Showing papers on "Magnetic field published in 1983"

Field and wave electromagnetics

Abstract: 1. The Electromagnetic Model. Introduction. The Electromagnetic Model. Si Units and Universal Constants. Review Questions. 2. Vector Analysis. Introduction. Vector Addition and Subtraction. Products of Vectors. Orthogonal Coordinate Systems. Integrals Containing Vector Functions. Gradient of a Scalar Field. Divergence of a Vector Field. Divergence Theorem. Curl of a Vector Field. Stoke's Theorem. Two Null Identities. Helmholtz's Theorem. Review Questions. Problems. 3. Static Electric Fields. Introduction. Fundamental Postulates of Electrostatics in Free Space. Coulomb's Law. Gauss's Law and Applications. Electric Potential. Conductors in Static Electric Field. Dielectrics in Static Electric Field. Electric Flux Density and Dielectric Constant. Boundary Conditions for Electrostatic Fields. Capacitances and Capacitors. Electrostatic Energy and Forces. Solution of Electrostatic Boundary-Value Problems. Review Questions. Problems. 4. Solution of Electrostatic Problems. Introduction. Poisson's and Laplaces' Equations. Uniqueness of Electrostatic Functions. Method of Images. Boundary-Value Problems in Cartesian Coordinates. Boundary-Value Problems in Cylindrical Coordinates. Boundary-Value Problems in Spherical Coordinates. Review Questions. Problems. 5. Steady Electric Currents. Introduction. Current Density and Ohm's Law. Electromotive Force and Kirchoff's Voltage Law. Equation of Continuity and Kirchoff's Current Law. Power Dissipation and Joule's Law. Boundary Conditions for Current Density. Resistance Calculations. Review Questions. Problems. 6. Static Magnetic Fields. Introduction. Fundamental Postulates of Magnetostatics in Free Space. Vector Magnetic Potential. The Biot-Savart Law and Applications. The Magnetic Dipole. Magnetization and Equivalent Current Densities. Magnetic Field Intensity and Relative Permeability. Magnetic Circuits. Behavior of Magnetic Materials. Boundary Conditions for Magnetostatic Fields. Inductances and Inductors. Magnetic Energy. Magnetic Forces and Torques. Review Questions. Problems. 7. Time-Varying Fields and Maxwell's Equations. Introduction. Faraday's Law of Electromagnetic Induction. Maxwell's Equations. Potential Functions. Electromagnetic Boundary Conditions. Wave Equations and their Solutions. Time-Harmonic Fields. Review Questions. Problems. 8. Plane Electromagnetic Waves. Introduction. Plane Waves in Lossless Media. Plane Waves in Lossy Media. Group Velocity. Flow of Electromagentic Power and the Poynting Vector. Normal Incidence of Plane Waves at a Plane Conducting Boundary. Oblique Incidence of Plane Waves at a Plane Conducting Boundary. Normal Incidence of Plane Waves at a Plane Dielectric Boundary. Normal Incidence of Plane Waves at Multiple Dielectric Interfaces. Oblique Incidence of Plane Waves at a Plane Dielectric Boundary. Review Questions. Problems. 9. Theory and Application of Transmission Lines Introduction. Transverse Electromagnetic Wave Along a Parallel-Plate. Transmission Line General Transmission-Line Equations. Wave Characteristics on Finite Transmission Lines. Transients on Transmission Lines. The Smith Chart. Transmission-Line Impedance Matching. Review Questions. Problems. 10. Waveguides and Cavity Resonators. Introduction. General Wave Behaviors Along Uniform Guiding Structures. Parallel-Plate Waveguide. Rectangular Waveguides. Circular Waveguides. Dielectric Waveguides. Cavity Resonators. Review Questions. Problems. 11. Antennas and Radiating Systems. Introduction. Radiation Fields of Elemental Dipoles. Antenna Patterns and Antenna Parameters. Thin Linear Antennas. Antenna Arrays. Receiving Antennas. Transmit-Receive Systems. Some Other Antenna Types. Review Questions. Problems. Appendix A: Symbols and Units. Appendix B: Some Useful Material Constants. Bibliography. Answers to Selected Problems. Index. Back Endpapers.

Anisotropy in MHD turbulence due to a mean magnetic field

Abstract: The development of anisotropy in an initially isotropic spectrum is studied numerically for two-dimensional magnetohydrodynamic turbulence. The anisotropy develops due to the combined effects of an externally imposed dc magnetic field and viscous and resistive dissipation at high wave numbers. The effect is most pronounced at high mechanical and magnetic Reynolds numbers. The anisotropy is greater at the higher wave numbers.

