Superconducting Magnets for M.H.D. Generators [and Discussion]

This paper will review the science and technology of superconducting magnets and the possibility of their application in large m.h.d. generators. The newly exploited hard superconductors offer, in principle, the most economic means of providing magnetic fields of several tens of kilo-oersteds over large volumes. Unfortunately these materials have exhibited certain peculiarities of behaviour when wound into coils (`training' and `degradation') which, until recently, had made it impossible to realize their full potential or to design larger coils with any confidence. The reasons for this are now fairly well understood and are seen to be fundamental to hard superconductors. A number of techniques have been developed to reduce these effects, the most recent, involving the intimate combination of the superconductor with a metal of low resistivity together with the provision of adequate cooling, circumvents the problem completely. A further problem with large superconducting magnets is to protect the windings, in the event of a coolant failure, against destruction by the release of the stored magnetic energy, 10$^{10}$ J in the case of a large m.h.d. generator magnet. It will be shown that this also dictates the use of composite conductors in which the superconductor is the minor component. Although a number of detailed problems of cryogenic engineering remain to be solved, it is now possible to design windings for large superconducting magnets that will operate predictably, safely and at the full potential of the superconductor.