Yasuzo Tanaka

Excess magnetization due to interfilamentary proximity coupling in NbTi multifilamentary wires

Abstract It is shown theoretically that the excess magnetization of a multifilamentary superconducting wire caused by interfilamentary proximity coupling, ΔMp, is proportional to the twist pitch of the wire, I. It is also shown that this theoretical prediction can be confirmed experimentally for Nb - Ti multifilamentary wires. The critical current density of the superconducting shielding current flowing through the matrix, which becomes a weak superconductor due to interfilamentary proximity coupling, is given by J cp = J 0 ( B B c2p ) −1.4 × [1 − ( B B c2p )] 1.4 with J 0 ∝ [ cosh ( d N 2ξ N )] −2 , where B is the magnetic flux density, and Bc2p, ξN and dN are the upper critical flux density, the effective coherence length and the width of the weakly superconducting matrix, respectively. It is concluded that ΔMp can be estimated quantitatively by measuring the I dependence of total magnetization, and also that the most effective method of decreasing ΔMp is to decrease ξN to the dirty limit.

50-m long HTS conductor for power cable

The long conductor fabrication is one of key technologies to realize the High-T/sub c/ superconducting power cable. A 50-m long conductor was fabricated by helically winding the High-T/sub c/ superconducting tape onto a former with a winding machine. The conductor consisted of ten layers of Ag-sheathed Bi-2223 tape which had a high critical current density of 10,000 A/cm/sup 2/ (at 77 K). AC losses and layer-by-layer current distribution were measured, feeding AC current of 100 to 2,000 Arms to the conductor cooled by LN2. The results showed that most of the current flowed in the outer layers where the impedance was low, and that AC losses were remarkably reduced by making the current distribution uniform.

Enhanced Jc properties in superconducting NbTi composites by introducing Nb artificial pins with a layered structure

High critical current densities (4250 A/mm2 at 5 T, 4.2 K, and 15 000 A/mm2 at 2 T, 4.2 K) were achieved by using an artificially designed pin structure composed of randomly oriented clusters of Nb/NbTi multilayer in a NbTi composite superconductor (with a round cross section). The maximum values were obtained when the Nb layer thickness was designed to be approximately twice the coherence length in NbTi, where the layer thickness is five to ten times thicker than that of the optimized α‐Ti ribbon thickness in a conventional NbTi system.

Synthesis of Superconductive Oxides by Vacuum Calcination Method

The vacuum calcination method (VCM) was developed to synthesize Ln-Ba-Cu-O group superconductive oxides represented by YBa2Cu3O7-x through solid phase reaction. When Y2O3, BaCO3 and CuO are used as raw materials in the form of powder in this method, the carbon content of the resultant calcinated product will be 1/10 that of the normal methods or lower. VCM powder is a mixture of Y2BaCuO5 and BaCuxOy, an unknown phase, and has extremely high activity because of oxygen content that is lower than the stoichiometric composition of YBa2Cu3O7-x. For this reason, it absorbs oxygen during sintering under an oxidizing atmosphere causing transition to the perovskite phase of YBa2Cu3O7-x, thus indicating excellent sinterability and high Jc and magnetization.

Effects of artificial pins on the flux pinning force and other superconducting properties in NbTi superconductors

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.<<ETX>>

Electromagnetic interfilament coupling of silver-sheathed Bi-2223 multifilamentary tapes in transverse AC magnetic fields

Abstract To discuss the electromagnetic interfilament coupling of silver-sheathed multifilamentary Bi-2223 (HTS) tapes, AC losses of HTS tapes with various twist pitches and sample lengths in transverse magnetic fields were measured at 77 K by a magnetization method. Both hysteresis loss and eddy current loss in fields perpendicular to the HTS tape surface were approximately one order of magnitude greater than those in parallel fields. While AC losses in parallel fields were extremely reduced with a decrease in the twist pitch or the sample length, those in perpendicular fields scarcely decreased. AC losses of the untwisted HTS tape, which were proportional to the frequency in a perpendicular field of 29 mT, were slightly smaller below 70 Hz and larger above 150 Hz than those of the HTS tape twisted at a pitch of 8.6 mm. It is concluded that the coupling loss of untwisted HTS tapes is much smaller than the eddy current loss at commercial frequencies because of complete electromagnetic interfilament coupling.

New Fabrication Method of High-Jc YBa2Cu3O7 Superconducting Films on Flexible Metallic Substrates

In plane-aligned, c-axis-oriented YBa2Cu3O7 (YBCO) films were deposited on polycrystalline Ni tapes buffered with self-oxidized NiO. The in-plane alignment of the NiO layer was achieved by the oxidation of textured pure (00l)Ni tapes in 1 atm O2 in the furnace. Significant improvement in (00l)NiO texturing was obtained in the temperature range above 1000°C. Pulsed laser deposited YBCO films on the NiO layer were not only c-axis oriented with respect to the film surface, but also strongly in-plane textured. The full width at half maximum of the x-ray diffraction peaks from the (103)YBCO ϕ scan is approximately 11 degrees. This is similar to the values reported with other texturing techniques.

YBCO superconducting coils operated at nitrogen temperature

Trial coils of YBa2Cu3Ox superconductor were prepared from oxide superconducting powder and binder mixtures. The critical temperature of the wire was more than 86 K in the four-probe measurement. Magnetic fields, 21.1 gauss, and 350 gauss were generated by dc operations at 77 K and 4.2 K, respectively. A critical current density in the coil was of 1,330 A/cm2, corresponding to a level of 77 K and zero magnetic field. Experimental results show that an oxide superconducting magnet with an appropriate protection circuit is feasible and stable at liquid nitrogen temperature.

Development of superconducting linear induction motor for steel making processes

A superconducting linear induction motor (SLIM) is being developed for steel-making processes using ultrafine filamentary NbTi wires. A SLIM can produce a strong magnetic field and large thrust in a wide gap. Applying SLIMs to a steel making plant could result in important innovations. A model of the SLIM has been made and tested. The model SLIM worked in a frequency range of 20-30 Hz. The feasibility of a SLIM as hardware has been demonstrated.

Magnetic flux pinning properties of oxide superconductors produced by melt processes

Abstract The flux pinning mechanism of YBaCuO superconductors produced using a melt process was investigated by measuring the magnetic field dependence of transport critical current densities ( J c ) at 77 K. Superconductors produced by the melt process had high transport J c values and the magnetic field dependence of the transport J c was quite small compared to that of bulk sintered materials. This suggests that, in this case, J c in a high field region is determined by flux pinning within the bulk of the material. Using procedures based on conventional flux pinning theory, the J c - B properties were analysed to estimate dominant pinning sites in solidified specimens. According to the theoretical analysis and microstructural observations, the expected site of dominant pinning is the normal precipitates dispersed in the YBaCuO grains, and the transport J c may be determined by their average size and density.