Showing papers on "Electromagnetic field published in 1980"

Transient Response of Multiconductor Transmission Lines Excited by a Nonuniform Electromagnetic Field

Abstract: The time-domain transmission-line equations for uniform multiconductor transmission lines in a conductive, homogeneous medium excited by a transient, nonuniform electromagnetic (EM) field, are derived from Maxwell's equations. Depending on how the line voltage is defined, two formulations are possible. One of these formulations is considerably more convenient to apply than the other. The assumptions made in the derivation of the transmission-line equations and the boundary conditions at the terminations are discussed. For numerical calculations, the transmission -line equations are represented by finite-difference techniques, and numerical examples are included.

Application of the Finite-Difference Time-Domain Method to Sinusoidal Steady-State Electromagnetic-Penetration Problems

Abstract: A numerical method for predicting the sinusoidal steady-state electromagnetic fields penetrating an arbitrary dielectric or conducting body is described here. The method employs the finite-difference time-domain (FD-TD) solution of Maxwell's curl equations implemented on a cubic-unit-cell space lattice. Small air-dielectric loss factors are introduced to improve the lattice truncation conditions and to accelerate convergence of cavity interior fields to the sinusoidal steady state. This method is evaluated with comparison to classical theory, method-of-moment frequency-domain numerical theory, and experimental results via application to a dielectric sphere and acylindrical metal cavity with an aperture. Results are also given for a missile-like cavity with two different types of apertures illuminated by an axial-incidence plane wave.

Kinetic equations for low frequency instabilities in inhomogeneous plasmas

Abstract: Kinetic equations for low frequency, short perpendicular wavelength, electromagnetic perturbations in an inhomogeneous, magnetically confined plasma are developed. The analysis makes use of the recently developed high toroidal mode number expansion to reduce the lowest‐order system of equations to a set of ordinary (along the field line) integro‐differential equations. Included in these equations are the effects of finite Larmor radius, magnetic shear, trapped particles, and nonuniform magnetic curvature drifts. Perturbed fields are represented by a scalar potential and two components of the vector potential. Thus, the effects of the compressional component of the perturbed magnetic field are retained and the equations are valid for arbitrary values of the plasma pressure. The extension of the high toroidal mode number expansion to nonaxisymmetric configurations is discussed.

Geometrical Theory of Diffraction for Electromagnetic Waves

Abstract: The continuous development of the Geometrical Theory of Diffraction (GTD), from its conception in the 1950s, has now established it as a leading analytical technique in the prediction of high-frequency electromagnetic radiation and scattering phenomena. Consequently, there is an increasing demand for research workers and students in electromagnetic waves to be familiar with this technique. In this book they will find a thorough and clear exposition of the GTD formulation for vector fields. It begins by describing the foundations of the theory in canonical problems and then proceeds to develop the method to treat a variety of circumstances. Where applicable, the relationship between GTD and other high-frequency methods, such as aperture field and the physical optics approximation, is stressed throughout the text. The purpose of the book, apart from expounding the GTD method, is to present useful formulations that can be readily applied to solve practical engineering problems. To this end, the final chapter supplies some fully worked examples to demonstrate the practical application of the GTD techniques developed in the earlier chapters.

Transient response of multiconductor transmission lines excited by a nonuniform electromagnetic field

Abstract: The time-domain transmission-line equations for uniform multiconductor transmission lines in a conductive, homogeneous medium excited by a transient, nonuniform electromagnetic (EM) field, are derived from Maxwell's equations. Depending on how the line voltage is defined, two formulations are possible. One of these formulations is considerably more convenient to apply than the other. The assumptions made in the derivation of the transmission-line equations and the boundary conditions at the terminations are discussed. For numerical calculations, the transmission -line equations are represented by finite-difference techniques, and numerical examples are included.

