Showing papers on "Electromagnetic field published in 1998"

Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz)

Abstract: IN 1974, the International Radiation Protection Association (IRPA) formed a working group on non-ionizing radiation (NIR), which examined the problems arising in the field of protection against the various types of NIR. At the IRPA Congress in Paris in 1977, this working group became the International Non-Ionizing Radiation Committee (INIRC). In cooperation with the Environmental Health Division of the World Health Organization (WHO), the IRPA/INIRC developed a number of health criteria documents on NIR as part of WHO’s Environmental Health Criteria Programme, sponsored by the United Nations Environment Programme (UNEP). Each document includes an overview of the physical characteristics, measurement and instrumentation, sources, and applications of NIR, a thorough review of the literature on biological effects, and an evaluation of the health risks of exposure to NIR. These health criteria have provided the scientific database for the subsequent development of exposure limits and codes of practice relating to NIR. At the Eighth International Congress of the IRPA (Montreal, 18–22 May 1992), a new, independent scientific organization—the International Commission on Non-Ionizing Radiation Protection (ICNIRP)—was established as a successor to the IRPA/INIRC. The functions of the Commission are to investigate the hazards that may be associated with the different forms of NIR, develop international guidelines on NIR exposure limits, and deal with all aspects of NIR protection. Biological effects reported as resulting from exposure to static and extremely-low-frequency (ELF) electric and magnetic fields have been reviewed by UNEP/ WHO/IRPA (1984, 1987). Those publications and a number of others, including UNEP/WHO/IRPA (1993) and Allen et al. (1991), provided the scientific rationale for these guidelines. A glossary of terms appears in the Appendix.

Plane wave integral representation for fields in reverberation chambers

Abstract: A plane wave integral representation is presented for well-stirred fields in a reverberation chamber. The representation automatically satisfies Maxwell's equations in a source-free region and the statistical properties of the fields are introduced through the angular spectrum, which is taken to be a random variable. Starting with fairly simple and physically appropriate assumptions for the angular spectrum, a number of properties of the electric and magnetic fields and the power received by an antenna or a test object are derived. Many of these properties and test object responses are in agreement with other theories or with measured results. An important result for radiated immunity testing is that the ensemble (stirring) average of received power is equal to the average over plane wave incidence and polarization.

Representation of electromagnetic fields over arbitrary surfaces by a finite and nonredundant number of samples

Abstract: It is shown that the electromagnetic (EM) field, radiated or scattered by bounded sources, can be accurately represented over a substantially arbitrary surface by a finite number of samples even when the observation domain is unbounded. The number of required samples is nonredundant and essentially coincident with the number of degrees of freedom of the field. This result relies on the extraction of a proper phase factor from the field expression and on the use of appropriate coordinates to parameterize the domain. It is demonstrated that the number of degrees of freedom is independent of the observation domain and depends only on the source geometry. The case of spheroidal sources and observation domains with rotational symmetry is analyzed in detail and the particular cases of spherical and planar sources are explicitly considered. For these geometries, precise and fast sampling algorithms of central type are presented, which allow an efficient recovery of EM fields from a nonredundant finite number of samples. Such algorithms are stable with respect to random errors affecting the data.

Electromagnetic Analysis and Design in Magnetic Resonance Imaging

Abstract: This book presents a comprehensive treatment of electromagnetic analysis and design of three critical devices for an MRI system - the magnet, gradient coils, and radiofrequency (RF) coils. Electromagnetic Analysis and Design in Magnetic Resonance Imaging is unique in its detailed examination of the analysis and design of the hardware for an MRI system. It takes an engineering perspective to serve the many scientists and engineers in this rapidly expanding field.Chapters present:an introduction to MRIbasic concepts of electromagnetics, including Helmholtz and Maxwell coils, inductance calculation, and magnetic fields produced by special cylindrical and spherical surface currentsprinciples for the analysis and design of gradient coils, including discrete wires and the target field method analysis of RF coils based on the equivalent lumped-circuit model as well as an analysis based on the integral equation formulationsurvey of special purpose RF coilsanalytical and numerical methods for the analysis of electromagnetic fields in biological objectsWith the continued, active development of MRI instrumentation, Electromagnetic Analysis and Design in Magnetic Resonance Imaging presents an excellent, logically organized text - an indispensable resource for engineers, physicists, and graduate students working in the field of MRI.

