Showing papers on "Magnetic flux published in 1999"

Hall effect sensor of displacement of magnetic core

Abstract: A sensor for measuring linear displacement has a core of magnetic material disposed for movement within a bobbin about which a coil of insulated, electrically conductive wire is wound. At least one Hall effect device is disposed about the coil. The sensor is contained with a magnetic housing. When the coil is excited by a DC power source, the direction of magnetic flux flowing through the Hall effect device is dependent upon the position of the core.

Numerical modeling of the geodynamo: Mechanisms of field generation and equilibration

Abstract: Numerical calculations of fluid dynamos powered by thermal convection in a rotating, electrically conducting spherical shell are analyzed. We find two regimes of nonreversing, strong field dynamos at Ekman number 10 -4 and Rayleigh numbers up to 11 times critical. In the strongly columnar regime, convection occurs only in the fluid exterior to the inner core tangent cylinder, in the form of narrow columnar vortices elongated parallel to the spin axis. Columnar convection contains large amounts of negative helicity in the northern hemisphere and positive helicity in the southern hemisphere and results in dynamo action above a certain Rayleigh number, through a macroscopic α 2 mechanism. These dynamos equilibrate by generating concentrated magnetic flux bundles that limit the kinetic energy of the convection columns. The dipole-dominated external field is formed by superposition of several flux bundles at middle and high latitudes. At low latitudes a pattern of reversed flux patches propagates in the retrograde direction, resulting in an apparent westward drift of the field in the equatorial region. At higher Rayleigh number we find a fully developed regime with convection inside the tangent cylinder consisting of polar upwelling and azimuthal thermal wind flows. These motions modify the dynamo by expelling poloidal flux from the poles and generating intense toroidal fields in the polar regions near the inner core. Convective dynamos in the fully developed regime exhibit characteristics that can be compared with the geomagnetic field, including concentrated flux bundles on the core-mantle boundary, polar minima in field intensity, and episodes of westward drift.

Measuring the Magnetic Field on the Classical T Tauri Star BP Tauri

Abstract: We examine several theories that describe how stellar magnetic fields on classical T Tauri stars (CTTSs) interact with their surrounding accretion disks. We demonstrate that these theories require magnetic field strengths ranging from a few hundred to several thousand gauss, depending on which model is used and more importantly on the properties of individual systems. For example, the CTTS BP Tau is predicted to have a relatively strong magnetic field (1.4-4.1 kG), which should be detectable. We present infrared (IR) and optical echelle spectra of BP Tau and several reference stars of similar spectral class. Using detailed spectrum synthesis and the latest model atmospheres, we fitted 12 absorption features in the optical spectrum, including the strong titanium oxide (TiO) band head at 7055 A. For BP Tau we determine key stellar parameters: effective temperature (Teff=4055 ± 112 K), gravity (log g=3.67 ± 0.50), metallicity ([M/H]=0.18 ± 0.11), projected rotational velocity (v sin i=10.2 ± 1.8 km s-1), and optical veiling (r=0.00-0.15). A similar analysis of 61 Cyg B (K7 V) is used to validate the methodology. We then use the IR spectra to look for Zeeman broadening, which has a more pronounced effect at longer wavelengths. A Zeeman sensitive Ti I line at 2.2233 μm appears significantly broadened in BP Tau, relative to several rotationally broadened standard stars. The observed line is also significantly broader than predictions based on our optical analysis. Interpreting this excess broadening as Zeeman splitting of the Ti I line, we fitted the spectrum and find a distribution of field strengths whose surface averaged mean is =2.6 ± 0.3 kG. We did not use the Zeeman sensitive Fe I line at 8468.4 A when determining stellar or magnetic parameters for BP Tau, so this line provides a test of our results. The observed line profile is indeed broader than the nonmagnetic prediction, but the 8468.4 A line gives a magnetic flux lower than what was obtained in the IR, perhaps indicating that strong fields are concentrated into cool spots. Finally, we investigate an ad hoc model in which the IR line is assumed to form in the accretion disk itself. We discuss several reasons why the magnetic model is preferred, but the disk atmosphere example illustrates that our magnetic field measurement must still be tested using several IR lines with a range of Zeeman sensitivities.

