Masanao Mimura

Uniform current distribution conductor of HTS power cable with variable tape-winding pitches

In the multilayer conductor, the inner layers have higher impedance than the outer layers. As a result, the current concentrates in the outer layers. Our early study showed that its AC losses were reduced to one-tenth by making the current of each layer uniform. From such a point of view, a trial to realize the uniform current distribution was made by adjusting the winding pitches of high-temperature superconducting (HTS) tapes layer by layer. A 1 m long conductor was fabricated, where the inner layer had longer winding pitch than the outer layer. Experimental results showed that the currents flowing in individual layers were almost the same and that this conductor had lower AC losses than the nonuniform current distribution conductor.

Design and testing of a pair of current leads using bismuth compound superconductor

The thermal behavior of current leads using an oxide superconductor for the low-temperature portion has been studied. Numerical calculations predict a reduction of the necessary coolant flow rate and refrigerator input power. A pair of current leads has been manufactured where the low-temperature portion consists of six sintered Bi compound cylindrical bars and the high-temperature portion consists of a Cu wire bundle. The lead, cooled by gaseous helium along its entire length, is 0.9 m long and designed to carry 1 kA. The leads have been tested in the same arrangement as practical applications. The helium flow rate necessary to hold thermal equilibrium was about 80% of that for conventional copper leads. The calculation shows that power consumption of the refrigerator needed to cool high-temperature superconductor current leads with an optimum cooling scheme will be about one-third of that for conventional current leads.<<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.

Measurement of AC losses of superconducting cable by calorimetric method and development of HTS conductor with low AC losses

To achieve compact and highly efficient HTS power transmission cables, reducing and evaluating AC losses is important. Furukawa is developing conductors with low AC losses of 1 W/m at 3000 Arms for the 66/77 kV class HTS cables as a part of the Super-ACE project. Specifically, we aim to achieve a superconducting conductor with minimum AC losses by optimizing the winding pitches and twisted filaments in the tape. The electrical method is generally used to evaluate AC losses in HTS cables. However, measures of AC losses were uncertain using only the electrical method of evaluation. Therefore, the calorimetric device was developed and was used for determining AC losses. Consequently, results from the two methods almost agreed. Moreover, AC losses in the fabricated HTS conductors were measured at the lowest levels ever achieved.

Measurement of AC losses in an HTS conductor by calorimetric method

To achieve compact and highly efficient HTS power transmission cables, reducing and evaluating AC losses is important. However, measures of appreciable AC losses were uncertain using only the electrical method of evaluation. Therefore, a calorimetric device that had low heat leakage and high-resolution thermometers was developed. Using this device, measurement accuracy of AC losses in an HTS conductor was 0.09 W/m within a length of 3 m. The 4-layer 3 m long HTS conductor was fabricated with uniform current distribution in each layer by using variable-winding pitches and twisted filaments in the tape. Using the calorimetric device, we confirmed that AC losses in this conductor were 1 W/m level at 3 kA rms (50 Hz).

Fabrication of Y-Ba-Cu-O films on surface-oxidation epitaxy (SOE) processed substrates

