WITH the advent of high field superconducting materials, 1 which exhibit volume rather than surface conduction, i.e. critical current densities proportional to cross-sectional area rather than circumference; one may no longer be forced to construct superconducting coils of only the finest of wire. Instead one might use relatively heavy wire or ribbon, increasing cross-sectional areas from 5 x 10 -4 cm 2 to 5 x 10 -2 cm 2 and at the same time reducing the length of the winding from a few kilometres to a few dekametres. The mechanical and electrical advantages of such an approach are obvious. The great disadvantage, however, lies in the corresponding increase of coil currents from tens of amperes to thousands of amperes.
Transformation schemes
To introduce currents much greater than 200 A with leads from room temperature into a liquid helium bath at 4 ยฐ K presents formidable engineering problems, and may be justifiable only with the very largest of magnet coil systems. The thought of powering a superconducting coil by some means of energy transformation is a rather appealing one.
Mendelssohn 2 discussed a direct current transformation scheme for powering small alloy coils and avoiding leads into the liquid helium altogether as early as 1933. An application of this method to a niobium stannide coil has been reported recently by Hempstead and Kim. 3 Quite different applications of d.c. transformers have been described by McFerran. 4 However, all these methods are limited by the stringent requirement of flux conservation within the superconducting circuit, and by the amount of energy that may be transferred in a single cycle.
Alternating current a.c. transformation and subsequent rectification might well remove these limitations. Olsen 5 has shown the feasibility of rectification with magnetically biased superconducting to normal switches in which the switching transition is triggered by the transformed currents themselves and their associated magnetic fields. Purcell and Payne 6 have discussed means of improving the rectification effi-ciency by detailed 'shaping' or control of the variation of the quenching magnetic field with time. None of the a.c. rectification schemes has as yet been developed to the point of producing kilo-amperes of direct current.