TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)

The AC loss characteristics of high T_c superconductors are reviewed. AC losses are organized according to their generating mechanism and the magnetic field components generating them. Useful analytical formulae of hysteresis losses are presented for the superconductor slab exposed to a parallel magnetic field, for the thin strip exposed to a transverse magnetic field, and for the strip and ellipse carrying transport currents. Experimental and numerical methods for evaluating AC loss are reviewed, and these methods are applied to quantify AC losses in some practical superconductors. The AC loss characteristics of single-coated conductors are shown together with the reduction of their AC losses through striation. The AC loss characteristics of mono-layer as well as multi-layer superconducting power transmission cables are explained, and reduction of their AC losses by means of using narrower coated conductors is discussed.

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In this article A15 compound wires other than Nb₃Ga and V₃Ga are described. Nb₃Al has better high-field performance and strain tolerance than those of Nb₃Sn. However, in the diffusion between Nb and Al, compounds richer in Al are predominantly formed. Moreover stoichiometric Nb₃Al is present at quite a high temperature approximately 2,000ºC. The rapid quenching from a high temperature produces the bcc phase supersaturated with Al, which can be transformed to nearly stoichiometric Nb₃Al by subsequent annealing. The liquid quenching process is successful to obtain high performance in Nb₃Al, Nb₃(Al, Si) and Nb₃(Al, Ge) through this route. Then a continuous Joule heating and quenching process has been developed, which produces long-length Nb₃Al wires with good homogeneity. A 19.5 T test magnet has been constructed by using the resulting wire. Moreover, high-field accelerator magnets are being developed using this wire. The continuously Joule heated and quenched Nb₃Ga wire shows an upper critical field of ~32 T at 4.2 K. Nb₃Ge exhibits the highest T_c among A15 compounds studied so far. Long length Nb₃Ge tapes with high Jchave been fabricated through the continuous CVD process. Multifilamentary V₃Si wires have been fabricated by the bronze process, adjusting the overall V / Si molar ratio in the wire. The T_c and especially B_c2 of the bronze-processed V₃Ge wire are significantly enhanced by the addition of Al to the V core. Nb₃Al wires are now available for practical use in addition to V₃Ga and Nb₃Sn. Nb₃Ga, Nb₃Ge and V₃Si compounds may keep high potential for future applications.

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