Takayo Hasegawa

Bi2Sr2CaCu2Ox/Ag multilayer tapes with Jc>500 000 A/cm2 at 4.2 K and 10 T by using pre-annealing and intermediate rolling process

Abstract The high transport critical current density ( J c )>500 000 A/cm 2 at 4.2 K, 10 T (critical current>700 A) is obtained in the Bi-2212/Ag multilayer tape fabricated by using the PAIR ( P re- A nnealing and I ntermediate R olling) and melt–solidification process. The combination of the improvement in geometric configuration of the tape and PAIR process make it possible to obtain the most preferable microstructure and enormous J c . Effect of processing and geometry of the tape on J c and Bi-2212 grain alignment has been studied. The enormous transport J c is attributed to highly textured grain alignment and uniformity in the tapes, which are achieved by PAIR process. After post annealing in flowing argon at 450°C for 4 h, transition temperature T c is increased from 83 K to 92 K and transport J c >25 000 A/cm 2 at 77 K is obtained. PAIR process is applicable for fabricating long Bi-2212 tapes with large transport J c for practical applications.

HTS conductors for magnets

We have successfully improved the J/sub c/ value of Bi-2212 round wire to 500 kA/cm/sup 2/ at 4.2 K in self-field. The J/sub c/ value strongly depended on the filament size, aspect ratio, and geometry. Adjusting the filament size made it possible to obtain wires with J/sub c/ values higher than 450 kA/cm/sup 2/ in a range of wire diameters from 0.8 to 1.3 mm. The J/sub c/ value remained constant up to an applied tensile strength of 150 MPa. A 1 + 6 stranded cable fabricated using 1.02 mm diameter wires carried an I/sub c/ value of 4.5 kA at 4.2 K in self-field. A 16-strand Rutherford cable could also be fabricated using this wire.

Fabrication and test results for Rutherford-type cables made from BSCCO strands

Wires based on the Bi-2212 HTS superconductor are becoming available commercially, with current densities that are attractive for some applications. The authors report here on their success in using these Bi-2212 wires to fabricate multistrand, kiloamp conductors that can be used to construct dipole and quadrupole magnets for particle accelerator applications. Multistrand cables have been made from several types of Bi-2212 wire supplied by two manufacturers. These cables were made with cores of various compositions and dimensions in order to optimize the fabrication process. In addition, cables have been made from aspected strands as well as round strands. Cable critical currents are reported and compared for the various cable parameters investigated in this study.

12 kA HTS Rutherford cable

One of the advantages of Bi-2212 is that a high J/sub c/ value can be obtained in wires with various shapes and filament configurations. Realization of a high J/sub c/ round wire made it possible to fabricate many kinds of cable developed for applications of low temperature superconductors (LTC). We successfully fabricated a 30-strand Rutherford cable using 0.8 mm/sup d/ round wire. Optimization of cabling factor enables to reduce the I/sub c/ degradation at the cable edge. I/sub c/ values were about 650-750 A at 65 K in a noninductive winding condition, which corresponds to 11-13 kA at 4.2 K in self-field.

Effects of PAIR (pre-annealing and intermediate rolling) process on superconducting properties of Bi2Sr2CaCu2Ox/Ag multilayer tapes

Abstract PAIR process, which is the combination of pre-annealing (PA) and intermediate rolling (IR) processes and is performed prior to melt-solidification, enhances J c of the Bi-2212/Ag composite tapes. The effects of pre-annealing and intermediate rolling process on microstructure and J c of the Bi-2212/Ag multilayer tapes fabricated by the oxide-painting method have been studied. The results show that J c has been increased strongly by performing pre-annealing at 800–880°C and intermediate rolling with >25% deformation. Transport J c of 3.3×10 5 A/cm 2 in magnetic field (4.2 K, 10 T) is obtained. The enhancement of J c is assigned to the microstructure with high grain alignment and uniformity, as well as the improvement of grain coupling by the PAIR process.