Effects of weak magnetic fields on free radical recombination reactions

Abstract: The radical pair mechanism is used to elucidate how applied magnetic fields that are weaker in strength than typical hyperfine interactions can influence the yields and kinetics of recombination reactions of free radicals in solution. The so-called low field effect is shown to arise from coherent superpositions of degenerate electron-nuclear spin states in a spin-correlated radical pair in zero field. A weak applied magnetic field causes these (zero-quantum) coherences to oscillate, leading to coherent interconversion of singlet and triplet electronic states of the radical pair and hence changes in the yields of recombination products and of the free radicals that escape into solution. For singlet geminate radical pairs, the low field effect leads to a boost in the concentration of free radicals, which may be relevant in the context of in vivo biological effects of electromagnetic fields. Using analytical approaches in limiting cases, the maximum possible low field effects are calculated for a variety of ...

Three-dimensional quantization of the electromagnetic field in dispersive and absorbing inhomogeneous dielectrics

Abstract: A quantization scheme for the phenomenological Maxwell theory of the full electromagnetic field in an inhomogeneous three-dimensional, dispersive and absorbing dielectric medium is developed. The classical Maxwell equations with spatially varying and Kramers-Kronig consistent permittivity are regarded as operator-valued field equations, introducing additional current- and charge-density operator fields in order to take into account the noise associated with the dissipation in the medium. It is shown that the equal-time commutation relations between the fundamental electromagnetic fields $\hat E$ and $\hat B$ and the potentials $\hat A$ and $\hat \phi$ in the Coulomb gauge can be expressed in terms of the Green tensor of the classical problem. From the Green tensors for bulk material and an inhomogeneous medium consisting of two bulk dielectrics with a common planar interface it is explicitly proven that the well-known equal-time commutation relations of QED are preserved.

MAGY: a time-dependent code for simulation of slow and fast microwave sources

Abstract: We present the newly developed Maryland Gyrotron (MAGY) code for modeling of slow and fast microwave sources. The code includes a time-dependent description of the electromagnetic fields and a self-consistent analysis of the electrons. The calculations of the electromagnetic fields are based on the waveguide modal representation, which allows the solution of a relatively small number of coupled one-dimensional partial differential equations for the amplitudes of the modes, instead of the full solution of Maxwell's equations. Moreover, the basic time scale for updating the electromagnetic fields is the cavity fill time and not the high frequency of the fields. The equations of motion of the electrons are formulated within the framework of the guiding-center approximation and solved with the electromagnetic fields as the driving forces. Therefore, at each time step, a set of trajectories are calculated and used as current sources for the fields. We present two examples for the operation of the code, namely the two-cavity gyroklystron and the backward-wave oscillator (BWO). These examples demonstrate the possible usage of the code for a wide variety of electron-beam systems.

Electromagnetic field theory fundamentals

Abstract: Preface 1. Electromagnetic field theory 2. Vector analysis 3. Electrostatics 4. Steady electrical currents 5. Magnetostatics 6. Applications of static fields 7. Time-varying electromagnetic fields 8. Plane wave propagation 9. Transmission lines 10. Waveguides and cavity resonators 11. Antennas 12. Computer-aided analysis of electromagnetic fields Appendix A. Smith chart and its applications Appendix B. Computer programs for various problems Appendix C. Useful mathematical tables Index.