A comparison of power density for axial flux machines based on general purpose sizing equations

Abstract: Based on the concept of the converter fed machine (CFM), an optimal machine design can be considered as the best match of the machine topology, the power electronic converter and the performance specification. To compare power production potential of axial flux machines with various topologies, different waveforms of back EMF and current, general purpose sizing and power density equations for such machines are needed. In this paper, a general approach is presented to develop and to interpret these equations. Sample applications of the sizing and power density equations are the axial flux toroidal permanent magnet utilized to compare the axial flux toroidal permanent magnet (AFTPM) machine and the axial flux two-stator permanent magnet (AF2SPM) machine.

Introduction to magnetic helicity

Abstract: This paper reviews several aspects of magnetic helicity, including its history from Gauss to the present, its relation to field structure, its role in Taylor relaxation, and how it is defined for sub-volumes of space. Also, its importance in solar physics will be discussed. Magnetic helicity quantifies various aspects of magnetic field structure. Examples of fields possessing helicity include twisted, kinked, knotted, or linked magnetic flux tubes, sheared layers of magnetic flux, and force-free fields. Helicity thus allows us to compare models of fields in different geometries, avoiding the use of parameters specific to one model. Magnetic helicity is conserved in ideal magnetohydrodynamics and approximately conserved during reconnection. Often, physical systems are described in terms of interacting parts: for example one might separate the solar magnetic field into an interior field and an atmospheric (coronal) field. We can obtain insight into how the parts of a magnetic system interact by describing how magnetic helicity is transferred from one part to another. This transfer is governed by an equation similar to Poynting's theorem for the transfer of energy through boundaries. In a confined volume, widespread reconnection may reduce the magnetic energy of a field while approximately conserving its magnetic helicity. As a result, the field relaxes to a minimum energy state, often called the Taylor state, where the current is parallel to the field. Such relaxation processes are important to both fusion and astrophysical plasmas. Recent observations show that structures in the northern hemisphere of the sun have predominantly negative helicity, and structures in the south have predominantly positive helicity. Helicity injection by differential rotation may explain this dependence.

Overview of the large helical device project

Abstract: The Large Helical Device (LHD) has successfully started running plasma confinement experiments after a long construction period of eight years. During the construction and machine commissioning phases, a variety of milestones were attained in fusion engineering which successfully led to the first operation, and the first plasma was ignited on 31 March 1998. Two experimental campaigns were carried out in 1998. In the first campaign, the magnetic flux mapping clearly demonstrated a nested structure of magnetic surfaces. The first plasma experiments were conducted with second harmonic 84 and 82.6 GHz ECH at a heating power input of 0.35 MW. The magnetic field was set at 1.5 T in these campaigns so as to accumulate operational experience with the superconducting coils. In the second campaign, auxiliary heating with NBI at 3 MW has been carried out. Averaged electron densities of up to 6 × 1019m-3, central temperatures ranging from 1.4 to 1.5 keV and stored energies of up to 0.22 MJ have been attained despite the fact that the impurity level has not yet been minimized. The obtained scaling of energy confinement time has been found to be consistent with the ISS95 scaling law with some enhancement.

Magnetic field based power transmission line communication method and system

Abstract: A method and system for communicating information between subscribers (20, 22, 24) over power lines (26) where the information is transmitted in the magnetic field component of the electromagnetic radiation carried over the power line (26) and the field is excited by a MASER (50) in order to enable communication between the subscribers (20, 22, 24) at the various electrical sites. The MASER (50) provides an inverted atomic population by pumping directly, through a Q-switch (58) and a synthetic aperture lens (60), into the atomic population of the electromagnetic wave carried over the power line (26) to produce acoustic wave oscillation at the appropriate atomic transition frequency. The MASER (50) output is transmitted within the existing magnetic flux envelope created by the magnetic field and the power line (26) acts as a magnetic waveguide for the coherent magnetic frame emission from the MASER (50). Inductive coupling is used to receive the transmitted information.

