Showing papers on "Magnetic flux published in 1995"

The flux-line lattice in superconductors

Abstract: Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.

The Flux-Line Lattice in Superconductors

Abstract: Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.

The Earth's magnetosphere is 165 RE long: Self-consistent currents, convection, magnetospheric structure, and processes for northward interplanetary magnetic field

Abstract: The subject of this paper is a self-consistent, magnetohydrodynamic numerical realization for the Earth's magnetosphere which is in a quasi-steady dynamic equilibrium for a due northward interplanetary magnetic field (IMF). Although a few hours of steady northward IMF are required for this asymptotic state to be set up, it should still be of considerable theoretical interest because it constitutes a 'ground state' for the solar wind-magnetosphere interaction. Moreover, particular features of this ground state magnetosphere should be observable even under less extreme solar wind conditions. Certain characteristics of this magnetosphere, namely, NBZ Birkeland currents, four-cell ionospheric convection, a relatively weak cross-polar potential, and a prominent flow boundary layer, are widely expected. Other characteristics, such as no open tail lobes, no Earth-connected magnetic flux beyond 155 R(sub E) downstream, magnetic merging in a closed topology at the cusps, and a 'tadpole' shaped magnetospheric boundary, might not be expected. In this paper, we will present the evidence for this unusual but interesting magnetospheric equilibrium. We will also discuss our present understanding of this singular state.

Speed control of interior permanent magnet synchronous motor drive for flux weakening operation

Abstract: A novel flux-weakening scheme for an interior permanent magnet synchronous motor (IPMSM) is proposed. It is implemented based on the output of the synchronous PI current regulator-reference voltage to PWM inverter. The onset of flux weakening and the level of the flux are adjusted inherently by the outer voltage regulation loop to prevent the saturation of the current regulator. Attractive features of this flux weakening scheme include no dependency on the machine parameters, the guarantee of current regulation at any operating condition, and smooth and fast transition into and out of the flux weakening mode. Experimental results at various operating conditions including the case of detuned parameters are presented to verify the feasibility of the proposed control scheme.

Ulysses observations of the radial magnetic field

Abstract: The radial component of the magnetic field in the southern hemisphere has been measured at Ulysses as it traveled from the equator to −80.2° latitude and returned. The radial component multiplied by the square of the radial distance, i.e., BRr², averaged over 77 day intervals (three solar rotations) is approximately constant at −3.5 nT and shows no evidence of a dependence on heliographic latitude. To discriminate against possible time variations, the measurements have been compared with simultaneous observations in the ecliptic by IMP-8. The two sets of observations agree very well confirming the absence of a significant latitude gradient. Since the sun's dipolar magnetic field component is strong at this phase of the sunspot cycle, it is inferred that magnetic flux from the polar cap is transported to lower latitudes in the solar wind source region to produce a uniform radial field. Such a configuration would be expected if magnetic stresses are influencing the solar wind flow near the sun and are contributing to a non-radial deflection. Estimates of the flux of open field lines (3 × 1014 webers), of the non-radial solar wind expansion (∼3) and the polar cap magnetic field strength (∼5 Gauss) are derived.

Flux-based active filter controller

Abstract: This paper presents a synchronous frame flux-based control method for a parallel active filter application. The flux-based controller directly implements the inverter switchings in the synchronous reference frame by a hysteresis rule-based carrier-less pulse-width modulation (PWM) strategy to achieve high current bandwidth. This paper addresses the issues and impact on parallel active filtering requirements for utility interface of commonly used harmonic front-ends. The synchronous frame flux-based controller provides additional insights for harmonic current compensation requirements. Simulation results provide the validation of the flux-based active filter controller to meet IEEE Standard 519 recommended harmonic standards for large rated nonlinear loads under balanced and unbalanced supply conditions.

A unified approach to main flux saturation modelling in D-Q axis models of induction machines

Abstract: Main flux saturation is most frequently modelled by selecting either stator and rotor d-q axis currents or stator and rotor d-q axis flux linkages as state-space variables. This paper attempts to unify main flux saturation modelling in d-q axis models of induction machines by presenting a general method of saturation modelling. Selection of state-space variables in the saturated machine model is arbitrary and appropriate models in terms of different state-space variables result by application of the method. A couple of models, obtainable with different selection of state-space variables, are presented. The cross-saturation effect is explicitly present in all the models, except for the one with stator and rotor flux linkage d-q axis components as state-space variables. The models are verified by simulation and experimental investigation of induction generator self-excitation. >

