Showing papers on "Magnetic flux published in 1980"

Wavefront dislocations in the Aharonov-Bohm effect and its water wave analogue

Abstract: The authors study the wavefronts (i.e. the surfaces of constant phase) of the wave discussed by Aharonov and Bohm, representing a beam of particles with charge q scattered by an impenetrable cylinder of radius R containing magnetic flux Phi . Defining the quantum flux parameter by alpha =q Phi /h, they show that for the case R=0 the wave psi AB possesses a wavefront dislocation on the flux line, whose strength (i.e. the number of wave crests ending on the dislocation) equals the nearest integer to alpha . When alpha passes through half-integer values, the strength changes, by wavefronts unlinking and reconnecting along a nodal surface. In quantum mechanics this phase structure is unobservable, but they devise an analogue where surface waves on water encounter an irrotational 'bathtub' vortex; in this case alpha depends on the frequency of the waves and the circulation of the vortex. Experiments show dislocation structures agreeing with those predicted. psi AB is an unusual function in which incident and scattered waves cannot be clearly separated in all asymptotic directions; they discuss its properties using a new asymptotic method.

Magnetic fields and coronal heating

Abstract: General considerations concerning the scaling properties of magnetic-field-related coronal heating mechanisms are used to build a two-parameter model for the heating of closed coronal regions. The model predicts the way in which coronal temperature and electron density are related to photospheric magnetic field strength and the size of the region, using the additional constraint provided by the scaling law of Rosner, Tucker, and Vaiana. The model duplicates the observed scaling of total thermal energy content with total longitudinal flux; it also predicts a relation between the coronal energy density (or pressure) and the longitudinal field strength modified by the region scale size.

Coronal heating by stochastic magnetic pumping

Abstract: Recent observational data cast serious doubt on the widely held view that the Sun's corona is heated by traveling waves (acoustic or magnetohydrodynamic). It is proposed that the energy responsible for heating the corona is derived from the free energy of the coronal magnetic field derived from motion of the 'feet' of magnetic field lines in the photosphere. Stochastic motion of the feet of magnetic field lines leads, on the average, to a linear increase of magnetic free energy with time. This rate of energy input is calculated for a simple model of a single thin flux tube. The model appears to agree well with observational data if the magnetic flux originates in small regions of high magnetic field strength. On combining this energy input with estimates of energy loss by radiation and of energy redistribution by thermal conduction, we obtain scaling laws for density and temperature in terms of length and coronal magnetic field strength.

Eruptive solar magnetic fields

Abstract: The quasi-steady evolution of solar magnetic fields in response to gradual photospheric changes is considered, with particular attention given to the threshold of a sudden eruption in the solar atmosphere. The formal model of an evolving, force-free field dependent on two Cartesian coordinates is extended to a field which is not force free but in static equilibrium with plasma pressure and gravity. The basic physics is illustrated through the evolution of a loop-shaped electric current sheet enclosing a potential bipolar field with footpoints rooted in the photosphere. A free-boundary problem is posed and then solved for the equilibrium configuration of the current sheet in a hydrostatically supported isothermal atmosphere. As the footpoints move apart to spread a constant photospheric magnetic flux over a larger region, the equilibria available extend the field to increasing heights.

Flux linkages of bipolar sunspot groups: A computer study

Abstract: Past studies of the structure of solar magnetic fields have used magnetograph data to compute selected field lines for comparison with the morphology of structures seen in various spectral wavelengths. While those analyses examine one of the integral properties of magnetic fields (field lines), they are not complete since they fail to determine the other important integral property: the boundaries of the flux of field lines of given connectivity. In the present analysis we determine such a system of boundaries, called separatrices, for the current free field of two p-f spot pairs so as to exhibit the line of self-intersection, called the separator. The analysis is compared with previous analytical work. These computer results, confirming earlier studies carried out using iron fillings, show that the separatrix has the form of two intersecting ovoids, defining four flux cells. New features which have emerged from this study include the observation that the projections of the separatrix in a plane perpendicular to the separator at its highest point do not intersect at 90° as has been widely believed, but rather closer to 60° in the case studied. The separator is very nearly circular over most of its length. The two neutral points (B = 0) which appear at the photospheric ends of the separator have the mixed radial-hyperbolic form as expected, a feature requiring every field line lying on the separatrix to connect with at least one of the two neutral points. The rotation of line direction with height (shear) is graphically illustrated in the potential field case studied here. We also exhibit a magnetic arcade.