Theory of thin‐filament motion in Earth's magnetotail and its application to bursty bulk flows

Abstract: An MHD theory has been developed for the motion of a thin magnetic flux tube through a two-dimensional stationary medium that is in MHD equilibrium. The flux tube is represented as a one-dimensional filament. Simple properties of the computed time development of the filament are explored analytically, including linear intermediate and slow-mode waves, rotational discontinuities, and slow shocks. One numerical solution is presented in detail for a filament in the Earth's magnetotail that is initially underpopulated relative to its neighbors. The computed filament motion displays the strong earthward flow and reduced field line stretching that characterize bursty bulk flows, which are frequently observed in the plasma sheet of the Earth's magnetosphere. The solutions also display the propagation of MHD waves from the equatorial magnetosphere to the ionosphere, then partial reflection from the conducting ionosphere. The ionospheric end of the bursty-bulk-flow flux tube moves equatorward, but that motion is delayed relative to the earthward motion in the equatorial plane. The simulations illuminate the relationship between the interpretation of a bursty bulk flow (BBF) as an underpopulated flux tube and the fact that BBFs typically do not have significantly lower particle pressure than the neighboring plasma sheet. Though the simulated filament started out with lower particle pressure than its neighbors and thus started out as a “bubble” in the plasma sheet, its particle pressure rose to values comparable to, or sometimes greater than, its neighbors once its strong earthward motion developed.

The Granular Magnetic Fields of the Quiet Sun

Abstract: We report new observations that combine high-precision infrared polarimetry and high-resolution imagery in the visible to demonstrate that most of the quiet solar surface contains a measurable magnetic field. We found that when observed at 1 arcsec2 resolution, 68% of the observed area contains magnetic flux higher than 5×1015 Mx (corresponding to an apparent average field of 1 G). The majority of these magnetic features have magnetic flux below 5×1016 Mx. Their magnetic field strengths range from below 200 to 1000 G, which means that their filling factors are on the order of 1%. The spatial distribution and time evolution of these magnetic features are closely associated with the solar granulation. The properties of these weak granular magnetic features we observed differ from those of the intranetwork fields described in earlier observations. We also observed the formation and disappearance of a kilogauss magnetic feature associated with the development of intergranular lanes, which may be evidence of convective collapse.

Magnetocentrifugal Launching of Jets from Accretion Disks. I. Cold Axisymmetric Flows

Abstract: We present time-dependent, numerical simulations of the magnetocentrifugal model for jet formation, in an axisymmetric geometry, using a modification of the ZEUS3D code adapted to parallel computers. The gas is supposed cold with negligible thermal pressure throughout. The number of boundary conditions imposed on the disk surface is that necessary and sufficient to take into account information propagating upstream from the fast and Alfven critical surfaces, avoiding overdetermination of the flow and unphysical effects, such as numerical "boundary layers" that otherwise isolate the disk from the flow and produce impulsive accelerations. It is known that open magnetic field lines can either trap or propel the gas, depending upon the inclination angle, θ, of the poloidal field to the disk normal. This inclination is free to adjust, changing from trapping to propelling when θ is larger than θ_c ~ 30°; however, the ejected mass flux is imposed in these simulations as a function of the radius alone. As there is a region, near the origin, where the inclination of field lines to the axis is too small to drive a centrifugal wind, we inject a thin, axial jet, expected to form electromagnetically near black holes in active galactic nuclei and Galactic superluminal sources. Rapid acceleration and collimation of the flow is generally observed when the disk field configuration is propelling. We parameterize our runs using a magnetic flux Ψ ∝ R^(-eΨ) and mass flux j = ρv_z ∝ R^(-ej). We show in detail the steady state of a reference run with parameters e_Ψ = -1/2, e_j = 3/2, finding that the wind leaves the computational volume in the axial direction with an Alfven number M_A ~ 4, poloidal speed v_p ~ 1.6v_(K0), collimated inside an angle θ ~ 11°. We show also the thrust T, energy L, torque G, and mass discharge Ṁ of the outgoing wind, and we illustrate the dependence of these quantities with the exponents e_Ψ and e_j.

