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.