Hirokazu Tsubouchi

New 25 T Cryogen-Free Superconducting Magnet Project at Tohoku University

The new high magnetic field research laboratory network is recognized as one of the Japanese Master Plans of Large Research Project by the Science Council of Japan. Recently, the project of the 25 T cryogen-free superconducting magnet (25 T-CSM), which is operated under a conductive cooling condition by cryocooler, was approved under the high magnetic field research laboratory network. We adopted a high strength CuNb/Nb3Sn Rutherford cable with a prebending treatment for the middle section coils of the 25 T-CSM. The central magnetic field of 14 T is generated by the operational current of 851 A by the Nb3Sn middle section and NbTi outer section coils in a 300 mm bore. The induced maximum hoop stress in the CuNb/Nb3Sn section is about 250 MPa. In addition, the 11.5 T high temperature superconducting insert coil is also designed using Gd123 tapes. Therefore, a total central magnetic field of 25.5 T can be achieved.

Prebending Effect for Mechanical and Superconducting Properties of Nb-Rod-Processed Cu–Nb Internal-Reinforced

We are planning to develop three large superconducting magnets, using CuNb-reinforced Nb3Sn Rutherford cables with the prebending treatment. CuNb-reinforced Nb3Sn wires using a new Nb-rod-processed CuNb composite were developed for the Rutherford cables. In this study, the prebending effects for the mechanical and the superconducting properties of the Nb-rod-processed CuNb/Nb3Sn wire were investigated. We found that the CuNb/Nb3Sn wire with 0.8% prebending treatment had largely enhanced critical current at 4.2 K. The reduction of the n-value was observed around 0.8% prebending strain. The mechanical properties of the prebent wires were enhanced. Youngs modulus and 0.2% proof stress of the wire with prebending treatment at 4.2 K are 180 GPa and 270 MPa, respectively. The residual strain was released from 0.30% to 0.15% by 0.8% prebending treatment.

\hbox{Nb}_{3} \hbox{Sn}

The design and production of high temperature superconducting (HTS) power transmission cables was studied. In the production of HTS cable, difficulties are mainly caused by the poor mechanical properties of HTS tapes, because critical currents of the HTS tapes deteriorate due to the strains applied during cable production and usage. Therefore, two basic characteristics of HTS cables were experimentally analyzed to improve HTS cable design and production: (1) the mechanical-electrical properties of the HTS cable; and (2) the properties of electrical insulation. The analysis results indicate that the most important technology is the control of the strains applied to the tape in the cable. Based on the results, the design of the HTS cable was then improved, and the machines at Furukawa Electric fabricated a three-phase prototype HTS cable of 30 m in length. The results of the performance test of the cable demonstrated the proposed design and the production method are appropriate.

Wires

New Nb-rod-processed CuNb reinforced Nb3Sn superconducting wires have been developed for large high field magnet applications. The critical current, Ic, mechanical properties and resistivity are investigated. The residual strain and irreversible strain are found to be 0.35% and 0.77%, respectively. We found that the Nb-rod-processed wire has a large strain sensitivity to Ic. However, the wire has good mechanical properties, having a 0.2% proof stress of 280 MPa and a Youngs modulus of 150 GPa. This means that the stress sensitivity of the Ic for the wire is small. These results show that the Nb-rod-processed wire is suitable as a strand of Rutherford cable for a high field superconducting magnet.

Design and production of high-Tc superconducting power transmission cable

We developed Cu-Nb reinforced Nb3Sn Rutherford cables for wide-bore high magnetic field coils that were able to be produced by React-and-Wind technique. The Cu-Nb/Nb3Sn strands were reinforced by internal Cu-20 vol%Nb composites fabricated by the Nb-rod-method and were manufactured through the bronze-process. The Rutherford cables composed of sixteen un-reacted Cu-Nb/Nb3Sn strands of 0.8-mm diameter were heat-treated at 670 °C for 96 hours. After that, repeated bending strains (pre-bending strains) were applied to the flatwise direction of the reacted Nb3Sn Rutherford cables at room temperature. Critical current (Ic) values of Cu-Nb/Nb3Sn strands extracted from the Rutherford cables were compared to those of the single strands that had undergone the same processing as the cables. Ic values under the applied tensile stress of the Rutherford cable were increased by the pre-bending treatment as similar to the single strand. The performances of Cu-Nb/Nb3Sn Rutherford cables were improved by utilization of the pre-bending process.

