External AC Magnetic Field Research Articles

Numerical and theoretical evaluations of AC losses for single and infinite numbers of superconductor strips with direct and alternating transport currents in external AC magnetic field

AC losses in a superconductor strip are numerically evaluated by means of a finite element method formulated with a current vector potential. The expressions of AC losses in an infinite slab that corresponds to a simple model of infinitely stacked strips are also derived theoretically. It is assumed that the voltage-current characteristics of the superconductors are represented by Bean’s critical state model. The typical operation pattern of a Superconducting Magnetic Energy Storage (SMES) coil with direct and alternating transport currents in an external AC magnetic field is taken into account as the electromagnetic environment for both the single strip and the infinite slab. By using the obtained results of AC losses, the influences of the transport currents on the total losses are discussed quantitatively.

AC Alternating-Current Loss Analyses of a Thin High-Temperature Superconducting Tube Carrying AC Transport Current in AC External Magnetic Field

Numerical simulations on the AC loss characteristics in a thin high-temperature superconducting (HTS) tube are presented. Geometry of the HTS conductor is modeled as a tube with negligible thickness, and assumed to carry a transport current with the same phase as an AC externally applied magnetic held perpendicular to the axis. Based on the classical theory of AC loss with the Bean critical current model, the distribution of critical current density jc and AC loss Q are obtained by means of numerical analysis. The results are in very good agreement with experiments. A double-peak profile is observed in the curve of the critical current density distribution along the azimuth angle. This numerical simulation method is suitable for a thin HTS tube, which may be applicable on a thin tube configuration consisting of coated superconductors.

Dynamic Simulation of High Temperature Superconductors Above a Spinning Circular Permanent Magnetic Guideway

Before a high temperature superconducting (HTS) magnetic levitation (Maglev) vehicle system can be fully applied and operational, the study of its dynamic characteristics is necessary. With the developed HTS Maglev dynamic measurement system (SCML-03), with a circular permanent magnet guideway (PMG) of 1.5 m in diameter, the vehicle’s translational motion above a PMG can be effectively simulated with the PMG allowed to rotate freely. Levitation force measurements of a high temperature superconductor (HTSC) array of seven YBa2Cu3O7−x bulks were carried out above regular (linear) and a simulated (circular) PMG. The levitation force above a linear PMG segment and a circular PMG segment in the static state is found to be in good agreement with each other. The levitation force in the dynamic state is found to slowly attenuate since the presence of a rotating circular PMG below the HTS array is found to be analogous to the application of an AC external magnetic field.

Nonlinear electrodynamics of vortex matter in hard superconductors (Review)

Nontrivial electrodynamic properties of vortex matter, which are due to a specific nonlinearity of the material equations, in hard superconductors are discussed: collapse of the transport current and the static magnetization of superconductors by an external orthogonal ac magnetic field; appearance of jumps in the time dependence of the electric field at the boundary of a sample as a result of nonlinear interaction of waves having different frequencies; specific staged penetration of an electromagnetic field into anisotropic superconductors. Nonlocal effects resulting in a large modification of nonlinear phenomena are also examined. Special attention is given to a discussion of the unique phenomenon of macroturbulent instability, associated with the flow of Abrikosov vortices, in hard superconductors. Most results presented are based on original experimental and theoretical investigations performed with the participation of the present authors.