Magnetic field concentration means and method for an implanted device

Abstract: A magnetic field concentration means and method for use in conjunction with an implanted device responsive to an externally generated magnetic field More specifically, a magnetic field concentrator is disclosed which includes a metallic slug located between a magnetic field generator and a magnetic pick-up coil contained in the implanted device The metallic slug concentrates magnetic lines of flux at the pick-up coil In a specific embodiment, the implanted device is an intracranial pressure monitoring device (ICPM) which is located within an orifice formed in a recipient's skull The magnetic field concentrator is positioned within the orifice and directly above the ICPM, thereby concentrating magnetic flux lines at the ICPM Two configurations of the slug are disclosed, one being in the form of a cylinder and the other being in the form of a truncated cone In another application of the concentrator, a bandage or other suitable holding means positions the slug externally to the implanted device, thereby again concentrating magnetic lines of flux at the implanted device

Method and apparatus for measuring a DC current in a wire without making a direct connection to the wire

Abstract: A pair of semi-circular, hard magnetic sensors are rigidly fixed within separate blocks which are in turn removably received in corresponding housings mounted to the jaws of a spring biased clamp. The clamp is adapted for positioning the sensors into a split ring around a wire carrying the DC current to be measured. According to a first embodiment of the method, longitudinally wound coils about each of the sensors are momentarily energized by an AC current source connected thereto through a variable transformer, while the sensors are around the wire. Thereafter the sensors are removed from the clamp and placed side by side beneath a compass so that the magnetic flux previously induced in the sensors deflects the compass needle from North. Circumferentially wound coils about each of the sensors are connected to a DC current source which is varied to move the compass needle back to North by cancelling the magnetic flux previously induced in the sensors. The amount of current required to move the compass needle back to North provides an indication of the amplitude of the DC current flowing through the wire about which the sensors were clamped. In a second embodiment of the method, after the sensors have been removed from around the wire, a ballistic galvanometer is connected to the circumferential coils about the sensors. The longitudinal coils about the sensors are energized to cause the magnetic flux previously induced in the sensors to be cancelled. The change in magnetic flux measured by the ballistic galvanometer provides an indication of the amount of DC current flowing through the wire.

Self-sustaining solenoid

Abstract: A self-sustaining solenoid which is adapted so that when applying an operating current to a coil, a moving iron core disposed in the coil is attracted into contact with a fixed receiver. A magnetic yoke is provided to extend between the fixed receiver and the end portion of the moving iron core projecting out of the coil, and a permanent magnet is disposed on the magnetic yoke at least at one end in the direction of movement of the moving iron core. When the moving iron core is held in contact with the fixed receiver, magnetic fluxes emanating from the permanent magnet mostly pass through the moving iron core, the fixed receiver and the magnetic yoke, and even if the operating current is cut off, the moving iron core is retained in contact with the fixed receiver. A magnetic gap is provided through which the magnetic fluxes of the permanent magnet mostly pass when the moving iron core is out of contact with the fixed receiver, and magnetic flux resulting from the application of the operating current pass through the magnetic gap.

Steady-state flux-line cutting in type II superconductors

Abstract: Flux-line cutting (intersection and cross-joining of adjacent nonparallel vortices) has been suggested as a mechanism for steady-state dissipation in current-carrying, type II superconductors in longitudinal magnetic fields. In this paper a specific theoretical flux-line-cutting model is proposed which generates a constant steady-state electric field in a current-carrying, ideal, type II superconducting slab in a longitudinal field. The assumed model consists of parallel vortex planes at different angles which periodically shuttle back and forth between the regions in which flux-line cutting occurs. The resulting macroscopic electric current and magnetic flux density distributions are calculated. The model yields a nonlinear voltage-current characteristic and a longitudinal paramagnetic moment.