Ultimate intrinsic signal-to-noise ratio in MRI

Abstract: A method to calculate the ultimate intrinsic signal-to-noise ratio (SNR) in a magnetic resonance experiment for a point inside an arbitrarily shaped object is presented. The ultimate intrinsic SNR is determined by body noise. A solution is obtained by optimizing the electromagnetic field to minimize total power deposition while maintaining a constant right-hand circularly polarized component of the magnetic field at the point of interest. A numerical approximation for the optimal field is found by assuming a superposition of a large number of plane waves. This simulation allowed estimation of the ultimate intrinsic SNR attainable in a human torso model. The performance of six coil configurations was evaluated by comparing the SNR of images obtained by the coils with the ultimate values. In addition, the behavior of ultimate intrinsic SNR was investigated as a function of main field strength. It was found that the ultimate intrinsic SNR increases better than linearly with the main magnetic field up to 10 T for our model. It was observed that for field strengths of 4 T or higher, focusing is required to reach the ultimate intrinsic SNR.

Electrodynamics: A Modern Geometric Approach

Abstract: Special relativity Maxwell's equation(s) Lorentz-Force equations electromagnetic waves in 1-D electromagnetic waves in vacuum polarization waves in media waves at boundaries the field of a moving charge the Lorrentz-Dirac equation rotations and spherical harmonics radiation multipoles.

Induced current densities from low-frequency magnetic fields in a 2 mm resolution, anatomically realistic model of the body

Abstract: This paper presents calculations of current density in a fine-resolution (2 mm) anatomically realistic voxel model of the human body for uniform magnetic fields incident from the front, side and top of the body for frequencies from 50 Hz to 10 MHz. The voxel phantom, NORMAN, has a height of 1.76 m and a mass of 73 kg. There are 8.3 million voxels in the body differentiated into 37 tissue types. Both the impedance method and the scalar potential finite difference method were used to provide mutual corroboration. Results are presented for the current density averaged over 1 cm2 in muscle, heart, brain and retina.

Numerical simulation of SAR and B/sub 1/-field inhomogeneity of shielded RF coils loaded with the human head

Abstract: The finite-difference time-domain (FDTD) method is combined with the method of moments (MoM) to compute the electromagnetic fields of shielded radio-frequency (RF) coils loaded with an anatomically accurate model of a human head for high-frequency magnetic resonance imaging (MRI) applications. The combined method can predict both the specific energy absorption rate (SAR) and the magnetic field (known as the B/sub 1/ field) excited by any RF coils. Results for SAR and B/sub 1/ field distribution, excited by shielded and end-capped birdcage coils, are calculated at 64, 128, 171, and 256 MHz. The results show that the value of SAR increases when the frequency of the B/sub 1/ field increases and the B/sub 1/ field exhibits a strong inhomogeneity at high frequencies.

QED commutation relations for inhomogeneous Kramers-Kronig dielectrics

Abstract: Recently a quantization scheme for the phenomenological Maxwell theory of the full electromagnetic field in an inhomogeneous three-dimensional, dispersive, and absorbing dielectric medium has been developed and applied to a system consisting of two infinite half-spaces with a common planar interface (H.T. Dung, L. Kn\"oll, and D.-G. Welsch, Phys. Rev. A 57, 3931 (1998)). Here we show that the scheme, which is based on the classical Green-tensor integral representation of the electromagnetic field, applies to any inhomogeneous medium. For this purpose we prove that the fundamental equal-time commutation relations of QED are preserved for an arbitrarily space-dependent, Kramers-Kronig consistent permittivity. Further, an extension of the quantization scheme to linear media with bounded regions of amplification is given, and the problem of anisotropic media is briefly addressed.

The Effects of Metals and Interfering Fields on Electromagnetic Trackers

Abstract: The operation of six degree-of-freedom electromagnetic trackers is based on the spatial properties of the electromagnetic fields generated by three small coils. Anything in the environment that causes these fields to be distorted will result in measurement noise and/or errors. An experimental investigation was undertaken to measure the effect of external fields present in a typical working environment (namely mains and computer monitor fields) and the presence of metals (25-mm cubes of various types of metals, a large steel bar, and a large steel sheet). A theoretical model is proposed to explain the observations. Two devices were used in this investigation: a Polhemus Fastrak and an Ascension Flock of Birds.

