Search

The Japanese national project started on standardization of the test method of the DC critical current of superconducting cables. A round robin test of critical current measurement is carried out to survey underlying problems for the standardization. The sample cables are made of Bi-2223 tapes and the inner core conductor has two layers and the outer shield conductor is composed of a single layer. The diameter of the shield layer is about 40 mm and the length of the cable is 2.00 m. The critical current was determined by the electric field criterion of 1 μV/cm. The n value is also measured for the inner core and the outer shield conductor. Three institutes participated in the test for two sample cables. The obtained experimental relative standard deviation for the critical current at 77.30 K is in the range of 0.2 to 1.3%, and is sufficiently small. Hence, it can be said that the present method that is widely used is suitable as a standard test method. On the other hand, the experimental relative standard deviation of the value is in the range of 2.8 to 18.6%, which is relatively large. This is caused by the measurements done at different temperatures.

Innovative dc superconducting cables with higher current-carrying capacity than conventional cables were proposed in previous works. The superior property is brought about by the longitudinal magnetic field that enhances the critical current density. In this work, the current-carrying capacity of a three-layer cable composed of Bi-2223 tapes was theoretically calculated based on the measured critical current density in the longitudinal magnetic field. The obtained current-carrying capacity of the proposed cable is 13.2% higher in comparison with the conventional cable of the same size.

We proposed the innovative force-free superconducting cable with higher current-carrying capacity than conventional superconducting DC cables utilizing the longitudinal magnetic field effect that enhances the critical current density. That is, the critical current density can be enhanced by the longitudinal magnetic field that the conductor itself and the outer shield conductor produce. It means that, if the longitudinal magnetic field can be reduced, the critical current density is appreciably reduced, resulting in an accelerated resistive transition. Such situation can be realized by arranging the copper conductor layers in a suitable structure that transport currents when the superconductors are in resistive state. We propose to arrange the copper conductor layers in the inner and outer conductors to reduce the longitudinal magnetic field by twisting in opposite directions to respective superconducting layers in each conductor region. Various conditions of fault-current limiting can be achieved by arranging the winding angle of the copper layers. Discussion will be given on the fault-current limiting properties of the proposed cable.

The improvement of critical current density properties of REBCO coated conductors for applications in superconducting power devices is desired. It is known that the critical current density Jc of REBCO thin films made by the TFA-MOD method in a magnetic field is greatly improved by the introduction of BZO-nanoparticles of a non-superconducting phase as artificial pinning centers. In addition, the field angle anisotropy can also be reduced by isotropic flux pinning interactions of nanoparticles. However, a detailed report on the mechanism of flux pinning with a quantitative investigation in such coated conductors has not been given. In this study, the field angle dependence of the critical current density is measured for TFA-MOD-processed YGdBCO coated conductor with a superconducting layer of 0.65 μm thick, and the results are theoretically analysed to clarify the flux pinning mechanism.

The MgB2 coil wound with a parallel conductor composed of two MgB2 tapes with insulation has been developed. The tape is obtained by the rolling process from the round wire which is fabricated by in-situ method. The performances of tapes, such as critical current density and ac losses, are higher than that of round wires in case that the transverse magnetic fields are applied to the tape in direction of the parallel to the flat face of the tape. We proposed as a candidate of applying the tape to a conductor with large current capacity that the parallel conductor is composed of insulated tapes. We fabricated a tape with aspect ratio of two from a round wire with diameter of 0.8 mm as a strand for the conductor. The coil is composed of two coils that are jointed each other in series. At the joint, two strands are transposed to be identical the inductances of two strands. The inner and outer diameters of the coil are 65 mm and 107 mm, respectively. The length of the coil is 53 mm. The total turn number is 540 turn. The magnetic flux density at the center in the coil is 1.31 T when the transport current is 200 A. In this paper, the design and the test results of the MgB2 coil wound with the conductor are shown and the critical current properties and ac loss properties of the coil are discussed.

