APPLICATIONS OF LOW TC SUPERCONDUCTORS FOR VERY HIGH MAGNETIC FIELDS

Nb₃Sn Conductors

The Japanese government through the Science and Technology Agency (STA) has been a very strong supporter of the development of a power reactor based on magnetic confinement of plasma, and STA has been a major sponsor of the ITER project. In this project, Nb₃Sn conductors that are used to produce very high fields (H » 13 T) in a large volume play a key role. The two processes used to produce the conductors are the bronze and the internal tin processes: the first is the oldest and most commonly used process to manufacture wire for high field applications; the latter is also quite extensively used, particularly in the United States.

Even though the internal tin process was developed at Mitsubishi Electric Company in Japan in the early 1970s, it was not used at all in Japan until very recently. It was interesting to the WTEC panel to find that Mitsubishi had reentered the field of manufacturing Nb₃Sn wires by the internal tin process after many years of being absent. In this process, coalescence of the Nb₃Sn filaments has been one of the major difficulties. The coalescence of the filaments causes enlargement of the effective size of the filaments and thus leads to increased hysteresis loss. Mitsubishi researchers performed an extensive metallurgical study of the mechanism for the coalescence and were able to find fabrication steps that minimized this problem. The result of this development is that Mitsubishi has now become one of the suppliers of Nb₃Sn wires to the ITER project, and, of importance to other manufacturers, the process appears to be less costly than the bronze process.

Nb₃Al Conductors

The other interesting development in Japan for high field conductors is the successful fabrication of large-sized Nb₃Al conductors suitable for use in the ITER project. This superconductor has been studied extensively in the past since it can be made to have a higher T_c (20 K) than that for Nb₃Sn. More importantly, its critical current is less sensitive to mechanical strains; however, Nb₃Al conductor has turned out to be very difficult to produce in large quantities. Because of the enormous forces involved in the ITER type of magnet, ITER engineers have been very interested in the mechanical properties of Nb₃Al wire, and those involved in the ITER project at the Japan Atomic Energy Research Institute (JAERI) have been encouraging Japanese manufacturers to develop Nb₃Al as a conductor for the ITER project. Sumitomo Electric Industries was the first to successfully produce ITER-sized conductor capable of meeting the critical current requirement. Following this, Hitachi in collaboration with Hitachi Cable also reported manufacture of the conductor. Some other companies in Japan are also known to be working on Nb₃Al. The fact that Japan is the only country working on this development is noteworthy; that no U.S. effort exists perhaps reflects the financial difficulties that the U.S. magnetic fusion community is experiencing.

The Nb₃Al conductors are fabricated using the so-called "jelly-roll" method, which has been known for a number of years. In principle, the process is very simple. Thin foils of Al and Nb are wrapped around a core, and a large number of these elements are then packed into a Cu tube and drawn to size and heat treated for forming Nb₃Al by interfacial reaction. In practice, it is difficult to uniformly reduce the thickness of the foils to tens of nanometers before the reaction. In spite of this, there are now two or three manufacturers in Japan that can produce this conductor on a large scale.