Optical gradient forces of strongly localized fields

Abstract: We present a new approach for determining optical gradient forces applied by strongly focused laser beams on dielectric particles. We show that when the electromagnetic field is focused to a diffraction limited spot a dipole approximation is valid for any particle size. We derive intuitive predictions for force-displacement curves, maximal trapping forces, and force constants. The theory fits well with recent measurements of particles trapped by laser tweezers. We also discuss effects of radiation pressure and gravity. [S0031-9007(98)06883-5]

Charged-Particle Motion in Electromagnetic Fields Having at Least One Ignorable Spatial Coordinate

Abstract: We give a rigorous derivation of a theorem showing that charged particles in an arbitrary electromagnetic field with at least one ignorable spatial coordinate remain forever tied to a given magnetic field line. Such a situation contrasts with the significant motions normal to the magnetic field that are expected in most real three-dimensional systems. It is pointed out that while the significance of the theorem has not been widely appreciated, it has important consequences for a number of problems and is of particular relevance for the acceleration of cosmic rays by shocks.

Reader/writer having coil arrangements to restrain electromagnetic field intensity at a distance

Abstract: The present invention provides a reader and/or writer for performing both the supply of operating power to an IC card and transmission of a communication signal to the IC card or transmission and reception thereof to and from the IC card, wherein a main coil or a spiral antenna for generating an electromagnetic field for the two or at least power supply, and auxiliary coils or spiral antennas for generating magnetic fields opposite in phase to the electromagnetic field generated by the main coil or spiral antenna to thereby restrain the intensity of the electromagnetic field in the distance are provided side by side.

Element-free Galerkin method for electromagnetic field computations

Abstract: Although numerically very efficient the finite element method exhibits difficulties whenever the remeshing of the analysis domain must be performed. For such problems utilizing meshless computation methods is very promising. In this paper, a kind of meshless method called the element-free Galerkin method is introduced for electromagnetic field computation. The mathematical background for the moving least square approximation employed in the method is given, and the numerical implementation is briefly discussed. Application of the proposed method for electromagnetic field computation and verification of the obtained results using theoretically known solution is also presented.

Signal-to-noise ratio in MRI.

Abstract: in RF receiving coil design. However, modern is proportional to 0 , so that their population imaging techniques often demand very high speed difference is larger at high values of 0 . Since the and spatial resolution, so that the highest possible ratio of the energy difference to thermal energy is SNR is still required to avoid poor image quality. very small, the population difference increases in In this commentary, the physics underlying the direct proportion to 0 , as does the size of the variation of SNR with static field strength will be nuclear magnetic dipole moment. Secondly, the briefly reviewed, practical methods of optimizing Larmor frequency at which the dipole moment SNR will be outlined, including a discussion of the precesses is proportional to 0 so that the rate of principles of quadrature and phased-array RF change of flux linked to the RF coil also increases coils, and methods of measuring SNR for acceptin proportion to 0 . Therefore, by Faraday’s law ance testing and quality assurance will be of electromagnetic induction, the signal produced discussed. by a dipole magnetic moment of constant magni-

Electronic and electromagnetic properties of nanotubes

Abstract: A nanotube is phenomenologically modeled as a chain of atoms wrapped helically on a right circular cylinder. The semiclassical Hamiltonian of an electron is derived, using the Wannier approach for the Schr\"odinger equation, when the nanotube is exposed to both constant (dc) and high-frequency (ac) electromagnetic fields. The Boltzmann kinetic equation is then solved in the framework of momentum-independent relaxation time approximation. An analytical expression for electric current in a nanotube is derived. The interaction of nonlinearity and chirality is analyzed, chiefly as the dependence of a current chiral angle on the amplitude of the ac electric field. The derived expressions for the electronic transport also help in stating anisotropic impedance boundary conditions on the nanotube surface. Surface wave propagation in a carbon nanotube (CN) is examined. The idea of using CN's as nanowaveguides in the infrared frequency range is established. Convective instability is shown to occur under special conditions in a CN exposed to an axial dc electric field.