Frequency and power windowing in tissue interactions with weak electromagnetic fields

Abstract: Effects of nonionizing electromagnetic (EM) fields that raise tissue temperature in general differ very little from effects of hyperthermia induced by other means. However, fields raising tissue temperature orders of magnitude less than 0.1°C may result in major physiological changes not attributable to raised temperature per se. These weak fields have been observed to produce chemical, physiological, and behavioral changes only within windows in frequency and incident energy. For brain tissue, a maximum frequency sensitivity occurs between 6 and 20 Hz. Two different intensity windows have been seen, one for ELF tissue gradients around 10-7V/cm, and one for amplitude modulated RF and microwave gradients around 10-1V/cm. The former is at the level associated with navigation and prey detection in marine vertebrates and with control of human biological rhythms; the latter is at the level of the electroencephalogram (EEG) in brain tissue. Coupling to living cells appears to require amplifying mechanisms that may be based on nonequilibrium processes, with long-range resonant molecular interactions. These cooperative processes are now recognized as important in immune and hormonal responses, as well as in nerve cell excitation. Polyanionic proteinaceous material forming a sheet on cell membrane surfaces appears to be the site of detection of these weak molecular and neuroelectric stimuli.

Quantum field theory at finite temperature in a rotating system

Abstract: The finite-temperature Green's-function formalism is extended to the case of relativistic rotating systems. Free scalar, spinor, and electromagnetic Green's functions for a rotating system are obtained. As an application of the formalism, the neutrino parity-violating current is calculated. The result agrees with previous calculations where different methods have been used.

Three-dimensional nonlinear electromagnetic field computations, using scalar potentials

Abstract: A formulation based on scalar potentials for the numerical solution of three-dimensional nonlinear static electromagnetic field problems is presented. The resulting equations are solved using imite elements, based on a Galerkin procedure. A general-purpose package called TOSCA has been developed, implemented and tested. Results are presented for three cases and compared with measured values.

Scattering and guiding of waves by dielectric gratings with arbitrary profiles

Abstract: Based on an exact solution of the pertinent boundary-value problem, a method is presented for finding the electromagnetic fields scattered or guided by lossy dielectric gratings having arbitrary profiles. This method unifies the treatment of both perpendicular (TE) and parallel (TM) polarizations by expressing the fields in terms of two coupled first-order differential equations. Their solution is obtained by resorting to difference equations in conjunction with the algorithm of Adams–Moulton, which easily leads to accurate results for a large variety of practical problems. To illustrate the application of this approach, quantitative results are presented for the scattering of plane waves by lossy corrugated structures and for the guiding of (leaky) surface waves by triangular gratings with symmetric or asymmetric profiles.

Finite-Difference Analysis of EMP Coupling to Lossy Dielectric Structures

Abstract: This article describes the 3-D EMP finite-difference time-domain computer code THREDE as generalized to calculate coupling to, and scattering from, lossy dielectric objects. The code primarily treats the scattered component of the electromagnetic fields (thus presuming linearity) and employs a radiating outer boundary. As sample scatterers, we use dielectric spheres of ? = 2?0 and 9?0 illuninated by an EMP plane wave of double exponential profile. Comparitive calculations were made using the inverse-Fourier transform of the Rayleigh-Mie spherical-hannonic expansion solution-agreement of the two solutions is very good.

Collective spontaneous emission (Dicke superradiance)

Abstract: The present state of theoretical and experimental research on collective spontaneous emission (superradiance) in a system of radiators (atoms, molecules, or nuclei) is reviewed. The distinction between superradiance and the amplification of spontaneous emission is discussed. There is also a discussion of conditions under which the effect can be observed and of various theoretical methods for describing superradiance: the quantum single-mode and multimode models and the semiclassical approach. Theoretical papers on superradiance in systems with dimensions smaller than the wavelength of the radiation and also in extended systems are reviewed. It is shown that superradiance may occur in weakly amplifying media. A situation in which the superradiance is oscillatory is described. The possible use of superradiance to generate coherent emission in the x-ray and ? ranges is discussed. The superradiance accompanying Raman scattering of light in atomic and molecular media is studied. The theoretical results are compared with experimental observations of superradiance in the optical range. In an appendix, the nature of the phase transition in a system of radiators interacting through an electromagnetic field is discussed.

Linear interferometric waveguide modulator for electromagnetic-field detection.

Abstract: A channel-waveguide interferometer with asymmetric arms, suitable for electromagnetic-field detection, is described. A π/2 intrinsic phase differential makes the output linearly proportional to the applied voltage (electromagnetic field). Horizontal and vertical electrode sets provide polarization-independent operation, with electrode lengths chosen to make the voltages approximately equal. In Ti-diffused devices in Z–X-cut LiNbO3, the TE- and TM-mode outputs are equally modulated with the horizontal voltage 43% of the vertical. Output linearity over a 40-dB range and operation up to 300 MHz are demonstrated.