Magnetic flux rope versus the spheromak as models for interplanetary magnetic clouds

Abstract: Magnetic clouds form a subset of interplanetary ejecta with well-defined magnetic and thermodynamic properties. Observationally, it is well established that magnetic clouds expand as they propagate antisunward. The aim of this paper is to compare and contrast two models which have been proposed for the global magnetic field line topology of magnetic clouds: a magnetic flux tube geometry, on the one hand, and a spheromak geometry (including possible higher multiples), on the other. Traditionally, the magnetic structure of magnetic clouds has been modeled by force-free configurations. In a first step, we therefore analyze the ability of static force-free models to account for the asymmetries observed in the magnetic field profiles of magnetic clouds. For a cylindrical flux tube the magnetic field remains symmetric about closest approach to the magnetic axis on all spacecraft orbits intersecting it, whereas in a spheromak geometry one can have asymmetries in the magnetic field signatures along some spacecraft trajectories. The duration of typical magnetic cloud encounters at 1 AU (1 to 2 days) is comparable to their travel time from the Sun to 1 AU and thus magnetic clouds should be treated as strongly nonstationary objects. In a second step, therefore, we abandon the static approach and model magnetic clouds as self-similarly evolving MHD configurations. In our theory, the interaction of the expanding magnetic cloud with the ambient plasma is taken into account by a drag force proportional to the density and the velocity of expansion. Solving rigorously the full set of MHD equations, we demonstrate that the asymmetry in the magnetic signature may arise solely as a result of expansion. Using asymptotic solutions of the MHD equations, we least squares fit both theoretical models to interplanetary data. We find that while the central part of the magnetic cloud is adequately described by both models, the 'edges' of the cloud data are modeled better by the magnetic flux tube. Further comparisons of the two models necessarily involve thermodynamic properties, since real magnetic configurations are never exactly force-free and gas pressure plays an essential role. We consider a polytropic gas. Our theoretical analysis shows that the self-similar expansion of a magnetic flux tube requires the polytropic index gamma to be less than unity. For the spheromak, however, self-similar, radially expanding solutions are known only for gamma equal to 4/3. This difference, therefore, yields a good way of distinguishing between the two geometries. It has been shown recently that the polytropic relationship is applicable to magnetic clouds and that the corresponding polytropic index is approximately 0.5. This observational result is consistent with the self-similar model of the magnetic flux rope but is in conflict with the self-similar spheromak model.

Flux distribution of solar intranetwork magnetic fields

Abstract: Big Bear deep magnetograms of June 4, 1992 provide unprecedented observations for direct measurements of solar intranetwork (IN) magnetic fields. More than 2500 individual IN elements and 500 network elements are identified and their magnetic flux measured in a quiet region of 300 × 235 arc sec. The analysis reveals the following results: (1) IN element flux ranges from 1016 Mx (detection limit) to 2 × 1018 Mx, with a peak flux distribution of 6 × 1016 Mx. (2) More than 20% of the total flux in this quiet region is in the form of IN elements at any given time. (3) Most IN elements appear as a cluster of mixed polarities from an emergence center (or centers) somewhere within the network interior. (4) The IN flux is smaller than the network flux by more than an order of magnitude. It has a uniform spatial distribution with equal amount of both polarities.

Stochastic resonance in a superconducting loop with a josephson junction

Abstract: A simple superconducting loop with a Josephson junction subject to a time‐sinusoidal magnetic flux embedded in a noise background is considered. Cooperative effects, arising from the interplay between the noise and modulation are described; they manifest themselves in the response, measured as an output signal‐to‐noise ratio. In particular, it is shown that the response displays the stochastic resonance effect, wherein the output signal‐to‐noise ratio passes through a maximum at a critical value of the noise strength. A simple theory, based on the characterization of the superconducting quantum interference device as a bistable switching element, is seen to yield good qualitative agreement with the experimental results.

Square and rectangular thin superconductors in a transverse magnetic field.

Abstract: The equation of motion for the sheet current in square and rectangular films with linear or nonlinear resistivity in a time dependent transverse magnetic field is presented. As examples, the linear susceptibility of rectangles with complex resistivity, the ideal diamagnetic moment, and the linear and nonlinear penetration of magnetic flux into high- ${T}_{c}$ superconductors are given. The recently observed cushionlike flux penetration and the discontinuity lines in the current flow are confirmed.