Giant ac electrical response of La0.7Sr0.3MnO3in sub-kilogauss magnetic fields

AbstractWe report ac electrical transport in the metallic ferromagnet La0.7Sr0.3MnO3. Both ac resistance (R) and reactance (X) were measured as a function of temperature (T= 400-100 K), frequency of the ac current (f= 100 kHz – 20 MHZ) and external dc magnetic field (H= 0-100 mT) applied parallel to the current direction. It is shown that, whileR(H= 0 T) decreases smoothly around the Curie temperature (TC) forf= 100 kHz, an abrupt increase followed by a peak close toTCoccurs forf≥ 500 kHz. The peak decreases in magnitude, broadens and shifts down in temperature with increasing values ofH. The peak inRis completely suppressed underH= 100 mT resulting in a huge low-field ac magnetoresistance (R/R= -53 % forf= 2MHz) whereas the dc magnetoresistance only -31 % even at H = 7 T. While the reactanceX(H= 0 T) also shows an abrupt increase atTCforf< 10 MHz, it decreases abruptly atTCforf≥ 12 MHz. The magnetoreactance is largest (X/X= -47 %) atf= 100 kHz and it changes sign from negative to positive with increasing frequency. It is suggested that the observed huge ac magnetoresistance arises from decrease of magnetic permeability which enhances skin depth under a magnetic field. Our results indicate that the extraordinary sensitivity of the ac magnetoresistance to low dc magnetic fields can be exploited for device applications.

A flat cable with resistively joined YBCO stripes

A model of a flat AC cable with resistively joined strips of aY1Ba2Cu3Ox (YBCO)-coatedconductor was prepared. I–V curves, losses in AC external magnetic field and losses due to AC transport current at variousfrequencies up to 453 Hz were measured. The critical current of the cable at 77 K and self-field was∼160 A. It is shown that the resistive losses in the joints between the stripes do not significantlycontribute to the total cable losses exposed to an AC external magnetic field.

Influence of Work Height on Guidance Force Decay of HTS Bulk Exposed to AC Magnetic Field Perturbation in a Maglev Vehicle System

Bulk superconductors have great potential for various engineering applications, especially in superconducting maglev vehicle systems using high-temperature superconducting bulks. In such a system, the HTS bulks were always exposed to AC external magnetic field, which was generated by the inhomogeneous surface magnetic field of the NdFeB guideway. In our previous work, it was observed that the guidance force of the YBCO bulk over the NdFdB guideway used in the high-temperature superconducting maglev vehicle system was decayed by the application of the AC external magnetic field. In this paper, we investigate the influence of the work height on the guidance force decay of HTS bulk exposed to an AC field perturbation in the maglev vehicle system and try to adopt a method to suppress the decay. From the experimental results, it was found that the decay rate of the guidance force was smaller at higher work height in the case of the same field-cooled height. So we could suppress the guidance force decay of HTS bulk exposed to AC external magnetic field perturbation in the maglev vehicle system by increasing the work height of the bulk.

Fabrication and characterization of Bi2223 tapes with interfilamentarySrZrO3+Bi2212 barriers for AC loss reduction

To suppress interfilamentary coupling in(Bi,Pb)2Sr2Ca2Cu3Ox (Bi2223) multifilamentary tapes under an AC external magnetic field it is necessary toincrease the transverse resistivity by using a high-resistivity Ag alloy sheath as a matrix.Alternatively, oxide layers can be introduced as resistive barriers among the filaments,together with filament twisting. In this paper, we fabricated twisted Bi2223 tapeswith interfilamentary oxide barriers and evaluated the effect of the introductionof such a barrier on the AC magnetization loss properties in a perpendiculartransverse magnetic field. The conventional powder-in-tube method was used for tapefabrication. In order to avoid affecting the Bi2223 phase during the sintering process,SrZrO3 was selected as the barrier material. Moreover, 20 wt%Bi2Sr2CaCu2Oy (Bi2212) wasmixed with SrZrO3 to improve its ductility for cold working. Monocore Ag-sheathed rodswere coated with an oxide barrier using a slurry, before being stacked intohoneycomb structures. By twisting the filament with a twist pitch lengthbelow 9 mm and introducing interfilamentary barriers, the coupling frequency (fc) in a perpendicular field, which is inversely proportional to the decay time constant (τc) of coupling current, exceeded 100 Hz and perpendicular field losses around thepower-grid frequency were reduced from the analytical prediction for fully coupledfilaments. Avoiding physical connections among the irregular deformed filaments intwisted barrier tapes should lead to further loss reduction in a perpendicularfield.

