The Stabilization of High-Current-Density High-Field Superconductive Magnets
Authors: Schrader E. R. , Thompson P. A. , Coles W. 
Published: February 2004

This paper is specifically aimed at the types of problems and their solutions for large, high-field magnets which require high-current-density operation for reasons of size and economy. It is apparent that magnet stability can be varied to some extent to suit the specific use to which the magnet is put without sacrificing vital packing factors. The treatment in this paper will include some aspects of the methods used and illustrations from tests on magnets with Nb3Sn ribbon lengths ranging from 0.7 to over 90 km. A magnet wound with over 90 km of ribbon for the Lewis Research Center consists of 22 modules arranged in five concentric rows, the placement of the modules determined by requirements of axial strength, cooling, and placement of versions of Nb3Sn ribbon. The magnet was connected such as to permit powering of up to four groups of modules with separate supplies. The highest field attained is 135 kG in the center of the 6-in. bore (140 kG at the inner windings); this using three power supplies. The electrical system is designed to provide combination of voltage and current ramping for each of the four powered sections. Upon going normal, the power supplies are disconnected and the magnet sections are shorted to permit circulating currents with slow field decay. The required degree of stabilization is achieved by a combination of silver-plating on the ribbon, insulated sheets of copper interleaving between each layer, copper strips distributed across each layer and shorted to the turns, and large cross-section copper turns which act as secondaries. This combination of stabilizing means had been determined chiefly by empirical methods. Of interest is the dynamic response of each of the voltages of the 22 modules of the 6-in.-bore magnet upon normalcy. Visicorder traces show a complex pattern of resistive and induced voltages from which a sequential spatial and temporal pattern of normalcy propagation can be derived. The initial voltage appeared on the inner (higher field) row of modules in a location approximately two thirds of the distance out from the magnet central plane, generally fanning out to include the remaining modules within the subsequent two seconds. Because of the special provisions taken for stabilizing, the total field decay occurs over the order of 10 sec and the induced voltages are no more than 3 V for any one module. Results are presented for 3¼-in. bore, 750 m test coils wound with axial shorting strips with a range of resistivities from 1×10−8 Ω·cm (copper) to 1×10−5 Ω·cm (phosphor bronze) to illustrate the relative degree of independence of critical current on coil-charging rate achieved with the shorting strips of higher resistivity. This is explained by a computer analysis which relates the local inductance of a coil section with the power expended in the shorting strips such that at least a qualatative balance can be made between these effects and the desireable voltage protection achieved with shorting strips of lower resistivity. Preliminary test results are given for 50-cm i.d. magnets wound with 50 km of ribbon which were tested in the superfluid region (1.92°K). Consistent with previously published results on smaller magnets, the critical current improved by approximately 30% over that at 4.2°. It becomes obvious that the methods of stabilization described, when referred to an optimum which is consistent with the magnet usage, will yield a high current density in the windings and permit very large bore, high-field magnets to be built.