Progress in Wind-and-React Bi-2212 Accelerator Magnet Technology

We report on our progress in the development of the technology for the application of Bi2Sr2CaCu2O8+x(Bi-2212) in Wind-and-React accelerator magnets. A series of superconducting subscale coils has been manufactured at LBNL and reacted at the wire manufacturer SWCC. Selected coils are impregnated and tested in self-field, even though the coils exhibited leakage during the partial melt heat treatment. Other coils have been disassembled after reaction and submitted to critical current (Ic) tests on individual cable sections. We report on the results of the current carrying capacity of the coils. Voltage-current (VI) transitions were reproducibly measured up to a quench currents around 1400 A, which is 25% of the expected performance. The results indicate that the coils are limited by the inner windings. We further compare possibilities to use Bi-2212 and Nb3Sn tilted solenoid, and YBa2Cu3O7-delta (YBCO) racetrack inserts to increase the magnetic field in HD2, a 36 mm bore Nb3Sn dipole magnet which recently achieved a bore magnetic field of 13.8 T. The application of Bi-2212 and/or YBCO in accelerator type magnets, if successful, will open the road to higher magnetic fields, far surpassing the limitations of Nb3Sn magnet technology.

The effect of Ag on the formation of Bi2Sr2Ca1Cu2Oy thick film

Abstract We studied the effects of Ag on the melting process, the solidification process and the microstructure of Bi 2 Sr 2 Ca 1 Cu 2 O y (2212). Samples were 2212 thick films prepared on Ag substrate and on MgO substrate prepared by the pyrolysis technique of organic acid salts. Addition of Ag-octylate or Ag powder to the 2212 layer was also tried for these samples. Ag enhanced the decomposition of the 2212 phase in the heating process, no matter how Ag was used. Ag substrate or Ag powder were more effective on the decomposition than Ag-octylate. The solidification temperature of 2212 was lowered by Ag substrate or by addition of Ag. The lower the solidification is, the faster the rate of solidification becomes. However, in a 2212 layer prepared on MgO substrate, many small swellings were observed and texturing was hindered at the swellings. A silver substrate was most effective to produce highly textured 2212 microstructures.

Development of 66 kV and 275 kV Class REBCO HTS Power Cables

A Japanese national project called “Materials & Power Applications of Coated Conductors (M-PACC)” started in FY2008. In this project, we are developing a 66 kV/5 kA large-current high-temperature superconducting (HTS) cable and 275 kV/3 kA high-voltage HTS cable, using rare-earth barium copper oxide (REBCO) tapes. These HTS cables are expected to offer a compact cable with a large capacity and low power transmission loss. After the cable design has been studied and elemental technologies for each component of the cable system, such as ac loss reduction, protection against over-current, and high-voltage electrical insulation have been developed, two cable systems will be constructed and verified to meet the required specifications in FY2012. This paper describes the progress and status of these HTS cable developments in the M-PACC project.

Investigation on Critical Current Properties of Quasi-Isotropic Strand Made From Coated Conductors

The high-temperature superconducting (HTS) strand, with high current capacity and a round or square cross section, is expected to have widely potential applications in current leads and superconducting power devices since it is easily insulated to avoid point charging. This paper describes a quasi-isotropic strand made from coated conductors. The magnetic field distribution and critical current characteristic of the strand in self-field are simulated by the finite-element method and tested in liquid nitrogen. The result indicates that this strand has high current-carrying capacity. The critical currents in different external magnetic fields and various orientations were measured. It is verified by the experiments that the strand has isotropic characteristics in low magnetic field and weak anisotropy in higher field. The potential of the strands with high engineering current density makes them also suitable for high-field magnets and high-voltage power applications, while maintaining flexibility.

Development of 10 kA Bi2212 conductor for fusion application

Recently, superconducting properties of high Tc superconductors (HTS) have been highly improved and using performance of HTS a conceptual design of a fusion device was performed by Japan Atomic Energy Research Institute (JAERI). HTS has a capability to produce a magnetic field of higher than 16 T, which is required in such a fusion power reactor. Aiming at development of the conductor, a trial fabrication of a 10-kA 12-T conductor was started using round Ag-alloy sheathed Bi-2212 strands, which has best performance at 4.2 K, 16 T at present. The conductor has about 34-mm diameter, and is composed of 729 HTS strands. Operating temperature is designed at not only 4 K but also 20 K. The cable of the conductor is solder-coated on the surface to use specific heat of the lead as much as possible, which at 20 K is almost comparable with specific heat of SHe at 4.5 K, 0.6 MPa. From the tests of the conductor, the fabrication of large HTS conductor and 10 kA operation at 12 T, 18 K /spl sim/ 20 K were successfully performed and the first step of developing work of HTS conductor for fusion application was achieved.