Particle simulation of plasmas

Abstract: For plasma with a large number of degrees of freedom, particle simulation using high-speed computers can offer insights and information that supplement those gained by traditional experimental and theoretical approaches. The technique follows the motion of a large assembly of charged particles in their self-consistent electric and magnetic fields. With proper diagnostics, these numerical experiments reveal such details as distribution functions, linear and nonlinear behavior, stochastic and transport phenomena, and approach to steady state. Such information can both guide and verify theoretical modeling of the physical processes underlying complex phenomena. It can also be used in the interpretation of experiments.

Magnetic field paralleliser for 2π electron-spectrometer and electron-image magnifier

Abstract: An electron-optical device has been constructed in which electrons originally emitted over 2 pi steradians from a region of small volume are formed into a beam of half-angle 2 degrees . The instrument makes use of a magnetic field that diverges from 1 to 10-3 Tesla. The energies of the electrons parallelised in this way have been measured with a time-of-flight technique, giving energy resolutions as low as 15 meV. Electrons of energy 0-3 eV, formed in multiphoton ionisation, were used for these tests. The device can also act as an electron-image magnifier, giving a spatial resolution of a few mu m in the source plane. Detailed theoretical and computational results on the properties of the new apparatus are given.

Cooling Neutral Atoms in a Magnetic Trap for Precision Spectroscopy

Abstract: A configuration of magnetic fields is exhibited which can harmonically trap paramagnetic particles in a shallow field minimum, superposed on a nearly uniform field which simplifies spectroscopic interactions with the particles. The potential energy of a trapped neutral particle depends on the particle's internal quantum number, as well as its position in the trap, permitting a combined rf-laser optical pumping cycle which can cool particles by a factor of 2 per cycle. Application of these ideas to the trapping of Na atoms is discussed.

Electrical control system

Abstract: The system of the present invention provides an electrical digital compass for use in a vehicle which includes a unique compensation and variation correction control which is readily set by the vehicle operator with the operation of push-button switches. The electrical display system and circuitry includes a microprocessor for comparing the information from a magnetic field sensor when the vehicle is aligned in a magnetically known direction and for providing a correction signal to assure the displayed output corresponds to the predetermined magnetic field alignment. In areas where magnetic variation is significant, the system generates, stores and processes a correction signal to assure the displayed output corresponds to the true heading of the vehicle. In one embodiment of the invention, a temperature sensor is provided to indicate the ambient temperature in which the vehicle is operating and which is digitally displayed to the operator.

A methodology for the determination of global electromechanical quantities from a finite element analysis and its application to the evaluation of magnetic forces, torques and stiffness

Abstract: The paper will present two general methods for the deduction of global information from the final result of the finite element computation of an electromagnetic device. The first one, called the local jacobian derivative, may be used for evaluation of the derivative of any integral quantity versus the parameter of motion of a rigid body. Typically, this method when applied to electromagnetic systems, can be used for the computation of magnetic force or torque by virtual-work principle. Compared with the popular Maxwell's tensor method, this procedure is easier to implement in a finite element package especially for 3D problems. The second method which is based on a stationary property of the field solution, allows the evaluation of a second order derivative of any integral quantity. For instance, computation of the stiffness of a magnetic system (derivative of a force or a torque) may be achieved as the second order derivative of the magnetic energy. It may be pointed out that this method requires the field computation once for a linear problem as well as for a non-linear one.

Centrifugally driven winds from contracting molecular disks

Abstract: We suggest that bipolar outflows in dense molecular clouds associated with young stellar objects are steady, centrifugally driven, hydromagnetic winds that arise from molecular disks (on scales < or =10/sup 16/ cm) in which the infrared source(s) embedded. A disk of mass approx.100 M/sub sun/ and rotational speed of approx.10/sup 6/ cm s/sup -1/ provides a resevoir of 10/sup 47/ ergs, which could power the most energetic outflows observed. Acceleration to supersonic speeds is accomplished by the magnetic field embedded in the disk (paralllel rotational and magnetic axes) and extending outward beyond the wind region to join the galactic field. The wind carries angular momentum and energy from the disk out to large distances. Our analysis treats the problem of magnetic braking and energy transport in a partially ionized (two-fluid) wind. The basic parameter which is shown to govern the flow is the coupling parameter ..beta.. = T/sub n/-i/T/sub n/, the ratio of the characteristic neutral-ion collision time to flow time. A general analysis of angular momentum and energy transport is given, and the wind equation is solved along flux tubes in the strongly coupled (..beta..<<1) limit. The centrifugally driven wind forms when an embedded protostar begins to ionize themore » disk core region. A disk envelope forms at the wind base at a pressure which adjusts to the wind requirements. Envelope heating may be maintained by magnetic flux loss from the dense core (which has a field approx.10/sup -3/ gauss) that is preferentially along the rotation axis for flattened disks. The structure of the field at the disk core surface is derived. The observational implications are discussed; in particular, we emphasize that such molecular disks with double profiles should be observed.« less