Paramagnetic moment in field-cooled superconducting plates: paramagnetic meissner effect

Abstract: We calculate the magnetic moment for thin superconducting plates after field cooling. We consider the situation when the whole magnetic flux is trapped within the strip and compressed due to inhomogeneous cooling. We demonstrate that the magnetic moment in such a state is paramagnetic. For a thin superconducting strip we found exact solutions for the field and current distributions at any degree of flux compression. The cases of complete and partial Bean states were considered. For a thin superconducting disk we found an approximate solution for the case of weak compression and a complete Bean state. The mechanism considered can explain the paramagnetic Meissner effect observed recently in some high-${\mathit{T}}_{\mathit{c}}$ superconductors and in Nb.

Radiation Pressure–driven Magnetic Disk Winds in Broad Absorption Line Quasi-stellar Objects

Abstract: We explore a model in which QSO broad absorption lines (BALS) are formed in a radiation pressure-driven wind emerging from a magnetized accretion disk. The magnetic field threading the disk material is dragged by the flow and is compressed by the radiation pressure until it is dynamically important and strong enough to contribute to the confinement of the BAL clouds. We construct a simple self-similar model for such radiatively driven magnetized disk winds, in order to explore their properties. It is found that solutions exist for which the entire magnetized flow is confined to a thin wedge over the surface of the disk. For reasonable values of the mass-loss rate, a typical magnetic field strength such that the magnetic pressure is comparable to the inferred gas pressure in BAL clouds, and a moderate amount of internal soft X-ray absorption, we find that the opening angle of the flow is approximately 0.1 rad, in good agreement with the observed covering factor of the broad absorption line region.

Anomalous diffusion and Lévy random walk of magnetic field lines in three dimensional turbulence

Abstract: The transport of magnetic field lines is studied numerically where three dimensional (3‐D) magnetic fluctuations, with a power law spectrum, and periodic over the simulation box are superimposed on an average uniform magnetic field The weak and the strong turbulence regime, δB∼B0, are investigated In the weak turbulence case, magnetic flux tubes are separated from each other by percolating layers in which field lines undergo a chaotic motion In this regime the field lines may exhibit Levy, rather than Gaussian, random walk, changing from Levy flights to trapped motion The anomalous diffusion laws 〈Δx2i〉∝sα with α≳1 and α<1, are obtained for a number of cases, and the non‐Gaussian character of the field line random walk is pointed out by computing the kurtosis Increasing the fluctuation level, and, therefore stochasticity, normal diffusion (α≂1) is recovered and the kurtoses reach their Gaussian value However, the numerical results show that neither the quasi‐linear theory nor the two dimensional per

A new design concept of permanent magnet machine for flux weakening operation

Abstract: In this paper, a new design concept of a PM machine for flux weakening operation is proposed. The feasibility of strong flux weakening capability without permanently demagnetizing the permanent magnet is investigated. Results from the finite element analysis on the proposed PM machine structure are presented to verify the new design concept. >

AC motors for high-performance drives: a design-based comparison

Abstract: Three AC motors are compared for application in high-performance controlled drives: induction, brushless, and synchronous reluctance motors. Their design is optimized under common design assumptions, Then, the given stall-torques are compared to each other. In addition, the quadrature reactances are compared, as affecting overload and flux-weakening performances. Finally, applicative considerations are given, thus defining the most suitable solution for each field.

Design optimization of electrical machines using genetic algorithms

Abstract: The application of genetic algorithms (GAs) to the design optimization of electromagnetic devices is presented in detail. The method is demonstrated on a magnetizer by optimizing its pole face to obtain the desired magnetic flux density distribution. The shape of the pole face is constructed from the control points by means of uniform nonrational b-splines. >

Ulysses observation of a noncoronal mass ejection flux rope: Evidence of interplanetary magnetic reconnection

Abstract: A well-defined, small-scale (≈0.05 AU) magnetic flux rope was observed by Ulysses at about 5 AU in close proximity to a heat flux dropout (HFD) at the heliospheric current sheet (HCS). This magnetic flux rope is characterized by a rotation of the field in the plane approximately perpendicular to the ecliptic (and containing the Sun and the spacecraft) and with a magnetic field maximum centered near the inflection point of the bipolar signature. The edges of the flux rope are well defined by diamagnetic field minima. A bidirectional electron heat flux signature is coincident with the magnetic flux rope structure. The event occurred during a time of slightly increasing solar wind speed, suggesting that the field and plasma were locally compressed. Unlike most coronal mass ejections/magnetic clouds, this event is characterized by high proton temperatures and densities, high plasma beta, no significant alpha particle abundance increase, and a small radial size. We interpret these observations in terms of multiple magnetic reconnection of previously open field lines in interplanetary space at the HCS. Such reconnection produces a U-shaped structure entirely disconnected from the Sun (which we associate with the HFD), a closed magnetic flux rope (which we associate with the counterstreaming electron event), and a closed magnetic loop or tongue connected back to the Sun at both ends. These observations suggest that magnetic reconnection, and its changes to magnetic field topology, can occur well beyond the solar corona in interplanetary space.

