Shielding Currents Research Articles

Laser-generated ultrashort multimegagauss magnetic pulses in plasmas.

We demonstrate ultrashort (6 ps), multimegagauss (27 MG) magnetic pulses generated upon interaction of an intense laser pulse (10(16) W cm(-2), 100 fs) with a solid target. The temporal evolution of these giant fields generated near the critical layer is obtained with the highest resolution reported thus far. Particle-in-cell simulations and phenomenological modeling is used to explain the results. The first direct observations of anomalously rapid damping of plasma shielding currents produced in response to the hot electron currents penetrating the bulk plasma are presented.

Theoretical analysis on ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands

The ac loss properties of two-strand parallel conductors composed of superconducting multifilamentary strands were theoretically investigated. The constituent strands generally need to be insulated and transposed for the sake of uniform current distribution and low ac loss. In case the transposition points deviate from the optimum ones, shielding current is induced according to the interlinkage magnetic flux of the twisted loop enclosed by the insulated strands and the contact resistances at the terminals. It produces an additional ac loss. Supposing a simple situation where a two-strand parallel conductor with one-point transposition is exposed to a uniform ac magnetic field, the basic equations for the magnetic field were proposed and the theoretical expressions of the additional ac losses derived. As a result, the following features were shown. The additional ac loss in the non-saturation case, where the induced shielding current is less than the critical current of a strand, is proportional to the square of the magnetic field amplitude and the square of deviation length in transposition and has a Debye-type frequency dependence. On the other hand, in the saturation case, the additional ac loss is nearly proportional to the field amplitude and the deviation length in transposition. It has no frequency dependence at a higher frequency than the critical frequency.

High-sensitivity atomic magnetometer unaffected by spin-exchange relaxation.

Alkali-metal magnetometers compete with SQUID detectors as the most sensitive magnetic field sensors. Their sensitivity is limited by relaxation due to spin-exchange collisions. We demonstrate a K magnetometer in which spin-exchange relaxation is completely eliminated by operating at high K density and low magnetic field. Direct measurements of the signal-to-noise ratio give a magnetometer sensitivity of 10 fT Hz(-1/2), limited by magnetic noise produced by Johnson currents in the magnetic shields. We extend a previous theoretical analysis of spin exchange in low magnetic fields to arbitrary spin polarizations and estimate the shot-noise limit of the magnetometer to be 2x10(-18) T Hz(-1/2).

Magnetic circuit switching element with superconducting coil

In an existing power system, high-current circuit breakers are widely used. Meanwhile, we propose a new switching device using a switching of magnetic paths. It is a magnetic circuit switch activated by on/off of superconducting coils wound on iron core-legs. We have carried out an experiment using a small experimental device with two superconducting coils (control coils). When the control coil is shorted, the voltage of the output coil in the shorted side becomes 0 V because the magnetic flux is blocked completely by the induced shielding current of the superconducting coil. It was confirmed that the current of the superconducting coil is decided by the turn-ratio of the coils both experimentally and theoretically. Therefore, it was assured that any tremendous overcurrent does not flow in the shorted superconducting control coils. After this, we applied this device into a model of two-circuit transmission system and confirmed the fundamental behavior in case of a line fault. The experimental result shows that the fault current is suppressed almost perfectly and the transmission system can continue the power transmission without any intermission.

Critical current across grain boundaries in melt-texturedYBa2Cu3O7−δrings

We determine the critical current ${J}_{\mathrm{cB}}$ of grain boundaries fabricated by ``welding'' of melt-textured YBCO with various degrees of $\mathbf{ab}$-plane misalignment. ${J}_{\mathrm{cB}}$ is determined by monitoring the magnetic moment due to persistent shielding currents, which were induced in rings containing two sections of a single grain boundary. The voltage drop across the junctions is estimated to be below ${10}^{\ensuremath{-}12}\mathrm{V},$ much smaller than in typical transport measurements. As the temperature or magnetic field is increased, an abrupt decrease (kink) is observed in the magnetic moment of the ring when the induced shielding current in the ring exceeds the critical current across the boundaries. The kink signals that flux has begun to penetrate through the grain boundaries into the bore of the ring. This behavior is confirmed by magneto-optical imaging. We observe that grain boundaries with [001] tilt mismatch angles up to 5\ifmmode^\circ\else\textdegree\fi{} have critical current densities in excess of ${10}^{4}{\mathrm{A}/\mathrm{c}\mathrm{m}}^{2},$ comparable to the bulk current density. At larger mismatch angles, the critical current rapidly decreases with increasing angle. The qualitative behavior of ${J}_{\mathrm{cB}}$ as function of temperature, magnetic field, and misorientation angle is similar to that observed YBCO grain boundaries manufactured by other methods.