We have proposed MgB2 multifilamentary tapes with high critical current densities and low ac losses under external magnetic fields which is parallel to the tape face. The purpose of this proposal is to develop the MgB2 new conductors composed of plural tapes as windings of superconducting coils with both large current capacities and low losses for fusion reactors, superconducting magnetic energy storage systems, and so on. Our MgB2 multifilamentary tapes are fabricated by rolling process from round wires. The tapes are fabricated by using in-situ method. The purpose of this paper is to clarify the effects of deformation to tape shape on the electromagnetic properties of MgB2 filaments in multifilamentary wires. For the purpose, measurements of both critical currents and magnetizations have been carried out on MgB2 fine single filament wires which have simulated a filament in multifilamentary wires. Measurements on wires with large diameter have been also carried out, in order to clarify the effect of diameters of original round wires. We prepared pure and SiC-added MgB2/Nb/Cu composite wires as the original round wires before rolling process. The diameters of round wires have been varied and then the aspect ratios of tapes have been varied. The observed critical current densities of the tapes are about 7 times larger than that of the original round wire at a maximum. The ratios of Jc of wires to tapes don't have dependence of the diameter of the original round wire. In addition, the results of measurements of magnetizations have shown that magnetization losses in MgB2 tapes decrease to 1/3 compared with original round wires.

Flux pining properties of columnar defects nucleated by gold ion irradiation was investigated for DyBCO coated conductor. The energy of irradiated gold ions was 320 MeV and 200 MeV and the matching field was 1.0 and 2.0 T, respectively. It was found that the flux pinning strength was significantly enhanced at high magnetic fields. These experimental results were analysed using the summation theory of the elementary pinning forces with the theoretical model of flux creep and flow. It was found that the experimental results can be explained by the theories. The obtained result suggests that columnar defects with larger radius are more effective for pinning because of the larger probability of catching flux lines.

Because of pick-up coil's flat design, relatively low operation frequency (/spl sim/ 23 MHz) and high relative resolution (10/sup -6/) a single-layer Open-Flat Coil-based tunnel diode oscillator (OFC-magnetometer) has advantages, compared to other test methods. They become crucial at nondestructive studies with thin flat HTS materials (with small signals), especially near T/sub c/ (at start of the 'Cooper' pairs' formation). Due to these a new 'Para-Magnetic' peculiarity of the normal-to-superconductive (N/S) transition was detected recently in YBaCuO films. It precedes the 'Meissner' ejection and specifies details of an actually complicated shape of the N/S transition curve. We discuss influence of the currents and magnetic field on the PM effect and relationship between the quality of HTS material and amplitude of the effect. Differences between PM effect and externally close 'paramagnetic Meissner' and resistive 'peak' effects are discussed too.

The critical current density and the irreversibility field were measured before and after nickel ion irradiation for Bi-2212 single crystals in an optimally doped or an overdoped state in a magnetic field parallel to the direction of irradiation. The critical current density was theoretically calculated by using the summation theory and the flux creep-flow model. From the comparison with experimental results the condensation energy density was estimated for each specimen. These results are compared with a similar analysis on three-dimensional Y-123.

We have investigated extended electric field-vs.-current density E-J characteristics in YBCO coated IBAD tapes. The results of a Monte-Carlo study on the depinning in a random pin medium were compared with measurements. Using a low temperature scanning laser microscope, we examined the percolative behavior of the local resistive transition in the YBCO tape. It was also shown that the depinning probability, which is proportional to the dynamic resistance of the E-J curves, is scaled as a function of reduced current density with the aid of a power index. Consequently, the E-J characteristics in the tapes can be described by the combination of the two kinds of scaling laws: one is the scaling law of the depinning probability predicted in a random network and the other is the scaling law for the flux pinning force. These properties agree well with the percolation model of depinned clusters.

The energy loss density was measured for a Bi-2223 tape wire with superconducting filaments of average thickness 2.5 /spl mu/m to confirm the reduction of energy loss density due to the reversible fluxoid motion. The energy loss density is compared with the prediction by the modified Kim model. It is lower than the prediction at AC field amplitudes below the penetration field at temperatures above 77 K. The slope of the minor magnetization curve is less than unity and the estimated AC penetration depth is longer than the filament thickness in the same regime. This result supports the speculation that the reduction of the energy loss density is due to the reversible fluxoid motion.