A flux-weakening strategy for current-regulated surface-mounted permanent-magnet machine drives

Abstract: Permanent-magnet synchronous machines fed from current-regulated converters feature nearly ideal performance at low-to-moderate speeds. However, as rotor speed increases the back emf rises which results in loss of current regulation and decreased torque. In buried-magnet machine drives, flux weakening is often used to extend the speed range. This paper sets forth a flux-weakening control specifically designed for surface-mounted permanent-magnet machines which is simple and does not require knowledge of the machine or system parameters. The proposed method is demonstrated both experimentally and through the use of computer simulation. >

Flux dam, a method to reduce extra low frequency noise when a superconducting magnetometer is exposed to a magnetic field

Abstract: This letter discusses the effects of a moderate magnetic field applied to a SQUID magnetometer and introduces the invention of the ‘‘flux dam’’ A flux dam is a weak link or Josephson junction in series with the pickup loop which allows magnetic flux to enter and leave the inside of the superconducting loop easily for moderate changes in magnetic field This largely prevents the flux lines from entering and being pinned in the superconducting parts of the magnetometer which would create extra low frequency noise SQUID magnetometers with flux dams in the pickup loop can be operated in the Earth’s magnetic field without a significant increase in low frequency flux noise

Phase equilibration in bubble collisions.

Abstract: In the context of an Abelian gauge symmetry, spontaneously broken at a first-order transition, we discuss the evolution of the phase difference between the Higgs fields in colliding bubbles. We show that the effect of dissipation, represented by a finite plasma conductivity, is to cause the phases to equilibrate on a time scale, determined by the conductivity, which can be much smaller than the bubble radii at the time of collision. Currents induced during the phase equilibration generate a magnetic flux, which is determined by the initial phase difference. In a three-bubble collision, the fluxes produced by each pair of bubbles combine, and a vortex can be formed. We find that, under most conditions, the probability of trapping magnetic flux to form a vortex is correctly given by the ``geodesic rule.''

Multisample magnetic separation device

Abstract: An apparatus for separating magnetically responsive particles from a non magnetic test medium in which the magnetically responsive particles are suspended is disclosed herein. The separator includes an array (22) of containers (24), which hold the nonmagnetic test medium, on or more pins (28) disposed substantially within the test medium, and an external magnet (34) for producing a magnetic field within the test medium. The external magnet is proximate the pins so as to create magnetic flux lines, magnetizing the pins and creating a magnetic field substantially parallel to the longitudinal axis of the pins thereof in the test medium.

A simple representation of dynamic hysteresis losses in power transformers

Abstract: The paper describes a procedure for the representation of hysteresis in the laminations of power transformers in the simulation of electromagnetic transient phenomena. The model is based on the recognition that in today's iron cores the hysteresis loops are narrow and therefore the modeling details are only important in relation to the incurred losses and the associated attenuation effects. The resultant model produces losses proportional to the square of the flux density, as expected from measurement data. It is formulated as a simple, linear relationship between the variation B-B/sub re/spl nu, of the magnetic flux density B after a reversal point B/sub re/spl nu and the resulting additional field intensity H/sub hyst/. This idea can be easily implemented in existing transformer models with or without frequency dependent modeling of eddy currents in the laminations. It has been found that in simulation tests the representation of hysteresis is not necessary those situations have been described where the modeling of hysteresis appears to be more meaningful. >