Magnetic write element having a thermally dissipative structure

Abstract: A magnetic read/write head having improved thermal characteristics. The read/write head includes a read element and a write element formed there over. The read element includes a read sensor embedded within a dielectric material and sandwiched between first and second shields. The write element includes first and second poles joined to form a yoke. The yoke is closed at one end and defines a write gap at the other. A coil having windings which extend through the yoke generates a magnetic flux within the yoke. This magnetic flux causes a magnetic fringe field at the location of the write gap, the fringe field being capable of imparting a magnetic signal onto a passing magnetic medium. The coil sits atop a thermally conductive, electrically insulating material which electrically isolates the coil from surrounding structure. When a current flows through the coil, heat will be generated. Such heat, which could be detrimental to read performance, is conducted through the thermally conductive material out of the yoke and away from the write element. The thermally conductive material provides a large heat sink for dissipation of the heat.

3D multifields FEM computation of transverse flux induction heating for moving-strips

Abstract: The numerical and experimental studies on induction heating of continuously moving strips in a transverse field are presented in this paper. The induced eddy current and its coupled thermal field in moving media is computed with FEM. The adopted mathematical model consists of a Fourier thermal conduction equation and a set of differential equations, which describes the steady-state eddy current problem in a configuration comprising a magnetic vector potential and an electrical scalar potential. The calculated results are in good agreement with the measurement.

Three-dimensional Solutions of Magnetohydrodynamic Equationsfor Prominence Magnetic Support: Twisted Magnetic Flux Rope

Abstract: The search for a background magnetic configuration favorable for prominence support has been given a great deal of attention for several decades. The most recent theoretical studies seem to agree that a promising candidate for the support of the dense and cooler prominence material, which fulfills several of the theoretical and observational requirements such as twist, shear along the neutral line, and dips, is a magnetic flux rope. The most convincing models take an infinitely long periodic configuration that consists of a linear constant-α force-free magnetic field. These models, however, assume values of α that are close to its maximum possible value. In this Letter, we report our recent results, which show that it is indeed possible to produce a configuration that consists of a twisted magnetic flux tube embedded in an overlaying, almost potential, arcade such that high electric currents (and therefore values of α) are confined to the inner twisted magnetic flux rope. We present two MHD processes—corresponding to two different types of boundary conditions—that produce such a configuration. Our results show that the process associated variations of Bz at the photospheric level by applying an electric field involving diffusion is much more efficient for creating a structure with more twist and dips.

Collective and plastic vortex motion in superconductors at high flux densities

Abstract: The ‘mixed state’ of type II superconductors occurs when magnetic flux penetrates the material (in the form of vortices) without destroying the superconducting ground state. Zero resistivity is retained if the vortices are pinned by crystalline defects, but is destroyed by vortex motion. This provides the practical motivation for studying vortices in random pinning potentials1,2,3,4,5,6,7. But theinsights so obtained also bear on the more general class of problems involving the dynamics of elastic media in the presence of quenched disorder8 (for example, mechanical friction). Moreover, the magnetic vortex system is highly tunable and permits questions concerning frictional, plastic and elastic flow9 to be investigated on the scale of single vortices. Remarkable results have been obtained on the dynamics of this system10,11,12,13,14,15,16,17, but have been largely restricted to well separated vortices at very low flux densities. Scanning tunnelling microscopy has the potential to resolve individual vortices at much higher flux densities18,19,20,21, and here we show that the imaging rates can be sufficiently high toresolve the dynamics in this flux regime. We find that, in thepresence of strongly pinning line defects, the vortex lattice remains pinned until the number of vortices is about twice that ofthe defects, at which point plastic creep commences. But in thepresence of weak intrinsic point disorder, the vortices creep coherently along one of the principle axes of the vortex lattice, where they exhibit striking and unanticipated velocity modulations that appear to be related to the lattice periodicity.

New Recording Method Combining Thermo-Magnetic Writing and Flux Detection

Abstract: We have developed a new perpendicular thermo-magnetic recording method coupled with magnetic flux detection. The resolution is substantially improved by flux detection. Compared to the latest magneto-optical disk drives, the carrier-to-noise ratio of a reproduced signal is 5 dB higher, and the recording density reaches 8.3 Gbit/inch2. Moreover, this recording method has a good affinity to near-field optics, and it is an effective technique for attaining a higher density beyond the super-paramagnetic limitation in longitudinal magnetic recording.