Supershielding: Confinement of Magnetic Fields

In open-shield systems, we have derived current conditions that lead to remarkable magnetic-field confinement, which we call ``supershielding.'' The freedom to vary both primary and shield currents is important in solving the conditions. Solutions leading to very effective shielding are illustrated by the simple system of two parallel flat strips. Generalizations include other geometries, internal shielding, and electric-field confinement. An industrial coil design serves as a reference for practical issues. We discuss mathematical limits, via a ``seesaw'' approach, to ``perfect shielding.''

A large scale approach of bulk HTS to the electric utility area

On the basis of a novel industrial-like YBCO seedless functional melt texture grain alignment process, various HTS devices for superconducting magnetic bearings (SMB) and high-current leads for transport and limiting purposes are fabricated. Critical shielding currents of 30 kA/cm/sup 2/ at 77 K are utilized to construct radial-passive magnetic bearings for 20 kg loads or to suspend and spin a O 4 cm rotor safely to 120000 rpm. Stiffnesses between 40 to 140 N/mm are measured. Multi-grain YBCO rods up to 20 cm length are tested for more than 6 kA/cm/sup 2/ transport currents. The connections to the copper braids are improved to less than 0.4 micro-ohm at 1000 Amperes and 77 K. The experiments show the practical potential of melt textured YBCO with macroscopic orientation of the grain structure for electric power purposes.

An analysis of normal‐mode noise caused by braided shield current flowing on coaxial cable attached by a ferrite core

Since coaxial cables have excellent electromagnetic shielding characteristics, they are widely used as interconnecting cables for digital information equipment. However, since braided lines are used in the shield layer of most coaxial cables, if an RF current flows, a noise voltage (normal mode noise) is induced between the core wire and the braided wire by electromagnetic coupling. Electromagnetic noise is also radiated from the common mode current of the braided shield. It has been considered that ferrite core loading is effective for reduction of the electromagnetic noise, and such loading is frequently used. In this article, in order to study whether ferrite core loading is effective in reducing normal mode noise, the normal mode noise generated by the braided shield current is analyzed with respect to the core loading location. Next, a formula for the noise-reduction effect due to ferrite core loading is derived and its validity is demonstrated experimentally. © 1998 Scripta Technica, Electron Comm Jpn Pt 1, 82(2): 1–10, 1999

An analysis of normal-mode noise caused by braided shield current flowing on coaxial cable attached by a ferrite core

Since coaxial cables have excellent electromagnetic shielding characteristics, they are widely used as interconnecting cables for digital information equipment. However, since braided lines are used in the shield layer of most coaxial cables, if an RF current flows, a noise voltage (normal mode noise) is induced between the core wire and the braided wire by electromagnetic coupling. Electromagnetic noise is also radiated from the common mode current of the braided shield. It has been considered that ferrite core loading is effective for reduction of the electromagnetic noise, and such loading is frequently used. In this article, in order to study whether ferrite core loading is effective in reducing normal mode noise, the normal mode noise generated by the braided shield current is analyzed with respect to the core loading location. Next, a formula for the noise-reduction effect due to ferrite core loading is derived and its validity is demonstrated experimentally. © 1998 Scripta Technica, Electron Comm Jpn Pt 1, 82(2): 1–10, 1999

Twist effect on hysteresis loss in Bi2223 multifilamentary wires exposed to an AC magnetic field

We fabricated Bi2223 multifilamentary sample wires with various twist pitches and investigated the electromagnetic properties experimentally. They showed monofilamentlike electromagnetic properties regardless of twisting due to the contacts among filaments and/or proximity effect. The observed AC losses in the non-twisted sample wire agreed roughly with the theoretical prediction for a homogeneous superconducting slab with the same thickness of the filamentary region on the basis of Irie–Yamafuji model. However the AC losses in the twisted wires deviated from the theoretical ones, especially for the amplitude around the theoretically predicted penetration field of the slab. We showed that the observed AC loss properties can be explained by both the twist effect for the macroscopic shielding current and the contribution of the local shielding current.