One- and two-dimensional crystallization of magnetic holes

Abstract: Holes are produced inside a thin layer of magnetic fluid with use of monodisperse polystyrene spheres with diameters in the micrometer range. With an external magnetic field an apparent magnetic dipole will be associated with each hole as a result of the displaced fluid. The apparent dipolar interactions between the spheres may be made attractive or repulsive. This Letter presents the first direct microscopic observations of the crystallization of magnetic holes forming a variety of different lattices.

Uniform magnetic field produced by three, four, and five square coils

Abstract: The optimal dimensions of thin coil systems of three and four square coils for producing uniform magnetic fields are calculated We find that for three square coils, of side d and separation s between the outer coils, the most uniform field distribution occurs with s/d=0821 116 and with I′/I=0512 797 I′/I is the ratio of the currents in the center coil to that of the outer coils With four square coils, the best uniformity is obtained when a/d=0128 106 and b/d=0505 492, where a is the distance from the center to the inner coils and b is the distance from the center to the outer coils The ratio of the current in the inner pair of coils to that in the outer pair must be I′/I=0423 514 We compare the uniformity of the field produced by these coil systems with each other and with Rubens’ five‐coil system, both on and off axis It is shown that the optimal four‐coil design is superior to the three‐ and five‐coil systems The sensitivity of the uniformity to the precision of construction is discussed Di

Hall current effect on tearing mode instability

Abstract: From a linear 2-D eigenmode analysis, it is found that the Hall current effect on collisional tearing mode instability becomes important for the thin magnetic reversal layer whose width is comparable to the ion inertia length; Hall currents produce a three-dimensional field structure and increase the reconnection (growth) rate. Since the magnetaic reversal layer widths both in the magnetopause and in the magnetotail are reported to become as thin as the ion inertial length (several hundred km) when the reconnection process is supposed to occur, the Hall current effect may explain the appearance of the dawn-dusk component of the magnetic field in the magnetotail reconnection region.

Magnetic neutral sheets in evolving fields. I - General theory. II - Formation of the solar corona

Abstract: The problem of the hydrostatic equilibrium of a large-scale magnetic field embedded in a fluid with infinite electrical conductivity is considered. It is pointed out that a necessary condition for static equilibrium is the invariance of the small-scale pattern in the field along the large-scale direction. A varying topological pattern implies that no fluid pressure distribution exists for which the field is everywhere static. Magnetic neutral sheets form, and dynamical reconnection of the field takes place. It is shown here that the invariance is also a sufficient condition for the existence of a fluid pressure distribution producing static equilibrium. Even in the simplest cases, however, the requirements on the fluid pressure are extreme and, a priori, are unlikely. It is concluded that almost all twisted flux tubes packed together produce dynamical nonequilibrium and dissipation of their twisting. This is the basic effect underlying the long-standing conjecture that the shuffling of the footpoints of the bipolar magnetic fields in the sun is responsible for heating the active corona. Attention is then given to the consequences of this general dynamical dissipation in the magnetic fields that produce the active corona of the sun. The footpoints of the field are continually manipulated by the subphotospheric convection in such a way that the lines of force are continually wrapped and rotated about one another.

Combined position sensor and magnetic motor or bearing

Abstract: A combined magnetic sensor and actuator device for applying a magnetic force to a magnetizable body and for sensing the distance between the device and the magnetizable body. The device includes a magnetizable pole piece separated from the magnetizable body by gaps. The pole piece, gaps, and magnetizable body form a magnetic circuit with the gaps preferably being the major reluctance of the circuit. Separate means are provided for generating a relatively large time-varying magnetic actuating flux in the magnetic circuit and for generating a relatively small time-varying magnetic sensing flux in the magnetic circuit. Each magnetic flux follows a flux path, such that the two flux paths have at least a portion in common. Detection means measure the relatively small magnetic flux in the magnetic circuit, and thereby measure the distance between the device and the magnetizable body.

