondary-extinction parameters may, however, destroy this information. Refinement with respect to the field gradients affects mainly the quadrupolar deformation terms, and has little influence on the X-ray scale factor (i.e. monopolar terms) and computed electrostatic fields (i.e. dipolar terms). Properties of the charge density with different angular symmetries are thus only weakly correlated.

Green's Function Methods for Potential Magnetic Fields

Abstract: The Green's function method to calculate potential magnetic field on the Sun, which was first established by Schmidt (1964) in the case that the field component normal to a flat boundary plane is specified, is extended to the following three cases: (a) The field component along the line of sight, which is not generally normal to the flat boundary plane, is specified; (b) the line of sight component on a spherical boundary surface is specified; (c) the normal component on a spherical surface is specified, together with the condition that the field becomes approximately radial on an outer spherical surface (the so-called source surface). Properties of these Green's functions are examined, and the applicability of these methods to solar magnetic data is discussed.

Alpha-particle-induced field and enhanced collection of carriers

Abstract: A simple physical model explains all the characteristics of the newly discovered funnelling phenomenon. An alpha strike results in significant field in the quasi-neutral regions to a depth that is equal to 1 + µ _{n} /µ _{p} times the depletion region width of an n+/p junction. This and the predicted current waveform agree with experiments and simulation results. The model also predicts the effects of the angle of alpha incidence, and that p+/n junctions should exhibit weaker funnelling phenomenon.

Antiferromagnetic long-range order in the anisotropic quantum spin chain

Abstract: This is a study of the ground-state properties of the one-dimensional spin-s (12⩽s<∞) anisotropic XYZ antiferromagnet in a magnetic field of arbitrary direction. It provides the first rigorous results for the general case of this model in non-zero field. By exact calculations we find the existence of an ellipsoidal surface h = hN in field space where the ground state is of the classical two-sublattice Neel type with non-zero antiferromagnetic long-range order. At hN there are no correlated quantum fluctuations. We give upper and lower bounds for the critical field hc where antiferromagnetic long-range order is suppressed by the field. The zero-temperature phase diagrams are discussed for a few representative cases.

A field test of the transfer function model for predicting solute transport

Abstract: A model proposed for predicting average values of solute concentration as a function of depth and time, based on a transfer function model, is tested on a field experiment over 1.44 ha. The experiment consisted of an initial calibration observing the transport of a concentrated solute pulse past 15 replicated solution samplers located at a depth Z = 30 cm. The model was subsequently used without further calibration to predict the shape of the pulse at 60, 90, 120, 180 cm, with good agreement between average predicted and measured values. A further test of the model was conducted by taking 36 soil cores between 180 and 360 cm at 170 days after the addition of the solute pulse to the soil surface. Good agreement was found between the predicted and observed depth of maximum concentration expressed as a relative probability. Appendix Tables A1-A5 and Bl are available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82-001; $1.00. Payment must accompany order.

Field statistics in some generalized Jaynes-Cummings models

Abstract: Photon statistics in some fully quantized models of the interaction of a two-level atom with a single-mode radiation field have been studied using the operator equations of motion. Expressions for the photon number distribution and the mean photon number are presented for various initial conditions. It is found that the mean photon number may show decays and revivals of coherence similar to those of the atomic inversion in the coherent-state Jaynes-Cummings model. Application of these models to the study of multiphoton laser, absorption, and emission processes is also discussed.

Zonal harmonic model of Saturn's magnetic field from Voyager 1 and 2 observations

Abstract: An analysis of the magnetic field of Saturn is presented which takes into account both the Voyager 1 and 2 vector magnetic field observations. The analysis is based on the traditional spherical harmonic expansion of a scale potential to derive the magnetic field within 8 Saturn radii. A third-order zonal harmonic model fitted to Voyager 1 and 2 observations is found to be capable of predicting the magnetic field characteristics at one encounter based on those observed at another, unlike models including dipole and quadrupole terms only. The third-order model is noted to lead to significantly enhanced polar surface field intensities with respect to dipole models, and probably represents the axisymmetric part of a complex dynamo field.

Asymptotic formulae of quantum electrodynamics

Abstract: This chapter describes asymptotic formulae of quantum electrodynamics. The asymptotic constancy of the total cross-section is a characteristic property of scattering processes whose diagrams can be cut across internal photon lines. It occurs even when more than two particles are present in the final state of the reaction. The double-logarithmic corrections occur in cases of two kinds. One kind includes scattering through a fixed finite angle. The cross-sections decrease in the asymptotic high-energy range. In such cases, the double-logarithmic corrections are associated with the infrared divergence. These cases include elastic scattering of electrons in an external Coulomb field. The other class of cases includes reaction cross-sections, which decrease with increasing energy for a given square of the momentum transfer, that is, for scattering angles, which asymptotically approach zero or π.

Vector potential and magnetic field of current-carrying finite arc segment in analytical form, Part III: Exact computation for rectangular cross section

Abstract: Analytical expressions for the components of the vector potential and magnetic field of a circular arc segment of a current-carrying conductor of rectangular cross section and arbitrary azimuthal length are derived. Components of the field gradient are also calculated. All the expressions developed consist of known functions such as Jacobian elliptic functions, complete and incomplete elliptic integrals of the first, second, and third kind, and thus permit a new timesaving efficient computation algorithm.

Conception of an air-gap element for the dynamic analysis of the electromagnetic field in electric machines

Abstract: In the design of electric machine working under unbalanced conditions or supplied by non sinusoidal current, a detailed knowledge of the magnetic field distribution is required to predict the machine performance. Under such conditions this field distribution must be calculated using a dynamic model for the machine air-gap: A particular element constituted by the uniform part of the air-gap (u.p.a.g.) allows such a dynamic model in the present paper. An analytical solution of the Laplace equation in the air-gap is done. The field distribution in a saturated machine can be obtained within this air-gap model, combined with the classical finite element solution. The results of field calculation obtained using such a method are more precise than those calculated when modelling the air-gap with classical elements. Examples are given in the paper, showing this fact for both the cases of static and dynamic calculations.