This year marks the 50-years anniversary of the discovery of Nb3Sn compound by Professor B.T. Matthias. The first Nb3Sn conductor was fabricated in 1960, and since then it has contributed much for generating fields over 10 T at 4.2 K. A variety of processes have been applied for the fabrication of Nb3Sn. The effect of Cu on the enhancement of Nb3Sn synthesis has yielded a significant breakthrough for the fabrication of multi-filamentary Nb3Sn conductors in industrial scale. Now Nb3Sn conductors are being widely used for NMR, fusion and other high-field facilities as well as for refrigerator-cooled superconducting magnets. Although Nb3Sn conductors have a long history, their high-field performance has shown a pronounced enhancement over the past decade. Further progress may be still expected through the improvement of fabrication parameters.
The progress of Nb3Sn conductor development for fusion power applications at JAERI is described. The superconductor development was carried out in connection with the research and development for four fusion coil development projects: Phoenix, Cluster test, Demo Poloidal Coil and ITER-EDA. In the Phoenix project, the reliable production of multi-filamentary Nb3Sn strands was established during the development work towards realizing a 18 T coil, and an 18 T generation coil (BC-18) with multi-filamentary Nb3Sn strands was successfully operated at 4.2 K for the first time in the world. In the Cluster Test project, a pool-cooled Nb3Sn conductor with a high heat-flux surface, and the first 1-m class bore Nb3Sn coil (TMC) in the world was constructed and operated up to 12 T. In the Demo Poloidal Coil project, which was constructed to demonstrate the possibility of a poloidal coil, a forced-cooled Nb3Sn conductor was developed. The Nb3Sn coil (DPC-EX) constructed with it was pulse-operated up to 7 T in 0.5 s. Finally, in the ITER-EDA project a CICC-type 13 T-46 kA Nb3Sn conductor was developed. A Central Solenoid Model Coil (CSMC) was fabricated using it and successfully operated up to 13 T with 0.4 T/s.
The development of high performance Nb3Sn wires with an internal-tin route was reviewed. The internal-tin wires have excellent productivity and low production cost because the drawing process does not require intermediate annealing. This process has the advantage of producing several types of wires with wide superconducting properties for practical applications. We have been producing wires with high critical current density (Jc) and low hysteresis loss for fusion applications such as the ITER and KSTAR. We have also developed internal-tin wires with Jc>1600 A/mm2 at 12 T, 4.2 K for general high field magnets. Recently, we have been developing a new wire configuration for next generation high energy accelerators in collaboration with the KEK High Energy Accelerator Research Organization. The wire, called a distributed-tin (DT) wire, has an original structure in which the multi-Nb filament modules and Sn rods are uniformly distributed in a Cu matrix. The values of Jc of the DT wire are 2120 A/mm2 at 12 T, 4.2 K, which was obtained by increasing the Nb and Sn fractions of the wires, and 207 A/mm2 at 20 T, 4.2 K, because of the improvements in Bc2 and Tc. It is anticipated that the DT wires will be applied to not only next generation accelerators, but also high resolution NMRs.
To clarify the Jc-compressive strain properties of Nb3Sn superconducting wires fabricated using the internal-tin process, an evaluation of the properties has been carried out by applying pre-compressive strain to the wires. The strain was induced by clothing the wires in stainless steel that has large thermal contraction and then subjecting the wires to a heat-treatment process. We succeeded in shifting the peak strain from 0.3% to 0.7- 0.8% and grasping the Jc-compressive strain properties. These experimental values agree well with the stress analysis results. We also examined the differences in Jc-strain properties including the compressive strain between internal-tin processed wires and bronze wires. As a result, it was found that Jc-strain properties are different between them; however when both wires are clothed in stainless steel, they show almost the similar strain-sensitivity. Using these data, the specification value on Jc for the strand wire of the ITER TF coils was examined. It was concluded that 9.7% higher Jc is needed for internal-tin processed wires as compared to bronze route Nb3Sn wires at T=4.2 K and B=12 T, assuming that both wires have the same Jc at T=6 K and B=11.8T.
A superconducting coil with high-strength CuNi-NbTi/Nb3Sn wire was fabricated using the react and wind (R&W) technique. The CuNi-NbTi/Nb3Sn wire indicated a high 0.2% proof stress at a room temperature of 340 MPa, which is much larger than the conventional Cu/Nb3Sn wire of 150 MPa. The dimensions of the Nb3Sn coil produced using the R&W technique are 260 mm in inner diameter, 289 mm in outer diameter and 319 mm in height. It is designed to generate a central field of 2.18 T at 180 A. The superconducting CuNi-NbTi/Nb3Sn coil was inserted into a previously fabricated cryocooled backup NbTi coil and a field of 5.85 T was generated in the coil center. The constructed cryocooled superconducting magnet has a room temperature bore of 220 mm in diameter and is designed to generate a central field of 8 T. Consequently, the superconducting magnet generated 7.5 T at its maximum and 7 T stably for 12 hr.