Numerical Evaluation of AC Magnetization Loss Characteristics of HTS Coated Conductor With Magnetic Substrate Using FEM

The AC magnetization loss of a HTS coated conductor under the external AC magnetic field is analyzed using the finite element method taking account of the nonlinear <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> - <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> characteristic and the nonlinear magnetic property of a substrate at 77 K. The AC loss characteristic including the loss of the substrate under the external AC magnetic field is examined. Furthermore, we compared our numerical results of the AC magnetization loss with experimental results and examined the validity of our calculation.

Reduction of AC Losses in Bi-2223 Tapes by Introducing Interfilamentary Oxide Barriers

AC loss reduction in Bi-2223 tapes is one of the key issues for the realization of power cables and transformers. The large loss generation is mainly attributed to the strong electromagnetic coupling between the filaments in AC external magnetic field. In this paper, we fabricated twisted Bi-2223 tapes with interfilamentary oxide barrier and evaluate their AC loss properties. To suppress the side effect on Bi-2223 phase formation during sintering, Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> CuO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> or SrZrO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> powders were used as barrier materials. Furthermore, small amounts of Bi-2212 powders were mixed with them to improve their ductility for cold working. AC loss characteristics were examined at 77 K in parallel and perpendicular transverse magnetic fields with various field amplitudes and frequencies. The effect of introducing different oxide barriers on AC loss reduction at different operating condition is discussed.

Suppression of Influence of AC External Magnetic Field on Trapped Magnetic Flux in the HTS Bulk by Use of HTS Shielding Rings

Trapped magnetic fields in high temperature superconductor (HTS) bulks are decayed when the bulks are subjected to perturbation of magnetic fields. This phenomenon is inconvenient for the applications of HTS bulks and it is necessary to develop methods to suppress the decay. It was found in a previous study that the decay is caused by temperature rises of the bulks due to AC losses in the bulks. Therefore, it is necessary to develop a method to reduce the AC losses. In this work, a method is proposed to reduce the AC losses by putting HTS shield rings around the bulk and shielding performance of the rings were investigated. The experimental result shows that the shielding rings can drastically suppress the decay, even if the bulk is used in the same cooling condition and the effectiveness of the method was demonstrated.

Reduction of AC losses in Bi2223 tapes with interfilamentary oxide barriers

In order to suppress the interfilamentary coupling under AC external magnetic field, increasing the transverse resistivity by using Ag-alloy sheath with high resistivity as a matrix or introducing oxide layers as resistive barriers among the filaments should be necessary, together with the filament twisting. In this paper, we fabricated twisted (Bi,Pb) 2Sr 2Ca 2Cu 3O x (Bi2223) tapes with interfilamentary oxide barriers and evaluated the effect of barrier introduction on AC loss properties in a perpendicular transverse field. The conventional powder-in-tube method was used for tape fabrication. To avoid the side effect on Bi2223 phase formation, SrZrO 3 was selected for barrier materials. In addition, 20 wt% Bi2212 was mixed with SrZrO 3 to improve its ductility for cold working. Monocore Ag-sheathed rods were dip-coated by the oxide barriers with slurry before stacking with multifilamentary structures. By applying both the tight twisting with twist pitch below 10 mm and introducing interfilamentary barriers, the coupling frequency in a perpendicular field was increased above 100 Hz and perpendicular field losses were reduced from the analytical prediction for fully coupled filaments around power-grid frequency range.

Effect of filament twist on the structure and properties of Ag-sheathed Bi2223 tape

Ag-sheathed Bi2223 tapes show good performances and are ready for many prototype applications. However, drastic reduction of AC loss is strongly required for practical AC power applications. To reduce the AC loss under AC external magnetic field, the introduction of filament twisting with high resistive barrier is effective, but the filament twisting tends to decrease the J c value. This decrease in J c is a problem to balance low AC loss with high I c value. In this study, the effect of filament twisting on the microstructure and J c property was investigated to overcome the J c reduction. The Bi2223 tapes sheathed with Ag–Cu alloy were fabricated with a standard PIT method. Although the formation of Bi2223 phase was not affected by filament twisting from the XRD analysis and SEM observation, the J c values of the twisted tapes decreased about 30% compared to that of non-twisted tape. In the case of the same twist pitch, the sample rolled under large pass reduction ratio showed relatively high J c value. This high J c would be caused by the high aspect ratio of the filament of those tapes, in which the grain alignment along the filament interface would be well. On the other hand, a reason for the J c reduction of twist tapes is proposed to be the misorientation at grain boundaries of Bi2223 grains in the transference region near the edges of the tape, where the filaments move up and down in the thickness direction.