A new approach to vector control for a linear induction motor considering end effects

Abstract: An equivalent circuit model in the rotor flux oriented frame is developed for the vector control of a linear induction motor (LIM) by adding the so called "end effect" to the rotary induction motor (RIM) model. We describe the effects of the end effect by introducing speed dependent scale factors to the magnetizing inductance and series resistance in the d-axis equivalent circuit. Torque and flux equations are correspondingly modified. With the vector control scheme based on the proposed equivalent circuit, one can resolve the thrust attenuation problem in a high speed. We have demonstrated through experiments the improvements achieved by the proposed scheme.

Solar Cycle Changes in GONG p-Mode Frequencies, 1995-1998

Abstract: We have analyzed 27 3 month sets of Global Oscillaiton Network Group (GONG) data from the end of cycle 22 and the beginning of cycle 23 and here present evidence of significant shifts in the central frequencies and the even a-coefficients of the frequency splittings of the modes. The temporal behavior of the even a-coefficients is better reproduced by the corresponding coefficients of a Legendre polynomial decomposition of the surface magnetic field than by the total flux; i.e., the temporal variation is strongly correlated with the latitudinal distribution of the surface magnetic activity. These changes are consistent with available data from previous solar cycles. The even a-coefficients, which sense the asphericity of the solar structure, appear to show similar temporal evolution at all depths. The odd a-coefficients, which sense the internal differential rotation, show no significant variation with time or depth. In particular they show no significant correlation with either the magnetic flux or with the corresponding odd Legendre components of the flux. This suggests that the solar cycle related variation of the oscillation frequencies is not due to contamination of observed Doppler shifts by the surface magnetic fields.

Waves in Twisted Magnetic Flux Tubes

Abstract: The modes of oscillation of a twisted magnetic flux tube in an incompressible medium are investigated analytically. An exact dispersion relation for the case of uniform twist is obtained. In contrast to the case of an untwisted incompressible tube, body, surface, and hybrid (surface-body) modes arise.

A new zero frequency flux position detection approach for direct field oriented control drives

Abstract: This paper presents a novel approach to estimate the instantaneous position of the air-gap flux in standard induction machines at low and zero speed. The proposed approach is based on the generation of a high-frequency rotating field that, interacting with the main field, generates a high-frequency zero-sequence flux component. By demodulating such a component, which is always present, irrespective of speed and load levels, it is possible to detect the air-gap flux position. The proposed method allows one to overcome some key drawbacks of sensorless approaches based on high-frequency voltage injection and, compared to previous sensorless schemes exploiting the third harmonic voltage, improves the accuracy and extends the operating area up to zero speed. Experimental tests are shown in order to practically confirm the expected features of the proposed system.

Phenomena in an Emerging Active Region. II. Properties of the Dynamic Small-Scale Structure

Abstract: The magnetic flux emergence in growing active region NOAA 5617, when it is about 8 hr old, shows an intricate fine structure. The small-scale emergence events are characterized by a coincident upflow and transient darkening (of about 2 Mm and 10 minutes) in the continuum and line-center intensity followed by the appearance of one, or in some cases two, new bright grains flanking the line-center darkening. The bright grains (faculae) coincide with magnetic flux concentrations and downflows. The footpoints move apart at on average 1.4 km s-1. Flux emergence happens recurrently in a number of locations widely distributed over the active region, which appear to form a pattern with a wavelength of about 8 Mm. A preferred orientation that fits Hale's polarity law is displayed by the spatial pattern in the emergence locations, the emergence events themselves, subsequent footpoint motion, and the Hα arch filament system. We find long (~15 Mm) alignments of unipolar faculae of each magnetic polarity that also follow the preferred orientation. We adapt the model for flux emergence to accommodate the observed dynamic fine structure. Essential new features are (1) the emerging bundle of flux tubes is frayed in two systems, in vertical stacks, arranged in slightly curved, nearly parallel sheets; and (2) many flux tubes emerge in multiple locations.

