Fig. 2-19 Manufacturing method for Nb3Al wire A jellyroll is formed by winding Nb and Al sheets around Cu rod, and about one hundred jellyrolls are assembled into a billet. The billet is drawn until the thickness of the Nb and Al sheets becomes less than 1 micrometer. Since the initial thickness of the sheets is about 0.1 micrometer, the reduction of the cross section is one in a few ten thousands. A few kilometer long Nb3Al wires were produced using this method.

Fig. 2-20 A large Nb3Al coil A large Nb3Al coil installed into a test facility.

Fig. 2-21 Critical current dependence of Nb3Al and Nb3Sn on strain The critical current of the Nb3Al and Nb3Sn degrade with strain. Since the reduction ratio of Nb3Al is smaller than that of Nb3Sn, the Nb3Al is suitable for a large magnet whose conductor is subjected to a large electromagnetic force. In addition, the coil fabrication process can be simplified because of less susceptibly to strain.

Composite superconductors, such as Nb3Sn and Nb3Al, are the best candidates for the high-field, large-current superconducting coils of fusion reactors. Although the fabrication of Nb3Sn conductor is already practicable, the maximum field generated by a large Nb3Sn coil is around 13 T. In contrast, a Nb3Al coil can generate a field up to 16 - 17 T. The Nb3Al conductor has therefore been developed as an advanced superconductor candidate. However, since the maximum unit-length of the Nb3Al wire was less than a few hundred meters because of breakage during drawing, no large Nb3Al coil could be fabricated until recently. To manufacture Nb3Al, Nb and Al are heat-treated together at more than 1800 degree Celsius in a conventional process. However, any copper (Cu) in the wire melts when it is exposed to a temperature exceeding the melting temperature of Cu, 1085 degree Celsius. We developed a technique to fabricate Nb3Al conductor having high superconducting performance by a heat treatment at 750 degree Celsius for 50 h by making the multiple layers of Nb and Al extremely thin (less than 1 micrometer), as can be seen in Fig. 2-19. We also developed a mass production technique for this process, and Nb3Al wire was mass-produced to fabricate a large Nb3Al coil (Fig. 2-20). In the fabrication of the Nb3Al coil, since Nb3Al has better critical current tolerance (the maximum current flowing in the superconducting state) over strain, as shown in Fig. 2-21, the conductor could be wound after its heat treatment. This greatly simplifies the coil fabrication process. The coil could stably be charged to a nominal point of 46 kA at 13 T. In addition, an extended charge to 60 kA at 12.5 T, in which the conductor was subjected to 125% electromagnetic force of nominal operation, was successively performed. These results show that the Nb3Al conductor is suitable for a large coil whose conductors are subjected to large electromagnetic forces and promise the feasibility of the magnets for a post-ITER reactor, Demo plant, which will be operated at a high field (more than 16 T) and large current (more than 80 kA).

Reference N. Koizumi et al., Development of a Nb3Al Conductor to Be Applied to a Fusion Reactor and Its Application to a Large Superconducting Coil, Teion Kogaku, 38(8), 391(2003).

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