Experimental study of thermoelectricity in superconducting indium

Abstract: This paper reports on a comprehensive study of thermoelectricity in pure superconducting indium near the transition temperature. The samples consisted of hollow bimetallic toroids of lead and indium. Upon application of a temperature gradient, a magnetic flux was produced within the hollow cavity of the toroids which could be coupled, via a superconducting flux transformer, into a superconducting quantum interference device magnetometer. The thermoelectric origins of the magnetic flux and the experimental method are discussed in detail. The magnetic flux was measured in seven specimens, and in each was found to diverge as the transition temperature was approached with a ${(1\ensuremath{-}\frac{T}{{T}_{c}})}^{\ensuremath{-}\frac{3}{2}}$ power-law dependence. The magnitude of the flux varied by about a factor of 40 among the different samples and appeared to scale with the normal-state thermoelectric properties of the indium. Tests intended to eliminate possible spurious causes for the magnetic flux are also discussed.

Fault detection apparatus for rotor winding of rotary machine

Abstract: A fault detection apparatus for a rotor winding of a rotary machine comprises a peak value hold circuit, a peak value processor and a monitor. The peak value hold circuit holds peak value of a pulsing magnetic flux waveform supplied from a magnetic flux sensor mounted in proximity to a surface of a rotor, for a necessary time period. The peak value processor sequentially compares the peak value held in the peak value hold circuit with corresponding one of pre-stored peak values of the pulsing magnetic flux waveform to determine if a difference therebetween exceeds a preset level. The monitor indicates the presence or absence of fault based on the result of the comparison of the processor.

Exact static equilibrium of vertically oriented magnetic flux tubes

Abstract: A method is prescribed for generating exact solutions of magnetostatic equilibrium describing a cylindrically symmetric magnetic flux tube oriented vertically in a stratified medium. Given the geometric shape of the field lines, compact formulae are presented for the direct calculation of all the possible distributions of pressure, density, temperature and magnetic field strength compatible with these field lines under the condition of static equlibrium. The plasma satisfies the ideal gas law and gravity is uniform in space. A particular solution is obtained by this method for a medium sized sunspot whose magnetic field obeys the similarity law of Schluter and Temesvary (1958). With this solution, it is possible for the first time to illustrate explicitly the confinement of the magnetic field of the cool sunspot by the hotter external plasma in an exact relationship involving both magnetic pressure and field tension as well as the support of the weight of the plasma by pressure gradients. It is found that the cool region of the sunspot is not likely to extend much more than a few density scale heights below the photosphere. The sunspot field approaches being potential in the neighbourhood of the photosphere so that the Lorentz force exerting on the photosphere is less than what the magnetic pressure would suggest. This accounts for how the sunspot field can be confined in the photosphere where its magnetic pressure is often observed to even exceed the normal photospheric pressure. The energy mechanism operating in the sunspot and the question of mechanical stability are not treated in this paper.

Hysteresis losses and magnetic phenomena in rotating disks of type‐II superconductors

Abstract: Hysteresis losses and the behavior of the magnetic flux threading superconducting disks of Nb oscillating slowly over various angular displacements in static magnetic fields H0 directed perpendicular to the axis of rotation have been investigated by continuously monitoring 〈Bz〉 and 〈By〉, the average components of the magnetic induction, in the plane of the disk, parallel and perpendicular to H0. A simple model proposed previously satisfactorily accounts for the initial changes, starting from various initial flux configurations, the subsequent cyclical behavior, and the hysteresis losses. Expulsion of flux from the disk occurs during part of the first oscillation, then the amount of flux threading the disk remains constant during the subsequent oscillations, although the orientation distribution of the flux profile undergoes spectacular periodic variations. The analysis shows that dψ/dx = ±k dB/dx describes the orientation of the sheets of flux lines with respect to H0 with distance from the flat surfaces....