Magnetic and Radiative Variability of Solar Surface Structures. I. Image Decomposition and Magnetic-Intensity Mapping

Abstract: In order to specify quantitatively the contributions to irradiance variability by specific types of solar surface structure, we analyzed full-disk magnetograms and Ca II K images from the National Solar Observatory and Big Bear Solar Observatory for two sets of several days in early 1992 and mid-1993. These test days were chosen at maxima and minima of the rotational modulation in the Lyα irradiance from the Upper Atmosphere Research Satellite Solar-Stellar Irradiance Comparison Experiment (UARS/SOLSTICE) spectrometer. For the eight days, we isolated active regions, decaying active regions, the enhanced network, the network, and the quiet atmosphere based on their magnetic flux strength and distribution, filling factor, and association with sunspots. Transfer of these image decompositions to Ca II K images gives magnetic flux versus intensity (|B| vs. δK) relationships for four structures with measurable magnetic flux. In the range 30-400 Mx cm-2, these log-log curves are linear with a slope of 0.5, which suggests that the Ca II K residual intensity is proportional to the half-power of the magnetic flux density. The separation into quiet and active Sun structures gives a prediction of the variation of the |B| versus δK relation for the ensemble of our four principal structures from minimum to maximum in the activity cycle of the Sun viewed as a star.

Quadrupolar Magnetic Reconnection in Solar Flares. I. Three-dimensional Geometry Inferred from Yohkoh Observations

Abstract: We analyze the three-dimensional geometry of solar flares that show so-called interacting flare loops in soft X-ray, hard X-ray, and radio emission, as previously identified by Hanaoka and Nishio. The two flare loops that appear brightest after the flare are assumed to represent the outcome of a quadrupolar magnetic reconnection process, during which the connectivity of magnetic polarities is exchanged between the four loop footpoints. We parameterize the three-dimensional geometry of the four involved magnetic field lines with circular segments, additionally constrained by the geometric condition that the two pre-reconnection field lines have to intersect each other at the onset of the reconnection process, leading to a 10 parameter model. We fit this 10 parameter model to Yohkoh Soft and Hard X-Ray Telescopes (SXT and HXT) data of 10 solar flares and determine in this way the loop sizes and relative orientation of interacting field lines before and after reconnection. We apply a flare model by Melrose to calculate the magnetic flux transfer and energy released when two current-carrying field lines reconnect to form a new current-carrying system in a quadrupolar geometry. The findings and conclusions are the following. (1) The pre-reconnection field lines always show a strong asymmetry in size, consistent with the scenario of newly emerging small-scale loops that reconnect with preexisting large-scale loops. (2) The relative angle between reconnecting field lines is nearly collinear in half of the cases, and nearly perpendicular in the other half, contrary to the antiparallel configuration that is considered to be most efficient for magnetic reconnection. (3) The angle between interacting field lines is reduced by ≈10°-50° after quadrupolar reconnection. (4) The small-scale flare loop experiences a shrinkage by a factor of 1.31 ± 0.44, which is consistent with the scaling law found from previous electron time-of-flight measurements, suggesting that electron acceleration occurs near the cusp of quadrupolar configurations. (5) The large-scale loop is found to dominate the total induction between current-carrying loops, providing a simple estimate of the maximum magnetic energy available for flare energy release because of current transfer, which scales as ΔEI ≈ 1029.63(r2/109 cm)(I2/1011A)2 (with r2 the curvature radius and I2 the current of the large-scale loop) and is found to correlate with observed flare energies deduced from soft X-ray and hard X-ray fluxes. Most of the energy is transferred to small-scale loops that have one-half of the large-scale current (I1 = I2/2). (6) The quadrupolar reconnection geometry provides also a solution of Canfield's dilemma of the offset between the maximum of vertical currents and the HXR flare loop footpoints. (7) The quadrupolar geometry provides not only a framework for interacting double-loop flares, but it can also be considered as a generalized version of (cusp-shaped) single-loop flares.

Methods for controlling the path of magnetic flux from a permanent magnet and devices incorporating the same

Abstract: A permanent magnet device includes a permanent magnet having north and south pole faces with a first pole piece positioned adjacent one pole face thereof and a second pole piece positioned adjacent the other pole face thereof so as to create at least two potential magnetic flux paths. A first control coil is positioned along one flux path and a second control coil is positioned along the other flux path, each coil being connected to a control circuit for controlling the energization thereof. The control coils may be energized in a variety of ways to achieved desirable motive and static devices, including linear reciprocating devices, linear motion devices, rotary motion devices and power conversion.