SPASYN-an electromagnetic relative position and orientation tracking system

Abstract: Two relatively remote independent body coordinate frames are related in both position and orientation (six degrees of freedom) using precise electromagnetic field measurements. Antenna triads are fixed in each body frame. Variously polarized excitations in one body are correlated with signals detected in the remote body. Near-field and far-field processing strategies are presented with applications.

Inversion of Surface and Borehole Electromagnetic Data for Two-Dimensional Electrical Conductivity MODELS*

Abstract: A method for inverting electromagnetic fields induced by a line source in an earth of two-dimensional conductivity structure is developed. Certain unique features of the finite element method are used to construct an efficient algorithm for the accurate calculation of the Jacobian matrix of partial derivatives, and the resulting linearized equations are solved using the damped least squares method. Case studies of theoretical data generated from a simple model of interest in geophysical prospecting show that, in general, it is impossible to obtain, from surface data alone, accurate estimates of the conductivity of structures buried deeper than 0.2 skin depths under a conducting overburden. The addition of borehole data to the surface data is found to increase the resolving power of the electromagnetic method dramatically. In particular, the borehole data appear to stabilize the inverse when only a poor initial estimate of the likely structure is given.

Gauge-invariant coupled gravitational, acoustical, and electromagnetic modes on most general spherical space-times

Abstract: The coupled Einstein-Maxwell system linearized away from an arbitrarily given spherically symmetric background space-time is reduced from its four-dimensional to a two-dimensional form expressed solely in terms of gauge-invariant geometrical perturbation objects. These objects, which besides the gravitational and electromagnetic, also include mass-energy degrees of freedom, are defined on the two-manifold spanned by the radial and time coordinates. For charged or uncharged arbitrary matter background the odd-parity perturbation equations for example, reduce to three second-order linear scalar equations driven by matter and charge inhomogeneities. These three equations describe the intercoupled gravitational, electromagnetic, and acoustic perturbational degrees of freedom. For a charged black hole in an asymptotically de Sitter space-time the gravitational and electromagnetic equations decouple into two inhomogeneous scalar wave equations.

Stationary electromagnetic fields around black holes. III. General solutions and the fields of current loops near the Reissner-Nordström black hole

Abstract: The stationary multipole solutions of the Einstein-Maxwell equations representing coupled electromagnetic and gravitational perturbations of the extreme Reissner-Nordstroem black hole are simple rational functions of the radial coordinate. These solutions are used to construct the axially symmetric field of a current loop in the equatorial plane and of a small current loop on the polar axis. The magnetic lines of force are calculated numerically and exhibited graphically. The field representing a general Reissner-Nordstroem black hole in an asymptotically uniform magnetic field is also given, and its relation to the exact solutions of Ernst describing Kerr-Newman black holes in Melvin's magnetic universe is discussed.

On the Response of a Terminated Twisted-Wire Cable Excited by a Plane-Wave Electromagnetic Field

Abstract: The currents induced in the terminations of a twisted-pair cable are obtained for a plane-wave electromagnetic-field. Various termination configurations are considered and comparisons are made with the corresponding results for untwisted pairs. In special cases, the twisted cable may have a larger induced signal than the untwisted. Under various common restrictions, the induced signal is inversely proportional to either 1) the square of the number of turns or 2) the number of turns.

Surface Brillouin scattering from acoustic phonons. I. General theory

Abstract: The theory of Brillouin scattering from the surface of a semi-infinite medium is presented. The electromagnetic field is calculated to first order in the phonon displacements using the Rayleigh method. Both s and p polarisations are considered and the scattering geometry is not limited to the plane of incidence. The cross-section formulae can be interpreted in terms of the ripple and elasto-optic mechanisms. The effects on the displacement field due to the surface are investigated in both the discrete and the continuous part of the frequency spectrum. In particular, a detailed analysis of the so-called 'mixed modes' belonging to the continuum is analytically given for the isotropic medium. The application of the theory to GaAs and the comparison with experimental data (including A1 coated surfaces) will be discussed in paper II.