The current-voltage curves are measured for a dip-coated Bi-2212 tape wire at various temperatures under the magnetic field parallel to the c-axis. It is found that the current-voltage curves are approximately scaled on two master curves by normalizing as predicted in the vortex glass-liquid transition theory. However, the obtained dynamic critical index was about 3 and too small in spite of the two-dimensional flux line system. These results are compared with the theoretical analysis based on the flux creep-flow model taking account of the distribution of pinning strength. It is found that the theoretical result approximately explains the experimental result on the scaling behavior, the critical indices and the transition line.

In order to assess the magnetic flux distribution in type II superconductors, the correlation function was derived from SANS intensities. The distribution in Nb single crystal was consistent with the result of magnetization measurements. An existence of short range ordering in the magnetic flux arrangement in Bi2212 crystal was suggested at 7.7 K under magnetic field of 0.25 T, which is a sufficiently high field larger than the critical field. >

The weak link property in a QMG (quench and melt growth)-processed single-grain Y-Ba-Cu-O specimen was investigated by measuring the dependence of the critical current density on temperature, history of magnetic field application, and its enhancement under a longitudinal field geometry. The results show that weak-link regions still remain in the specimen. It is speculated that these link regions, which can transport superconducting currents at low temperatures, are degraded with increasing temperature and lose superconductivity around 20 K. >

A marked enhancement of global pinning strength in Nb-50 wt.%Ti alloy was obtained by introducing ribbon-shaped artificial pins. Simultaneously, however, depression of B/sub c2/ was observed. This behavior is analyzed theoretically, and a satisfactory explanation of the experimental results is obtained. >

It has been found that the critical current density, J/sub c/, in high-T/sub c/ oxide superconductors is reduced to zero at high temperatures and/or high magnetic fields even in a superconducting region. The irreversibility line was investigated in weakly pinned superconducting Nb-Ta with low T/sub c/ and was compared with that in oxide superconductors. Large flux flow noise with a peak around 0.3 Hz was observed in Nb-Ta specimens. The noise power integrated in the frequency range of 0.1-1.0 Hz had a sharp peak, when it was plotted against the magnetic field. The mechanism of such a characteristic noise is examined.

It has been recognized that BaHfO3 (BHO) pinning centers introduced into GdBa2Cu3O6 +y (GdBCO) coated conductors show high pinning performance at high magnetic fields. This pinning property was ascribed to the strong pinning force of the BHO nano-rods and also to the improved upper critical field, Bc2, probably due to the scattering of electrons by the interface between the nano-rods and the superconducting matrix. In this paper the critical current properties of GdBCO coated conductors with different amounts of BHO nano-rods are theoretically investigated. It is found that BHO nano-rods are promising to improve the high field performance of GdBCO coated conductors.

AC losses are measured applying magnetic fields of frequencies from 8.6 to 75 Hz and of 3 kG peak values to wires of Nb-60.2Ti-2.8Zr alloy mounted in slots of a bakelite bobbin. The temperature is varied between 1.9 K and the superconducting transition temperature T c = 8.9 K using supercritical He as coolant for the higher temperature region. Anomalous increments of AC losses caused by flux jumps are observed at certain field values H fj . H fj is almost constant at temperatures T = 4.2 to 6 K and decreases if the temperature is reduced below 4.2 K. The temperature dependence of H fj can be explained qualitatively by the temperature depencence of both the specific heat and the pinning force of the sample. The quantitative difference as well as a very weak frequency dependence of H fj and a reduction of H fj by embedding the sample in grease may be due to the influence of the special heat transfer conditions.

Critical-current densities of superconducting oxides are theoretically estimated for single-crystalline thin films and polycrystalline bulk materials with oriented and random textures. The percolation theory is used, and the effect of depression of the transport current through grain boundaries is taken into account. The results are compared in detail to previously published experimental results. >

AC losses, flux jumps and quench behaviour of copper-clad single core wires of Nb-50%Ti are investigated under coil simulation conditions. At a frequency of 27 Hz an ac magnetic field and an inphase ac current up to 0.525 T and 99 A, respectively, are applied. The temperature is varied between 4.2 and 1.9 K. Measurements for several ratios of current field and external field are reported. It results, that ac losses and flux jump values are determined only by the effective surface field and the temperature. The quench behaviour is different. Obviously, it is independent of the occurance of flux jumps and influenced by the heat transfer conditions.