Permanent magnet d.c. motor having a radially-disposed working flux gap

Abstract: A P.M. d.c. motor which may utilize "brushless" commutation. The motor includes a stator assembly with a plurality of upstanding discrete spaced-apart core components formed in a circular locus generally parallel with the motor axis. Each core component includes a relatively lengthy winding association region and extending therefrom a pole piece region providing a flux interaction surface. A field winding mounted upon a bobbin is positioned over the core component at the winding association region and when excited, generates electromagnetic flux at the flux interaction surface. A permanent magnet component is carried by a rotor such that its interaction surface is adjacent to that at the pole piece region, and is spaced therefrom to define a flux working gap at a desirably lengthy working gap radius from the motor axis.

Excitation of Oscillations in Photospheric Flux Tubes through Buffeting by External Granules

Abstract: We examine the excitation of transverse (kink) and longitudinal (sausage) waves in magnetic flux tubes by granules in the solar photosphere The investigation is motivated by the interpretation of network oscillations in terms of flux tube waves We model the interaction between a granule, with a specified transverse velocity, and a vertical flux tube in terms of the Klein-Gordon equation, which we solve analytically as an initial value problem for both wave modes, assuming the same external impulse The calculations show that for magnetic field strengths typical of the network, the energy flux in transverse waves is higher than in longitudinal waves by an order of magnitude, in agreement with the chromospheric power spectrum of network oscillations observed by Lites, Rutten, & Kalkofen But for weaker fields, such as those that might be found in internetwork regions, the energy fluxes in the two modes are comparable This result implies that if there are internetwork oscillations in magnetic flux tubes, they must show the cutoff periods of both longitudinal and transverse modes at 3 minutes and at 7 minutes or longer We also find that granules with speeds of about 2 km s-1 can efficiently excite transverse oscillations in frequent short-duration (typically 1 minute) bursts that can heat the corona

Chapter?8: Plasma operation and control

Abstract: Wall conditioning of fusion devices involves removal of desorbable hydrogen isotopes and impurities from interior device surfaces to permit reliable plasma operation. Techniques used in present devices include baking, metal film gettering, deposition of thin films of low-Z material, pulse discharge cleaning, glow discharge cleaning, radio frequency discharge cleaning, and in situ limiter and divertor pumping. Although wall conditioning techniques have become increasingly sophisticated, a reactor scale facility will involve significant new challenges, including the development of techniques applicable in the presence of a magnetic field and of methods for efficient removal of tritium incorporated into co-deposited layers on plasma facing components and their support structures. The current status of various approaches is reviewed, and the implications for reactor scale devices are summarized. Creation and magnetic control of shaped and vertically unstable elongated plasmas have been mastered in many present tokamaks. The physics of equilibrium control for reactor scale plasmas will rely on the same principles, but will face additional challenges, exemplified by the ITER/FDR design. The absolute positioning of outermost flux surface and divertor strike points will have to be precise and reliable in view of the high heat fluxes at the separatrix. Long pulses will require minimal control actions, to reduce accumulation of AC losses in superconducting PF and TF coils. To this end, more complex feedback controllers are envisaged, and the experimental validation of the plasma equilibrium response models on which such controllers are designed is encouraging. Present simulation codes provide an adequate platform on which equilibrium response techniques can be validated. Burning plasmas require kinetic control in addition to traditional magnetic shape and position control. Kinetic control refers to measures controlling density, rotation and temperature in the plasma core as well as in plasma periphery and divertor. The planned diagnostics (Chapter?7) serve as sensors for kinetic control, while gas and pellet fuelling, auxiliary power and angular momentum input, impurity injection, and non-inductive current drive constitute the control actuators. For example, in an ignited plasma, core density controls fusion power output. Kinetic control algorithms vary according to the plasma state, e.g. H- or L-mode. Generally, present facilities have demonstrated the kinetic control methods required for a reactor scale device. Plasma initiation - breakdown, burnthrough and initial current ramp - in reactor scale tokamaks will not involve physics differing from that found in present day devices. For ITER, the induced electric field in the chamber will be ~0.3V??m-1?- comparable to that required by breakdown theory but somewhat smaller than in present devices. Thus, a start-up 3MW electron cyclotron heating system will be employed to assure burnthrough. Simulations show that plasma current ramp up and termination in a reactor scale device can follow procedures developed to avoid disruption in present devices. In particular, simulations remain in the stable area of the li-q plane. For design purposes, the resistive V?s consumed during initiation is found, by experiments, to follow the Ejima expression, 0.45?0 RIp. Advanced tokamak control has two distinct goals. First, control of density, auxiliary power, and inductive current ramping to attain reverse shear q profiles and internal transport barriers, which persist until dissipated by magnetic flux diffusion. Such internal transport barriers can lead to transient ignition. Second, combined use poloidal field shape control with non-inductive current drive and NBI angular momentum injection to create and control steady state, high bootstrap fraction, reverse shear discharges. Active n = 1 magnetic feedback and/or driven rotation will be required to suppress resistive wall modes for steady state plasmas that must operate in the wall stabilized regime for reactor levels of ? ? 0.03.