Inclusion of shielded Birkeland currents in a model magnetosphere

We have developed a technique for including large‐scale magnetospheric current systems in magnetic field models. In this paper, we incorporate the Region I and Region II Birkeland current systems into the Source Surface Model (SSM) of the terrestrial magnetosphere. The original SSM was a prescribed‐magnetopause model that used a spherical harmonic expansion and a variational principle to obtain the magnetic field in the near‐Earth region. Field lines in the tail were constructed geometrically. This model is ideal for studying the effects of Birkeland currents since it has a well‐defined boundary between open and closed field lines, magnetopause shielding currents separate the geomagnetic and interplanetary magnetic fields in the absence of a specified connection between the two, and Birkeland currents are not included in the model either implicitly or explicitly. The Region I and II Birkeland currents are added to the model using a series of field‐aligned, infinitely thin wire segments. The magnetic field produced by these currents is calculated from the Biot‐Savart law and its normal component on the surface of the magnetopause is shielded (minimized) by image currents carried on wires placed outside the magnetosphere. The result of these added currents is a sunward shift of the separatrix between open and closed field lines. Since Region I currents are regarded here as flowing on the separatrix, this shift necessitates an iteration of our procedure. Three iterations were required to reach a satisfactory solution in our case. We find that inclusion of Birkeland currents in the SSM results in a northward Bz in the near‐midnight tail region, and a southward Bz at the flanks. The closure of previously open flux near midnight accounts for the sunward shift of the separatrix. Moreover, the sunward shift is found to increase with increasing Birkeland current strength by an amount that agrees with observations of the auroral oval.

Low-critical-current applications of high-Tc superconductors

The short-term large-scale applications of high-Tc superconductors are not limited only to the high critical-current ones (electrotechnical, levitation). Several low-critical-current applications are possible. Some, which already have industrial potential, are reviewed in this paper: magnetic shielding, thermal shielding and magnet current leads, all making use of extended thick ( > 50 μm) films. The state-of-the-art of the results is briefly summarized.

Principles in transmission line magnetic field reduction

The basic principles of transmission line magnetic field generation are addressed in order to enhance understanding when designing for low fields. For structured analysis, the B-field RMS-value is represented as a product of autonomous factors. The range of applicability spans phase-split arrangements, active shielding by impressed shield currents, and passive shielding using induced currents with and without series compensation of the shield-conductor loop. An innovative four-conductor scheme is presented.

Experimental evaluation of the total shielding current in a bulk high-Tc superconductor [YBaCuO

Total shielding currents of a melt processed bulk high-Tc superconductor (HTS) are evaluated from the measured voltage of a Rogowski coil when a permanent magnet is moved towards the HTS. The total shielding current and the levitation force between the magnet and the HTS show rapid growth and decay depending on the speed of the magnet. The dynamic levitation force and the magnetic relaxation are discussed from the experimental results.

Paramagnetic Meissner effect of high-temperature granular superconductors: Interpretation by anisotropic and isotropic models