Physical interpretation of weak localization: A time-of-flight experiment with conduction electrons

Abstract: The resistance of two-dimensional electron systems such as thin disordered films shows deviations from Boltzmann theory, which are caused by quantum corrections and are called "weak localization." Theoretically, weak localization is originated by the Langer-Neal graph in the Kubo formalism. In the present paper this graph is translated into a transparent physical picture. It represents an interference experiment with conduction electrons split into pairs of waves interfering in the backscattering direction. The intensity of the interference (integrated over the time) can be easily measured by the resistance of the film. A simple derivation for this quantum correction to the resistance is given. A magnetic field introduces a magnetic phase shift in the electronic wave function and suppresses the interference after a "flight" time proportional to $\frac{1}{H}$. Therefore, the application of a magnetic field allows observation of the fate of the electron as a function of time. Spin-orbit coupling rotates the spin of the electrons and yields an observable destructive interference, thereby demonstrating the change of sign of the electron-spin function by rotation. Magnetic impurities destroy the coherence of the phase. Therefore, with magnetoresistance measurements one can determine the inelastic lifetime, the spin-orbit coupling time, and the magnetic scattering time of the conduction electrons.

The Magnetic Field in the Prominences of the Polar Crown

Abstract: The Hanle effect method has been applied to the determination of the magnetic field in 120 prominences of the polar crown observed during the 1974–1980 period, which is the ascending phase of cycle XXI. The average field strength which was about 6 G at the beginning of the cycle reached twice this value just before the maximum. There is also a clear trend for a increase of the prominence field with the altitude. We confirm the fact that the magnetic vector makes a small angle (25 °) with the long axis of the prominence. As to the field orientation, we show that the most striking feature lies in the regular pattern of the component which is parallel to the axis of the filament; its direction seems to depend closely on the polarities of the high latitude photospheric field.

Localization, percolation, and the quantum Hall effect

Abstract: Noninteracting electrons in a smooth two-dimensional random potential are localized in the large magnetic field limit. In contrast to Anderson localization, eigenstates with large localization lengths occur with a probability proportional to a universal power of their size, with the power given in terms of percolation critical exponents. Adding a parallel electric field $\stackrel{\ensuremath{\rightarrow}}{\mathcal{E}}$ causes extended states to appear in numbers proportional to a power of $\mathcal{E}$. This implies a nonlinear broadening of steps in the quantized Hall conductivity. The results for a parallel electric field are obtained by considering a graded percolation problem, in which the probability that a site is occupied varies with position.

The equilibrium and stability of rotating plasmas

Abstract: In a rotating equilibrium state, the velocity and magnetic fields are shown to share the same flux surfaces. A simplified derivation is given of a second‐order (not necessarily elliptic) partial differential equation which determines axisymmetric equilibrium states. For general configurations, equations on flux surfaces which determine the Alfven and cusp continuous spectrum are derived and the stability investigated. These equations are written without the use of any particular coordinate system. Similar equations yield a sufficient condition for global stability of axisymmetric equilibria if the flow is parallel to the magnetic field up to a rigid rotation of the plasma. This condition is also necessary for stability in a mirror configuration with no toroidal field and a pure rigid rotation.

Transport effects associated with turbulence with particular attention to the influence of helicity

Abstract: The action of turbulence on a passive convected scalar field (e.g. temperature) or vector field (e.g. the magnetic field in an electrically conducting fluid) is reviewed, with particular attention paid to anomalous effects that can arise through the influence of Coriolis forces in a rotating system on the statistics of the turbulence. The simplest such effect (which corresponds to a breaking of the Onsager symmetry relations) is a 'skew-diffusion' effect, i.e. the appearance of a component of turbulent heat flux perpendicular to the local mean temperature gradient. The famous alpha effect of magnetohydrodynamic dynamo theory is also in this category, as is the more subtle Radler effect (the appearance of a mean electromotive force perpendicular to the mean current in a plasma). These effects are all associated with the helicity of a turbulent flow, i.e. the correlation between the velocity field u(x,t) and the vorticity field omega (x,t)=curl u.

Fractional Quantization of the Hall Effect

Abstract: Fractional quantization of the Hall effect and the two-dimensional structures in which it occurs are described The fractional quantization to date has been observed in high mobility modulation-doped AlxGa1−xAs/GaAs heterostructures at high magnetic fields and low temperatures where all electrons lie in the lowest Landau level At certain fractional Landau level occupations v = p/q (q = 3, 5, 7; p = 1, 2, 3) minima and Hall resistance plateaus develop The value of the Hall resistance, ρ xy, approaches h/ve2 at the plateaus The observed phenomena and their temperature dependence suggest the occurrence of a series of correlated electron states at fractional occupation of the lowest Landau level Both two-dimensional electron and hole systems exhibit fractional quantization

Method and apparatus for offshore electromagnetic sounding utilizing wavelength effects to determine optimum source and detector positions