Mechanical and superconducting properties of Nb3Sn wires with Nb-rod-processed CuNb reinforcement

A Rutherford flat cable composed of sixteen 0.8-mm-diameter Nb3 Sn strands with CuNb-reinforced stabilizers (CuNb/Nb3Sn) has been developed for use as an outsert for a 47-T hybrid magnet. To guarantee the safety of the magnet in the event of a quench, the low residual resistance ratio characteristic of arc-melted-in-situ CuNb composite had to be improved upon, and to obtain high electric conductivity, a new CuNb fabrication method using a Nb rod was attempted. The thermal runaway characteristics of chemical vapor deposition- YBa2Cu3O7-δ (Y123) coated-conductor tapes with varying Cu stabilizer thicknesses were measured in magnetic fields of up to 10 T at temperatures ranging from 17 to 60 K. It was found that the ratio of the thermal runaway over-current to the critical current decreases as temperature decreases and that thermal runaway occurs just above the critical current level at 5 K. The properties of a cryogen-free, 20-T superconducting outsert for a 47-T hybrid magnet utilizing CuNb/Nb3Sn Rutherford flat cables and Y123 tapes and having a room temperature bore of 400 mm were investigated.

Development of Nb-Rod-Method Cu–Nb Reinforced Nb 3 Sn Rutherford Cables for React-and-Wind Processed Wide-Bore High Magnetic Field Coils

A large-bore 14-T CuNb/Nb3Sn Rutherford coil was developed for a 25 T cryogen-free superconducting magnet. The magnet consisted of a low-temperature superconducting (LTS) magnet of NbTi and Nb3Sn Rutherford coils, and a high-temperature superconducting magnet. The Nb3Sn Rutherford coil was fabricated by the react-and-wind method for the first time. The LTS magnet reached the designed operation current of 854 A without a training quench at a 1 h ramp rate. The central magnetic field generated by the LTS magnet was measured by a Hall sensor to be 14.0 T at 854 A in a 300 mm cold bore.

Upgrade Design to a Cryogen-Free 20-T Superconducting Outsert for a 47-T Hybrid Magnet

A Rutherford flat cable that is applicable for operational currents up to 1000 A at 13–14 T was developed using a bronze-processed high-strength Nb3Sn strand with CuNb reinforcement (CuNb/Nb3Sn). The critical current for a 0.8-mm diameter CuNb/Nb3Sn strand is 98 A at 14 T and 4.2 K in the residual strain state. A test coil using the CuNb/Nb3Sn Rutherford flat cable composed of 16 CuNb-reinforced Nb3Sn strands was fabricated. We measured the critical current properties of the Rutherford test coil and obtained an excellent critical current of 1840 A at 14 T and 4.2 K. By using the strain gauges attached onto the stainless-steel reinforcement tape that was co-wounded with the Rutherford flat cable, it was found that a 300-MPa hoop stress at 4.2 K was applied to the CuNb/Nb3Sn strand. This implies that the critical current for a CuNb/Nb3Sn strand is enhanced to be 115 A at 14 T and 4.2 K through the stress-strain effect of the critical current at 300 MPa.

Performance of a 14-T CuNb/Nb3Sn Rutherford coil with a 300 mm wide cold bore

The prebending strain effect is that the repeated bending load at room temperature enhances superconducting properties of practical wires. The authors are now investigating the application of the effect to practical superconducting cabling conductors for high field superconducting magnets. Large current capacity and high strength are required for superconducting cabling conductors to make large scale and high field magnets. The superconducting cabling conductor with high strength wires will be useful for making high field magnets. The prebending strain effect was applied to a cabling technique with wires. High strength wires reinforced with CuNb composite and conventional wires without reinforcement were prepared, which were heat-treated at 943 K for 345.6 ks. Both wires were bent by 10 pulleys to give 0.8% prebending strain. After the prebending treatment the wires were assembled and fabricated 3-strand cables and 7-strand cables. The cables with high strength wires showed the enhancement of critical currents even after the cabling process. The results imply that the prebending treatment is applicable to the fabrication of cabling conductors for making a high field superconducting magnet.

Rutherford flat cable composed of CuNb-reinforced Nb3Sn strands

An application of the prebending effect to a cabling process using Nb₃Sn strands was demonstrated. The prebending effect is the enhancement effect of superconducting properties due to the repeated prebending treatment for practical bronze-route Nb₃Sn wires. CuNb/Nb₃Sn and Cu/Nb3Sn strands were applied prebending treatment using 10 fixed pulleys with 0.8% prebending strain. Four kinds of triplets, i.e., prebent CuNb/Nb₃Sn, no-prebent CuNb/Nb₃Sn, prebent Cu/Nb₃Sn and no-prebent Cu/Nb₃Sn triplets were fabricated. Critical currents were measured for the four kinds of triplets in magnetic fields up to 11 T at 4.2 K. The obvious critical current enhancement due to the prebending effect was maintained for the prebent CuNb/Nb₃Sn triplet. The results imply that the prebending treatment for high-strength Nb₃Sn strands is applicable to the cable conductor fabrication.