The field-cooled magnetization of high-Tc superconducting ceramics measured in low magnetic fields exhibits the paramagnetic Meissner effect (PME), i.e., the diamagnetic signal initially increases with decrease in temperature but reaches a maximum at temperatureTd and later decreases with decrease in temperature. Even in some samples the signal is ultimately able to transform inversely into a paramagnetic regime once the sample is cooled below a temperatureTp as long as the applied field is sufficiently small. This PME has been observed in various high-Tc cuprates and is explained by disparate aspects. An anisotropic model, in which the granular superconductors are assumed to be ideally anisotropic, was first alternatively proposed in the present work so as to theoretically account for this effect. On the other hand, an isotropic model, suitable for granular superconductors with randomly oriented grains, was proposed to deal with the samples prepared by a conventional solid-state reaction method. The anomalous magnetization behavior in the present model was demonstrated to be the superposition of the diamagnetic signal, which occurs as a result of the intragranular shielding currents, over the paramagnetic one due to the induction of the intergranular component induced by these currents where the intergranular one behaved as the efective pinning centers. The PME was demonstrated by this model to exist parasitically in granular superconductors. This intergranular effect is therefore worthy of remark when evaluating the volume fraction of superconductivity for the samples from the Meissner signal, in particular, at a low magnetic field.

Experimental investigations of the currents collected by metal shields in high-current vacuum arcs

This paper presents experimental results of currents collected on the three-element condensation shield connected to the cathode potential in high-current vacuum arcs. The arc current had 900 Hz, 150 Hz, or 50 Hz half-cycle sinusoidal shapes and was conducted between the CuCr40 contacts with a diameter of 50 mm (cathode) and 30 mm (anode) spaced 10 mm apart. Most of the measurements were made for the current of 900 Hz with peak values up to 9 kA. Arc voltage, floating shield potential, and distribution of shield currents were measured. It was found that the current collected by the shield and also the arc voltage and floating shield potential are greater for higher frequency currents, and that they are affected by the arc mode. Considerable shield current is observed during a high-amplitude (HA) oscillation sequence of arc voltage while its mean value is increased. For 900-Hz arcs at the 9-kA peak value (I/sub am/), the ratio of shield current (i/sub s/) to an instantaneous value of arc current (i/sub a/) reaches even 40% near I/sub am/ value. In the initial half-cycle period (before the initiation of high-voltage oscillation), the ratio of i/sub s//i/sub a/ increases with i/sub a/ and current frequency. A close relationship was found between arc voltage and current distribution on a three-element shield.

Interference between shielding and trasport currents in Bi 2Sr 2CaCu 2O x single crystals

Current-voltage characteristics of Bi 2Sr 2CaCu 2O x single crystals have been studied by pulsed current method. At low temperatures (T<12K for H=20kOe, H // c) the voltage for a current along the c-axis is dependent on magnetic prehistory. This effect is explained by an interference between shielding currents within the ab plane and transport current along the c-axis.

Surface currents and bulk pinning in Bi 2Sr 2CaCu 2O 8

The mechanisms giving rise to the irreversibility line (IL) in the phase diagram of Bi 2Sr 2CaCu 2O 8 and the mechanism behind the second maximum in the magnetization loops were determined using a novel Hall-sensor array. The IL is caused by bulk pinning at lower temperatures and by irreversible shielding currents above 22K. The second maximum is caused by vortex lattice melting, taking place at the same local magnetic induction B m throughout the sample.

Shielding current density in Mg-doped YBCO-123 as determined by a.c. magnetic-susceptibility measurements

High-resolution a.c. magnetic-susceptibility measurements have been performed on sintered samples of Mg-doped YBCO superconductors YBa2(Cu1−xMgx)3O7−δ (δ≤0.08, 0≤x≤0.1). An estimation of the intergranular shielding currents Js (T, Hd.c.) has been obtained by separating the grains and coupling-junctions contributions to the measured susceptibility. The results show that Mg doping increases the weak links weight and reduces the values of Js. At Hd.c.=0, Js (T, 0) follows a power law of the type (1−T/Tc)β, with β varying from 2 (undoped sample) to about 5 (x=2.5%). The dependence of Js on the applied d.c. magnetic field can be fitted to a Lorentzian lineshape whose width appears to be independent of the substitution level.

Magnetic instability in HTSC films

We have investigated the short time dynamics in the magnetic flux distribution of thin YBa 2Cu 3O 7-x films by means of time resolved magnetooptics. Local heating of the superconductor with a focussed laser pulse in a region with high shielding currents leads to an instability in the magnetic flux distribution. A pump probe technique allowed to visualize changes in the flux distribution on a nanosecond time scale. The instability develops in two steps giving rise to additional flux penetration and to a branching of the flux front into the superconductor.