Abstract: An improved method and apparatus for electromagnetic surveying of a subterranean earth formation beneath a body of water. An electric dipole current source is towed from a survey vessel in a body of water substantially parallel to the surface of the body of water and separated from the floor of the body of water by a distance less than approximately one-quarter of the distance between the surface and the floor. Alternating electric current, preferably including a plurality of sinusoidal components, is caused to flow in the source. An array of electric dipole detectors is towed from the survey vessel substantially collinearly with the current source. Each electric dipole detector of the array is separated from the current source by a distance substantially equal to an integral number of wavelengths of electromagnetic radiation, of frequency equal to that of a sinusoidal component of the source current, propagating in the water. A gradient detector array is also towed by the survey vessel in a position laterally separated from, or beneath, the mid-point of the current source. Additionally, an array of three-axis magnetic field sensors mounted in controllable instrument pods are towed by the seismic vessel on the flanks of the current source. Frequency-domain and time-domain measurements of magnetic and electric field data are obtained and analyzed to permit detection of hydrocarbons or other mineral deposits, or regions altered by their presence, within subfloor geologic formations covered by the body of water.

The "TRIFOU" Code: Solving the 3-D eddy-currents problem by using H as state variable

Abstract: In [1] and [2], a variational formulation of the eddy-currents problem using the magnetic field h as state variable was introduced. We describe here two applications of the method: computation of the impedance of a probe in non-destructive testing and prediction of the characteristics of a new design of an iron-free machine. The basic idea is first exposed, leaving aside difficulties like multiple connectedness. These are treated more thoroughly in Part 3, after the description of applications in Part 2.

Influence of nuclear spin on chemical reactions: Magnetic isotope and magnetic field effects (A Review)

Abstract: The course of chemical reactions involving radical pairs may depend on occurrence and orientation of nuclear spins in the pairs. The influence of nuclear spins is maximized when the radical pairs are confined to a space that serves as a cage that allows a certain degree of independent diffusional and rotational motion of the partners of the pair but that also encourages reencounters of the partners within a period which allows the nuclear spins to operate on the odd electron spins of the pair. Under the proper conditions, the nuclear spins can induce intersystem crossing between triplet and singlet states of radical pairs. It is shown that this dependence of intersystem crossing on nuclear spin leads to a magnetic isotope effect on the chemistry of radical pairs which provides a means of separating isotopes on the basis of nuclear spins rather than nuclear masses and also leads to a magnetic field effect on the chemistry of radical pairs which provides a means of influencing the course of polymerization by the application of weak magnetic fields.

Transport and isomorphic equilibria

Abstract: It is shown that large classes of plasma equilibria can have identical drift orbits and associated transport. Such equilibria are called isomorphic. In particular, the neoclassical coefficients are given for all equilibria in which the magnetic field strength depends on one helicity.

Plasma transport in stochastic magnetic fields. Part 3. Kinetics of test particle diffusion

Abstract: A discussion is given of test particle transport in the presence of specified stochastic magnetic fields, with particular emphasis on the collisional limit. Certain paradoxes and inconsistencies in the literature regarding the form of the scaling laws are resolved by carefully distinguishing a number of physically distinct correlation lengths, and thus identifying several collisional subregimes. The common procedure of averaging the conventional fluid equations over the statistics of a random field is shown to fail in some important cases because of breakdown of the Chapman-Enskog ordering in the presence of a stochastic field component with short autocorrelation length. A modified perturbation theory is introduced which leads to a Kubo-like formula valid in all collisional regimes. The direct-interaction approximation is shown to fail in the interesting limit in which the orbit exponentiation length LK appears explicitly. A higher-order renormalized kinetic theory in which LK appears naturally is discussed and used to rederive more systematically the results of the heuristic scaling arguments.

A theory of long period magnetic pulsations, 3. Local field line oscillations

Abstract: The local magnetic field is shown to oscillate at its Alfven resonance frequency (ies) in response to a wide band source whose frequency range covers the resonance frequency (ies). The proposed mechanism explains certain observations of magnetic pulsations where the frequency is found to vary continuously as a function of latitude for a given event.

Coded surveillance system having magnetomechanical marker

Abstract: A magnetic article surveillance system marker is adapted, when armed, to resonate at preselected frequencies provided by an incident magnetic field applied within an interrogation zone. The marker is a plurality of elongated ductile strips of magnetostrictive ferromagnetic material each of the strips being disposed adjacent to a ferromagnetic element which, upon being magnetized, magnetically biases the strips and arms them to resonate at the preselected frequencies. A substantial change in effective magnetic permeability of the marker at the preselected frequencies provides the marker with signal identity.