Current and field pattern in rectangular and inhomogeneous superconductors

Abstract: The penetration and exit of magnetic flux in type-II superconductors is investigated for the realistic situation where a transverse magnetic field is applied to a square or rectangular plate or film. In rectangular specimens the pattern of the sheet current and of the density of the perpendicular flux has some common features with the one-dimensional distributions in circular disks or long strips. Other features, however, are characteristic for the rectangular shape, e.g., the starlike pattern of the penetrating flux and, in the fully penetrated critical state, the discontinuity lines at which the current stream lines perform sharp bends and at which the perpendicular magnetic field ${\mathit{H}}_{\mathit{z}}$(x,y) exhibits sharp ridges. These typical features have to be calculated from a genuine two-dimensional theory. Such a theory based on a highly nonlinear current-voltage law is outlined. The field patterns obtained by this general theory are compared with patterns observed magneto-optically at the surface of square and rectangular single crystals or films of high-${\mathit{T}}_{\mathit{c}}$ superconductors with homogeneous and inhomogeneous critical-current distribution. It is shown that the analysis of the current-discontinuity lines is essential to understand the flux dynamics in superconductors. In samples with inhomogeneous critical current density ${\mathit{j}}_{\mathit{c}}$(r), a strong concentration of flux motion and electric field can occur along the lines where ${\mathit{j}}_{\mathit{c}}$ changes abruptly. This may trigger flux jumps.

A systematic approach to selecting flux references for torque maximization in induction motors

Abstract: This paper addresses the problem of optimal selection of the flux reference in induction machines. The results are based on a standard differential equation model of the induction machine and give the value of the flux reference which maximizes the torque at constant speed subject to voltage and current limits. Both the motor and generator modes are considered. The formulas are useful for the real-time programming of digital controllers designed to achieve maximum performance of AC drives at high speeds. Experimental results are given to validate the analysis and demonstrate the usefulness of the optimal flux reference in high-performance servo applications.

A Well Data Telemetry System

Abstract: A well data telemetry system for transmitting data along at least a part of a well bore 2 containing tubing of magnetically permeable material comprises a transmitting coil 9 located to transmit a magnetic flux signal along the modulation circuitry connected to the coil 9 for modulating tubing and/or the surrounding geological formations, the magnetic flux signal in response to a data signal, and an ac magnetometer 10 located to detect and respond to the magnetic flux signal at a position spaced from the transmitting coil 3 By using an ac magnetometer instead of a conventional receiver coil to detect and respond to the magnetic flux signal, the system is capable of responding to the low levels of signal which are likely to be received in practice Furthermore, the response of the ac magnetometer is independent of frequency so that the system may be used at low frequencies as well as high frequencies and demodulation of the transmitted signal can be effected without difficulty

Principles of quasistatic magnetic shielding with cylindrical and spherical shields

Abstract: The basic principles that underlie materials-based quasistatic magnetic shielding are described. Shielding mechanisms are identified and shown in specific examples that involve long cylindrical and spherical shield geometries. Analytic results are given both for shields that enclose the shielded region and for shields that enclose the source. The two configurations are reciprocal in the sense that identical shields have identical shielding factors for a uniform externally applied field and for a dipole source within the shield. Flux shunting and induced current shielding mechanisms are each described quantitatively and qualitatively in shielding situations where only one mechanism is involved. Situations where the mechanisms are simultaneously involved are then analyzed, and the relevances of the descriptions that involve only one mechanism are identified as regimes within the general descriptions.

Model-based measurement and control of electromagnetic actuator movements

Abstract: The linear electromagnetic actuator consists of an armature (1), stator (2) and excitation coil (3). The voltage (Uspule) applied to the coil produces a magnetic flux (phi) through the stator, the air gap and the armature. This causes a force (F magnet) to act on the armature, moving it in the direction of the stationary stator and closing the air gap. The load or the return spring (4) returns the armature after the voltage has been switched off. The sensor (5) measures the magnetic flux (phi) or the flux density (B) and, with the cross-section of the armature (A), the magnetic force on the magnet can be calculated and also the other parameters mentioned.

Comparisons between theory and observation of active region tilts

Abstract: Active regions in the Sun are generally tilted relative to the azimuthal direction, with the leading side being closer to the equator than the following side. This tilts is known to increase with latitude. Recently, theoretical calculations of the dynamics of emerging, initially toroidal active-region flux tubes have been done, showing that the observed tilts can be explained by the Coriolis force acting on a diverging flow field in emerging flux loops. The calculations of Fan, Fisher, & McClymont predict that alpha proportional to Phi(exp 1/4) B(sub 0 exp -5/4) sin theta, where alpha is the tilt angle of the active region, B(sub 0) is the magnetic field strength of the active-region flux tube near the base of the convection zone, and phi is the amount of magnetic flux in the tube. We compare these theoretical predictions with the behavior of a sample of 24,701 sunspot groups observed at Mount Wilson over a period of 68 yr, using the polarity separation distance d as a proxy for phi. Our major findings are given.