AC loss characteristics of a hex-cable in an azimuthal external AC magnetic field

Recently, power transmission cables made of a high temperature superconductor (HTS) are being developed, showing progress in field tests on a power network of distribution level. The HTS power cable consists of a conductor and a shield, which are composed of dozens of HTS tapes. In addition, the HTS cable with a large current capacity has a multi-layered structure in both the conductor and the shield. In such a complicated HTS cable with a multi-layered structure, the AC loss estimation is very difficult and thus not cleared yet. In this study, a hex-cable sample was prepared by arranging six Bi-2223(Bi 2Sr 2Ca 2Cu 3O 10) tapes on the surface of a normal copper-rod in order to experimentally investigate the AC loss characteristics of an HTS cable. The AC loss characteristics of the Bi-2223 tape in the hex-cable were estimated in different transport modes of the tape. The AC loss characteristics of the hex-cable were also examined in different cable transport modes in order to experimentally simulate the various transport situations of the conductor and the shield in an actual HTS cable. The measured AC loss of Bi-2223 tape in the hex-cable strongly depends on the transport mode. In particular, the measured AC losses for the simultaneous mode increase as the arrangement of tapes approaches a polygonal shape. The measured AC loss when the current of the same direction flows in the hex-cable and the copper-rod is about two times larger than the transport loss. On the contrary, it is almost half of the transport loss for the current flowing in the opposite direction, especially in the region of high currents.

Effect of AC Magnetic Field on the Levitation Force of YBCO Bulk above NdFeB Guideway

In the present High Temperature Superconducting (HTS) maglev vehicle system, the nonuniformity of the magnetic field along the movement direction above the NdFeB guideway is inevitable due to the assembly error and inhomogeneous of the material property of the NdFeB magnet. In order to investigate the influence of the nonuniformity on the levitation performance of the HTS bulk, an electromagnet supplied by AC current is used to simulate the nonuniformity of the external magnetic field. The levitation force of the HTS bulk is measured when applying AC currents to the electromagnet coils. Experimental results indicate that the levitation force changes abruptly and then oscillates after applying AC external magnetic field, and the levitation force is attenuated by the AC magnetic field after withdrawing the AC field. Moreover, the oscillation amplitude and the attenuation rate of the levitation force increase with the amplitude of the AC external magnetic field.

Quantum dynamic simulations for single molecular magnets using anisotropic spin models

We performed quantum spin dynamics in anisotropic electron-spin systems to clarify the mechanism in single molecular magnets (SMM) theoretically. Usually, SMMs have large negative anisotropic D value and the large total spin angular moment ( S total). The splitting ground spin states in one SMM molecule are usually caused by the anisotropic terms ( D and E), which become more important as well as isotropic term ( J). In order to examine the effects of such anisotropic magnetic terms, model systems with large spin multiplicity of the ground state ( S total = 5 and 2) were treated. First, energy diagram in external magnetic field is shown, since negative D terms are responsible to the zero-field magnetic moment. Next, the quantum Liouvile approach is used for our spin dynamics simulation involving the relaxation processes of the system. Our simulations in S total = 5 system successfully reproduced magnetic quantum tunneling (MQT) behavior in DC-external magnetic field. Moreover, in S total = 2 system under AC-external magnetic field, frequency dependence of real ( χ′) and imaginary ( χ″) parts of χ can be shown with Fourier transformation. Our theoretically drawn half circles in Cole-Cole plot ( χ′ versus χ″) will give information to many experimental studies.