Research and development activities and some recent results related to Nb3Sn superconducting wires produced by Kobe Steel, Ltd. and Japan Superconductor Technology Inc. (JASTEC) are introduced. An outline of the activities is described from a historical point of view. Improvements in the characteristics (i.e., critical current density (Jc), n-value and mechanical properties) of bronze-processed Nb3Sn wires are reviewed. Finally, the status of development for the Ta-Sn powder-in-tube (TS-PIT) process newly proposed by Tachikawa is described.
In order to develop a Nb3Sn coil using a react and wind method, it is important to clarify the influence of bending strain because the superconducting characteristics of Nb3Sn are very sensitive to stress and strain. We investigated the effect of prebending strain εpb, which means repeated bending loads, from 0% to 1.5% for superconducting wires. We found that Ic, Tc and Bc2 were enhanced by prebending strain. When the prebending strain was 0.8%, Ic values showed maximum enhancement at a whole magnetic field for CuNb/Nb3Sn wires. For instance, Ic was approximately twice at 19 T. Bc2 increased about 1.5 T at a whole temperature when εpb was 1.0%. Tc increased from 17.5 K to 17.9 K at a prebending strain of 1.2%.
Lorentz force is applied to coil windings when the magnet is charged. The coil windings are subjected to huge hoop tensile stress and transverse compressive stress in both the radial and axial directions due to the Lorentz force. The higher the magnetic field and the larger the bore, the larger the Lorentz force applied to the coil windings. Thus, not only the hoop stress, but also transverse compressive stress should be taken into account when designing the magnet. In this paper, we focus on transverse compressive stress, and the influence of this stress on critical current is explored for a standard Nb3Sn wire and a Cu-NbTi-reinforced Nb3Sn wire. It was confirmed that Cu-NbTi sufficiently reinforced the superconducting wire, not only for tensile stress, but also for transverse compressive stress. The effect of reinforcement arrangement in the cross section was also investigated. In the case that Cu-NbTi reinforcement is arranged at the center of the wire, there was less deterioration in superconducting property than in the case of the reinforcement being arranged in the outer part of the wire.
This paper reviews the stress/strain characteristics of some typical highly strengthened bronze-processed practical Nb3Sn superconducting wires examined in the HFLSM, IMR of Tohoku University up to the magnetic field of 14.5 T at a temperature of 4.2 K. Internal reinforcement of the superconducting composite wires using the materials with high strength and high electrical conductivity such as in situ processed or jelly rolled Cu-Nb microcomposite, Ta, and recently developed Cu-Nb-Ti compound was proved to be promising, although the stability of the wire is decreased to some extent. Although the intrinsic axial tensile strain sensitivity of Ic was not markedly changed, the reinforcements changed not only mechanical properties such as the 0.2% proof stress but also the parameters relevant to the tensile strain characteristics of Ic, the strain corresponding to the Ic peak, εm, and reversible strain limit, εirr, where permanent damage such as cracking starts to take place in the Nb3Sn filaments. Furthermore, the transverse compressive stress sensitivity of Ic decreased and the reversible stress limit increased. The effect of parameters on the Cu-Nb composite reinforced bronze-processed Nb3Sn wires and the strain characteristics are intensively discussed.
Nb3Sn superconducting wires reinforced with a Cu-NbTi compound have been developed. In laboratory-scale (2 kg) fabrication of the Nb3Sn superconducting wire with a Cu-NbTi reinforcer (Cu-NbTi/Nb3Sn), its critical current density is the same as conventional Nb3Sn wire without any reinforcer, but 0.2% proof stress is two times higher than the conventional one. The influence of the copper fraction on the strength of Cu-NbTi/Nb3Sn wires was also investigated. The strength of the wires is proportionally inverse to the copper fraction of the wire, and there are no significant effects on critical current density caused by the difference in copper fraction. According to these laboratory-scale developments, an industrial-scale Cu-NbTi/Nb3Sn wire was successfully fabricated up to a length of 13 km. The critical current density and 0.2% proof stress at room temperature of the industrial-scale wire was 604 A/mm2 at 12 T and 350 MPa, respectively. These values are the same as the laboratory-scale case. The high-strength Nb3Sn wires are going to be applied for the cryocooler-cooled hybrid magnets at Tohoku University and other high-field magnets.