Continuous vortex cutting in type II superconductors with longitudinal current

Abstract: The consequences of stipulating translational symmetry for a type II superconductor to which longitudinal electric current and longitudinal magnetic field are applied are investigated. The magnetic flux lines must cut each other continuously in order to generate an electric field in this symmetry. We describe the steady state by two interpenetrating vortex lattices moving into and out of the sample. We find for the slab and cylinder geometries that cutting, cross-joining, and subsequent straightening of the flux lines reduce the electric field, as compared with the normal conducting state, by a factor which is of the order of one over the total number of flux lines in the cylinder. We conclude that the much larger voltages observed in cylinders of several millimeters diameter can be explained only by a breakdown of translational symmetry. With translational symmetry, the voltage initially increases as the third power of the current. The resulting vortex configuration is force-free. The transverse flux component increases and the longitudinal component decreases from the axis to the surface, leading to a paramagnetic moment. The drift of oscillation velocity of the flux lines is reduced by the same factor as the electric field. We predict low-frequency oscillations of the vorticesmore » near the surface of thin superconducting wires.« less

Sunspots and the physics of magnetic flux tubes. X - On the hydrodynamic instability of buoyant fields

Abstract: The hydrodynamics of cylindrical buoyant bubbles (magnetic flux tubes) in an atmosphere is examined. Their dynamical stability is considered, with particular attention to the Rayleigh-Taylor and Kelvin-Helmholtz instabilities. The instabilities are sufficient to cause fragmentation in the absence of some strong stabilizing effect such as surface tension or an internal azimuthal magnetic field. The results of the stability analysis are first applied to rising gas bubbles in liquids. Then, since agreement with the experimental data is found, the results are applied to the similar problem of the rising magnetic flux in the sun. Magnetic flux tubes in a fluid velocity field are intrinsically unstable, unless twisted. With the known dynamical limits on the degree of twisting, an upper limit on the diameter of the order of a few hundred kilometers is found for the stability of individual flux tubes in the photosphere. Observations show that tubes, except when clustered to form sunspots, are generally not larger than this amount. Therefore, it is suggested that the hydrodynamic instability of larger tubes is a prime cause of their absence from the solar photosphere.

Electromagnetic flow meter

Abstract: An electromagnetic flow meter with less power consumption in which an instantaneous pulsate current with alternately changing polarity at given intervals is fed into an exciting coil, and a velocity of flow of fluid is calculated by using a voltage produced between electrodes in accordance with a residual magnetic flux of a magnetic circuit and a flow velocity of the fluid when no exciting current flows.

Low magnetic leakage flux brushless pulse controlled D-C motor

Abstract: To reduce a unidirectional leakage flux emanating from a two-filament, pulse-energized motor, the stator (21, 22) is constructed at least as a four-pole stator and each filament (40,41) of the windings (45; 74, 75, 78, 79) associated with respective stator poles is wound on the stator core elements (21) such that the number of North poles and South poles arising at the circumference of the core elements, upon energization by a current source (48, 49) of the respective filaments, will be identical, and the sum of the magnetic flux due the energization of the respective filament arising, for example, at the center or shaft of the motor, will be essentially zero, so that the effect of unidirectional, pulsating leakage flux upon a galvano-magnetic sensor (16) or other magnetic apparatus, e.g. magnetic tape transducing devices, will be essentially eliminated.

Ground fault indicator

Abstract: A ground fault indicator for a multi-phase power distribution system includes a split-core magnetic pole assembly having a gap within which is developed a magnetic flux representative of the vector sum of the currents in the individual phase conductors of the system. A magnetic reed switch positioned within the gap responds to the summation current in the gap to trip an indicating circuit in a remotely mounted indicating module in the event of a ground fault.

Transducer for sensing string vibrational movement in two mutually perpendicular planes

Abstract: Two magnetically permeable pole pieces have pole faces of predetermined configuration formed thereon, and the pole pieces conduct magnetic flux which interacts with a magnetically permeable string of a stringed instrument. The positioning of the pole faces, the geometric configuration of the pole faces relative to the string, and the predetermined pattern of magnetic flux emanated from the pole faces are arranged so that vibrational movement of the string in one plane creates significant magnetic flux changes in a first pole piece and minimal or no flux changes in the second pole piece. String vibrational movement in a second plane mutually perpendicular to the first plane creates significant magnetic flux changes in the second pole piece and minimum or no flux changes in the first pole piece. The magnetic flux changes are sensed and electrical signals related to the flux changes are derived. The electrical signals are supplied directly to audio amplifying equipment.

