This paper presents a review of the research and development of low loss AC composites with very fine superconducting filaments as well as of their applications at 50/60Hz frequency. At first a review of formulae for calculating AC losses in a superconducting multifilamentary composite (MFC) placed in an AC external transverse magnetic field is given and the energetical feasibility criterion for an AC superconducting winding is formulated. Then it is shown how this criterion has influenced the development of AC MFC. Short description of the technology and of characteristic parameters of actually produced AC MFC, cables and windings is presented. The results of feasibility studies and of R & D works on superconducting AC devices and machines such as superconducting fault current limiter, power transformer, fully superconducting generator, linear induction motor, magnetic energy storage system and thermally controlled superconducting switches are reviewed. The quench current degradation of several kA-class cables as well as of epoxy impregnated windings with non-cylindrical form is the most important issue of the further development of superconductor applications at 50/60Hz frequency.
In order to explain the basic structure of cryostats, several types of cryostats are illustrated and the functions of their parts are also described. The principal paths of heat leak from ambient temperature to a cryostat are described together with a structure to intercept this heat flow. In recent times, cryogenic applications have progressed with the result that they are much demanding of conditions, which are not favorable for cryostat design. One must solve this problem by understanding the heat flow paths in cryostats and tranding off the requirements of applications.
The effects of transverse compressive stress on the critical current, Ic, of two different superconductors, bronze processed Nb3Sn single-core wires and Nb-tube processed Nb3Al multifilamentary wires, have been measured. It has been found that both type of wires show basically the same stress dependence of Ic. Ic's decrease monotonically with externally applied stress and Ic-degradations become larger with increasing magnetic field. The process of Ic-degradations can be divided into 2 stages. In the first stage the Ic-degradation rate with stress is relatively small and lowered Ic's due to applied stress completely recover to the unstressed value when stress is removed, while in the second stage the Ic-degradation rate with stress is large and permanent Ic-degradations remain after removal of load. In the first stage of Ic-degradations due to stress, from an analysis based on the elasticity theory of the stress state of superconducting filaments of the multifilamentary wire under transverse compressive stress, it is suggested that the sensitivity of the critical current to transverse stress is substantially the same as that to axial stress. On the other hand, in the second stage it can be speculated that nonuniform deformation of the matrix may cause local bends or kinks in the superconducting filaments and these damages to the filaments may be chiefly responsible for the Ic-degradations. The stress beyond which the second stage starts are 150MPa and 700MPa for Nb3Sn and Nb3Al wires, respectively. This stress seems to correspond to the yield stress of the matrix material of the wires where superconducting filaments are embedded.