The performance of stabilized Nb-Zr wire in coils

The problem of achieving full designability in superconducting coils can be broken up into two parts: 1) achieving reproducible long lengths of superconducting conductor, and 2) winding this conductor into a coil in such a way as to result in a predictable coil performance. This paper discusses first the basic conductor used, namely a copper strip 0.040 inch by 0.500 inch which has imbedded in it nine 0.010-inch diameter heat treated Nb-Zr wires. Typically this conductor can carry 1800 A (200 amperes per wire) at 20 kG and 720 A (80 amperes per wire) at 40 kG and has a current carrying capacity which decreases approximately linearly with increasing magnetic field. (Since the amount of conductor required to produce a given magnetic field is inversely proportional to the current the conductor carries, the use of heat treated Nb-Zr, which has a much higher current carrying capacity, results in a very significant economic gain.) When measurements are taken on short samples of this conductor, the voltage across the sample remains equal to zero up to the critical current, and if enough cooling is present the voltage rises gradually at higher currents. The second part of this paper is concerned with the performance of the stabilized conductor when wound into magnets. Several magnets have been built using the stabilized Nb-Zr strip. These coils include round as well as saddle-shaped coils and produce magnetic fields over dimensions of a few inches to a few feet. The performance of these magnets is discussed with regard to the current carrying capacity of the conductor before the appearance of resistance. In all magnets tested the appearance of resistance did not precipitate a quench. The resistance appeared at a predetermined value of current and upon lowering the current it disappeared at essentially the same current as it had appeared.