Spontaneous emission from a dielectric slab

Abstract: The electromagnetic field in a dielectric slab bounded by two dielectric half spaces with arbitrary refractive indices is quantized by computing the complete set of orthonormal electromagnetic modes. The zero-point fluctuations of the electromagnetic field are determined as a function of position. The dependence of the rate of spontaneous emission of thin dielectric films on the thicknesses of the films and the refractive index of the substrate is studied and compared with experimental results.

Complex image method for calculating electric and magnetic fields produced by an auroral electrojet of finite length

Abstract: . The electromagnetic field due to ionospheric currents has to be known when evaluating space weather effects at the earth's surface. Forecasting methods of these effects, which include geomagnetically induced currents in technological systems, are being developed. Such applications are time-critical, so the calculation techniques of the electromagnetic field have to be fast but still accurate. The contribution of secondary sources induced within the earth leads to complicated integral formulas for the field at the earth's surface with a time-consuming computation. An approximate method of calculation based on replacing the earth contribution by an image source having mathematically a complex location results in closed-form expressions and in a much faster computation. In this paper we extend the complex image method (CIM) to the case of a more realistic electrojet system consisting of a horizontal line current filament with vertical currents at its ends above a layered earth. To be able to utilize previous CIM results, we prove that the current system can be replaced by a purely horizontal current distribution which is equivalent regarding the total (=primary + induced) magnetic field and the total horizontal electric field at the earth's surface. The latter result is new. Numerical calculations demonstrate that CIM is very accurate and several magnitudes faster than the exact conventional approach. Key words. Electromagnetic theory · Geomagnetic induction · Auroral ionosphere

Lightning-induced overvoltages in power lines: validity of various approximations made in overvoltage calculations

Abstract: The validity of different approximations used in the calculation of induced overvoltages in power lines are investigated. These approximations are as follows: (1) neglect the distortions introduced by the finitely conducting ground on the electromagnetic (EM) fields; (2) the horizontal electric field at ground level is calculated by using the wavetilt approximation, which is valid for radiation fields and for grazing incidence; (3) the horizontal field at the line height is obtained by adding the horizontal field calculated at ground level to the horizontal field at the line height calculated over a perfectly conducting ground; (4) the transmission line equations derived by assuming that the ground is perfectly conducting are used with the horizontal field present over a finitely conducting ground as a source term in calculating the induced overvoltages; and (5) the propagation effects on the transients as they propagate along the line are either neglected or modeled by replacing the line impedance due to the ground by a constant resistance. The results presented show that in the calculation of induced overvoltages the approximation (3) is justified and approximation (2) is justified if the interest is to estimate the peak value of the induced overvoltage. Approximation (4) is probably justified for short lines and/or for highly conducting grounds. But it can introduce significant errors if the line is long and ground conductivity is low. Approximations (1) and (5) may lead to significant errors in the peak value, risetime, and derivative of the lightning-induced overvoltages.

Three-dimensional relativistic electron scattering in an ultrahigh-intensity laser focus

Abstract: The relativistic dynamics of an electron submitted to the three-dimensional field of a focused, ultrahigh-intensity laser pulse are studied numerically. The diffracting field in vacuum is modeled by the paraxial propagator and exactly satisfies the Lorentz gauge condition everywhere. In rectangular coordinates, the electromagnetic field is Fourier transformed into transverse and longitudinal wave packets, and diffraction is described through the different phase shifts accumulated by the various Fourier components, as constrained by the dispersion relation. In cylindrical geometry, the radial dependence of the focusing wave is described as a continuous spectrum of Bessel functions and can be obtained by using Hankel's integral theorem. To define the boundary conditions for this problem, the beam profile is matched to a Gaussian-Hermite distribution at focus, where the wave front is planar. Plane-wave dynamics are verified for large f numbers, including canonical momentum invariance, while high-energy scattering is predicted for smaller values of f at relativistic laser intensities.