The surface-oxidation-epitaxy (SOE) method promotes simple and cost-effective process to form a biaxially textured NiO template for epitaxial growth of YBa/sub 2/Cu/sub 3/O/sub x/ (Y-Ba-Cu-O) film. SOE tapes of 100 m in length were prepared by the reel-to-reel oxidation system with the tape moving speed of 10 m/h. A highly biaxially textured NiO layer, which had /spl Delta//spl phi/ (full width at half maximum from X-ray /spl phi/-scan) of less than 23/spl deg/ throughout the whole length, was successfully formed under appropriate conditions of temperature and atmosphere. In order to improve surface conditions for obtaining high critical current density (J/sub c/) for Y-Ba-Cu-O film on the SOE-tapes, we have investigated the effect of thin oxide cap layers such as MgO, SrTiO/sub 3/, CeO/sub 2/, and so on. When BaZrO/sub 3/ was used as a cap layer, J/sub c/ and critical current (I/sub c/) of Y-Ba-Cu-O films grown by the pulsed laser deposition (PLD) at 77 K were measured to be 1.3 MA/cm/sup 2/ and 137 A, respectively. These values were for a 10 mm-width short tape. A continuous deposition of YBCO on 10-cm long /spl times/10-mm wide SOE tape with SrTiO/sub 3/ or BaZrO/sub 3/ cap layer was also investigated. The end-to-end I/sub c/ was 1 A (J/sub c/ of 1.6 /spl times/10/sup 4/ A/cm/sup 2/) for SrTiO/sub 3/ cap layer and 4 A (J/sub c/ of 5.6 /spl times/10/sup 4/ A/cm/sup 2/) for BaZrO/sub 3/ one.

AC losses to be reduced by twisting filaments in a silver-sheathed Bi-2223 multifilamentary tape

An investigation is carried out to manifest types of AC losses in silver-sheathed Bi-2223 multifilamentary tapes that can be reduced by twisting filaments. The AC losses, in external magnetic fields parallel to the tape surface, consist mainly of the AC losses due to macroscopic screening currents in the multiple filaments and the AC losses due to the microscopic screening currents in the grains. The former loss depends greatly on the filament twist pitch, while the latter does not. Below the critical changing rate of a magnetic field at which screening currents at the outer filaments begin to saturate, the inter-filament coupling losses are added to the hysteresis losses for the equivalent filament size. Consequently, an analytical model can successfully explain experimental results that the macroscopic AC losses are considerably reduced by twisting filaments.

Solenoid Coil Using Multilayered Composite Wire of Bi-Based Superconductor

Fabrication of long wires is one of the key technologies for electric power application of high-Tc superconducting materials. For this, we fabricated a long wire with a multilayer structure of the Jelly-Roll type using Bi-based superconductor. The wire has high critical current and no anisotropy of a critical current density to a direction of an applied magnetic field. As a practical application of the wire, solenoid coils were fabricated and their characteristics were evaluated. At 4.2K, the coils with a 10 m long wire and a 22 m long wire quenched at experienced maximum fields (Bm) of 1.08T and 1.65T, respectively. And long wires wound in the coils had excellent superconducting properties in higher fields above 10T, which is similar to that of a shorter wire. Therefore, the wire is suitable for a high field solenoid type magnet.

The Transport Critical Current Property of High Tc Superconducting Wires

The Ag-sheathed high Tc superconducting wires were prepared by the powder- in-tube method. The transport critical current density (Jc) was measured by the four-probe method with 1 μ v/cm criterion. And the magnetic field dependence of transport Jc at 77K and 4.2K were evaluated. In YBCO system, we obtained the Jc of 103 A/cm2 order, but we could not obtain the higher Jc because of the weak link problem. In case of BSCCO system, the Ag-sheathed tapes showed strong grain alignment which could be attained by adjusting the process parameters. So we obtained the maximum Jc of 3.5x104A/cm2 at 77K and 0T. But this Jc decreased drastically under high magnetic field over 0.1T, because of the weak pinning force and resulting flux creep. At 4.2 K, the Jc of BSCCO Ag-sheathed tape under 10T showed 1.7x 105 A/cm2.

Thermal Performance of a Pair of Current Leads Incorporating Bismuth Compound Superconductors

Thermal performance of current leads incorporating oxide superconductors has been studied. 1 kA model leads were fabricated and tested. Helium flow rate to keep steady state was about 3/4 of conventional copper leads. System thermal performance was analyzed in terms of refrigerator input power. If higher temperature coolant is used e.g. liquid nitrogen or gaseous helium at 50–70 K, refrigerator power for HTS current leads can be reduced to 1/3 that for conventional copper leads.