Dynamic magnetic structure of large amplitude Alfvénic variations in the solar wind

Abstract: The dynamic structure of large-amplitude Alfven disturbances of the interplanetary magnetic field is examined by transforming one-hour intervals of Explorers 33 and 35 magnetometer data from the solar ecliptic coordinate system to a coordinate system defined by the principal axes of the variance matrix. It is demonstrated how some interplanetary magnetic field fluctuations observed by both Explorers are consistent with local properties theoretically predicted for plane large-amplitude Alfven waves by Barnes and Hollweg (1974). The different types of angular motion of the magnetic field component normal to the direction of minimum variance may be indicative of the detailed conditions of the solar coronal plasma in the regions generating the Alfven waves, or some aspect of local generation.

Shielded electromagnetic acoustic transducers

Abstract: Disclosed is an electromagnetic acoustic transducer, including a source of magnetic flux for establishing a static magnetic field, an electrical conductor for conducting an alternating current in the static magnetic field, and an electrically conductive, nonmagnetic shield disposed between the source of magnetic flux and the conductor.

Generation of intense, high-energy ion pulses by magnetic compression of ion rings

Abstract: A system based on the magnetic compression of ion rings, for generating intense (high-current), high-energy ion pulses that are guided to a target without a metallic wall or an applied external magnetic field includes a vacuum chamber; an inverse reflex tetrode for producing a hollow ion beam within the chamber; magnetic coils for producing a magnetic field, Bo, along the axis of the chamber; a disc that sharpens a magnetic cusp for providing a rotational velocity to the beam and causing the beam to rotate; first and second gate coils for producing fast-rising magnetic field gates, the gates being spaced apart, each gate modifying a corresponding magnetic mirror peak (near and far peaks) for trapping or extracting the ions from the magnetic mirror, the ions forming a ring or layer having rotational energy; a metal liner for generating by magnetic flux compression a high, time-varying magnetic field, the time-varying magnetic field progressively increasing the kinetic energy of the ions, the magnetic field from the second gate coil decreasing the far mirror peak at the end of the compression for extracting the trapped rotating ions from the confining mirror; and a disc that sharpens a magnetic half-cusp for increasing the translational velocity of the ion beam. The system utilizes the self-magnetic field of the rotating, propagating ion beam to prevent the beam from expanding radially upon extraction.

X-ray bright points and the solar cycle

Abstract: Soft X-ray filtergrams show the presence on the Sun of large numbers of small, closed regions of coronal emission. These features, called 'X-ray bright points' correspond to small, short-lived regions of emerging magnetic flux. As a function of size or lifetime they form a broad spectrum of activity which is continuous with the active regions. The shape of the Sun's activity spectrum is such that most of all magnetic flux emerging at the surface comes in the form of bright points. From this viewpoint, active regions may be viewed as the long lifetime tail end of the bright point spectrum. Examination of soft X-ray data obtained from 1970 to 1978 shows that the number of bright points appears to be anticorrelated with traditional activity indices such as sunspot number; the anticorrelation persists after corrections are made for obscuration by active regions. Comparison of X-ray data with KPNO magnetograms shows that to within a factor of 2, the average total amount of magnetic flux emerging over the full Sun is constant through the entire period of observation. The solar cycle therefore appears to be more an oscillation in the wavenumber distribution of emerging flux than of the total quantity of magnetic flux produced.

Parametric decay of extraordinary electromagnetic waves into two upper hybrid plasmons

Abstract: The effects of self-generated magnetic field in laser produced plasmas on the parametric decay of an extraordinary electromagnetic wave into two upper hybrid plasmons is examined for arbitrary magnetic field intensity and arbitrary ratio k/k0-sub 0. Due to the presence of magnetic field, the linear Landau damping is greatly reduced and the spectrum of unstable modes is significantly modified for k lambda sub D approximately 0.2.