Irradiation of Prolate Spheroidal Models of Humans in the Near Field of a Short Electric Dipole

Abstract: Analysis of the near-field irradiation of prolate spheroidal models of humans and animals by a short electrical dipole is described. The method of solution involves an integral equation formulation of the problem in terms of the transverse dyadic Green's function and expanding the fields irradiated by a short dipole in terms of the vector spherical harmonics. The extended boundary condition method (EBCM) is employed to solve the integral equations. The power distribution and the average specific absorption rate (SAR) are calculated and plotted as a function of the separation distance. It is shown that for a dipole placed along the major axis of the spheroidal (k-polarization), and for a very short separation distance, d= 0.15 lambda, the relative power values at both ends of the spheroid are about 40 compared with the ratio of 15 in the planewave exposure case. Furthermore, the calculated average SAR values as a function of the separation distance were found to oscillate around the constant value obtained from the planewave irradiation case. Differences between the near-and far-field exposure cases occurred only at separation distances shorter than 0.5 lambda where the magnitudes of the electric and magnetic energy densities are higher than the time-average radiation power density.

Type I, II, and III spatially homogeneous cosmologies with electromagnetic field

Abstract: The purpose of this article is to investigate cosmological models satisfying the general relativistic field equations for electromagnetic field and perfect fluid, with particular reference to the nature of the 4-current. The investigation is concerned with tilting models as well as those whose 4-velocity is of the comoving form, and it is shown how the ''canonical tetrad,'' i.e., the tetrad with respect to which the energy-momentum tensor takes its canonical form, can be used to solve the problem of the possible forms of the 4-velocity. After a discussion of the canonical tetrad components of the general 4-current, including both convection and conduction currents, the Maxwell equations in spatially homogeneous cosmological models of types I, II and III are soved and the resulting 4-current in each case is compared with the general expression found earlier for the 4-current. In most cases it is found that the 4-current is either spacelike or zero and the conductivity is finite. The results are illustrated by finding all solutions of the combined Einstein-Maxwell perfect fluid field equations in which the metric functions are powers of t. All of these solutions necessarily have spacelike or zero 4-current and finite conductivity.

Theory of a distributed feedback laser

Abstract: This paper treats distributed feedback laser operation in which a classical electromagnetic field obeying Maxwell's equations interacts with an active medium according to the laws of quantum mechanics. The theory describes arbitrarily intense two-mode operation of fundamental and higher modes in both index and gain grating configurations. Spacial hole burning and finite atomic linewidth are included in the analysis and are shown to give marked changes in some laser intensity profiles.

Linear theory of an electron cyclotron maser operating at the fundamental

Abstract: The linear theory of an electron cyclotron maser (ECM) operating at the fundamental is developed. A set of analytic expressions, valid for all TE cavity modes, is derived for the starting current and frequency detuning using the Vlasov-Maxwell equations in the weakly relativistic limit. These results are applicable for an arbitrary electron velocity distribution as well as any longitudinal distribution of the RF field. It is shown that the starting current can be expressed in a simple form which contains the Fourier trans-form of the longitudinal field distribution. Analytic results are presented for specific longitudinal field variations, including uniform, sinusoidal, and Gaussian. It is found that the starting characteristics of an ECM are strongly influenced by the axial dependence of the RF field, but weakly affected by the velocity spread of the electron beam. The problem of multimode oscillation is treated in the linear theory by using a Slater expansion of the cavity field. The complete formulation for mode competition based on this expansion is presented and preliminary results are derived. This comprehensive analysis of ECM linear theory should be useful as a diagnostic of ECM performance and should facilitate comparison between theory and experiment.

Variable field coupled cavity resonator circuit

Abstract: In a resonant chain of coupled cavities such as used in a standing-wave linear particle accelerator it is often desirable to change the field strength in some cavities relative to some others. For example, if the output particle energy of an accelerator is changed by varying the fields of all cavities, the distribution of energies of output particles is disturbed. This distribution is largely controlled by the fields in the first group of cavities traversed by the particle beam. According to the invention, the fields can remain constant in the first group and be varied in following cavities. This is done by varying the distribution of electromagnetic field in one cavity asymmetrically with respect to the preceding and the following cavity. The asymmetric coupling produces different acceleration fields in one part of acceleration structure relative to another part. In an accelerator whose accelerating cavities are coupled via non-interacting side cavities, the different coupling may be produced by making the standing-wave field in one side-cavity asymmetric with respect to its coupling irises.