Novel and efficient MR active aqueous colloidal Fe3O4 nanoassemblies

This work demonstrates a new class of magnetic resonance (MR) active aqueous Fe3O4 magnetic nanoparticle nanoassemblies (Fe3O4 MNNA) of ∼40 nm size comprising ∼6 nm particles. They exhibit enhancement in T2 MR contrast as compared to 6 nm isolated counterparts (Fe3O4MNP) and commercial contrast agent, ferumoxytol. This significant improvement in the T2 MR signal arises from the synergistic magnetism of multiple Fe3O4nanoparticles assembled in Fe3O4 MNNA. These nanoassemblies also show better colloidal stability, higher magnetization, good specific absorption rate (under external AC magnetic field) and cytocompatibility with cells. Further, the functional groups (–NH2) present on the surface of Fe3O4 particles can be accessible for routine conjugation of biomolecules through well-developed bioconjugation chemistry. Specifically, a new MR active colloidal amine-functionalized Fe3O4 nanoassembly with enhanced T2 contrast properties has been fabricated, which can also be used as an effective heating source for hyperthermia treatment of cancer.

Revealing the Anisotropy in MgB2 Superconductor and Investigation of Critical Current near Critical Temperature by Using Hall Probe Susceptibility Technique

We report the results of an investigation on critical current measurements in bulk MgB2 superconductor by using Hall probe ac susceptibility. The temperature versus ac susceptibility has been measured as a function of external ac magnetic field with no dc part and magnitude in the range 240–1200 A/m by using the Hall probe method. The real part of the Hall probe susceptibility showed two-step transitions near critical temperature. We attributed this behavior to the anisotropic nature of the MgB2 compound. Close to the critical temperature the estimated anisotropy parameter is γ≈2.5, a value that is in fair agreement to the ones reported in the literature for polycrystalline and single crystals MgB2. We have also analyzed the susceptibility data within the framework of a critical state model (using the real part of the susceptibility) and an anisotropic nature of MgB2 compound. The inter-grain critical current is found to be 4×106 A/m2 at 39.59 K by using the critical state model. The temperature dependence of the critical current density was found to be in the form of J c (T)∝(1−T/T c ) β , where β was calculated as 2.30 by using critical state model, respectively. Scanning electron microscopy was used for grain size estimation.

Suppression of guidance force decay of HTS bulk exposed to AC magnetic field perturbation in a maglev vehicle system

Superconducting maglev vehicle was one of the most promising applications of HTS bulks. In such a system, the HTS bulks were always exposed to AC external magnetic field, which was generated by the inhomogeneous surface magnetic field of the NdFeB guideway. In our previous work, it was observed that the guidance force of the YBCO bulk over the NdFdB guideway used in the high-temperature superconducting maglev vehicle system was decayed by the application of the AC external magnetic field. In this paper, we adopted a method to suppress the decay by altering the field–cooled height of the bulk. From the experimental results, it was found that the decay rate of the guidance force was smaller at lower field–cooled height. So we could suppress the guidance force decay of HTS bulk exposed to AC external magnetic field perturbation in the maglev vehicle system by reducing the field–cooled height of the bulk. Furthermore, all the experimental results in this paper were explained based on Bean critical-state model.

Numerical Considerations About Using Finite-Element Methods to Compute AC Losses in HTS

In this paper, we investigate the influence of various numerical parameters on the precision and the speed of ac loss computations in high-temperature superconductors using the finite-element method. The case considered here is an infinite slab subjected to an external ac magnetic field. This problem can be modeled by a 1-D partial differential equation (diffusion equation). This relatively simple case allowed investigating the influence of the various parameters in a reasonable time. The findings of this work can be used as a starting point for optimizing the numerical settings of more complex models. As the main results, it is shown that choosing first-order elements for approximating the flux density ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</i> ) is the most stable option, and that using high order adaptive time-stepping methods provides good accuracy and fast simulations. A new and simple self-check test for validating the computed ac losses is also proposed. Finally, a detailed analysis about the behavior of the numerical solution near flux/current fronts is provided.