Modeling of electromagnetic phenomena in soft magnetic materials under unidirectional time periodic flux excitations

Abstract: We report on recent advances in the modeling of magnetic losses in steel laminations used in electromagnetic devices. The integrated-lamination moving dynamic Preisach model, used to evaluate the dynamic magnetization loops under distorted unidirectional flux patterns, is described. The main goal is the comparison of two numerical procedures, using the finite element-finite difference technique and the finite element-fixed point technique, respectively, each properly taking into account the hysteresis characteristics by the Preisach theory. Moreover, attention is paid to the identification of the material parameters entering the moving dynamic Preisach model. Finally, the two techniques are validated by the comparison of numerical experiments and measurements on two different materials. Here, global as well as local quantities in the lamination structure are evaluated.

An analytical model of switched reluctance machines

Abstract: An analytical model is introduced to describe switched reluctance machines (SRM). The nonlinear function of flux linkage depending on the winding current and rotor position is considered to be composed by three components: aligned and unaligned flux linkages, and an angular function, where the two flux linkages are only determined by phase current, and the angular function depends only on the rotor position. Examples of the application of this model in simulating the phase current, calculating the instantaneous torque and estimating the rotor position are presented. The results are compared with measurements.

High-performance motion control of an induction motor with magnetic saturation

Abstract: Generalizes the authors' work on high-performance control of induction motors to machines that exhibit significant magnetic saturation. The controller design is based on the standard d-q model of the induction motor which has been modified to account for the saturation of the iron in the main (magnetic) path of the machine. An input-output linearization controller is used to provide independent (decoupled) control of the speed and flux. With this controller, the flux reference becomes an extra degree of freedom for the designer to help achieve performance objectives. Taking into account saturation along with the voltage and current constraints, the flux reference is chosen to achieve the optimal torque (maximum for acceleration and minimum for deceleration) at any given speed. Experimental results are given to demonstrate the input-output controller's effectiveness in providing the tracking of a given position and speed trajectory while simultaneously tracking the optimal flux reference. The set of experiments are fast point-to-point motion control moves with an inertial load comparing the input-output controller based on the saturated magnetics model with that based on the linear magnetics model.

Modeling the magnetosphere for northward interplanetary magnetic field: Effects of electrical resistivity

Abstract: We develop a simple analytic model and use global simulations of Earth's magnetosphere to investigate the effects of electrical resistivity on the topology of the magnetosphere for northward interplanetary magnetic field (IMF). We find that for low resistivity values (≲104 Ω m) the magnetosphere remains open after 6 hours of northward IMF. For larger values (≳2×105 Ω m) the magnetic flux of the tail lobes decreases rapidly on the timescale of ∼1 hour. In this case the tail becomes closed, tadpole-shaped, steady state, and of finite length. The tail length decreases with increasing resistivity and becomes as short as about 50 RE for a resistivity value of 106 Ω m. Reconnection between IMF and lobe field lines occurs in all cases and is not significantly affected by the resistivity. However, large values of the resistivity annihilate lobe flux and break the frozen-in condition for closed tail flux tubes, leading to a decoupling of the flux tube motion from plasma convection. These effects make the development of a steady, closed tail of finite length possible. Because resistivity values larger than 102 Ω m are unrealistic for the quiet time tail, we conclude that the magnetosphere is unlikely to ever close and that models which predict the rapid closure and a steady, finite length tail are possibly in error due to numerical resistivity.