The role of the large coil program in the development of superconducting magnets for fusion reactors

The central element in the Office of Fusion Energy´s development of superconducting toroidal field magnets for fusion reactors is the Large Coil Program (LCP). Toroidal field coils in a tokamak reactor face special problems of heat generation by pulsed poloidal fields, demands for continuity of operation, structural design to handle the asymmetric in-plane loading and the out-of-plane forces repeatedly imposed, and space competition that makes high current densities desirable. Several design concepts have been advanced but large coils meeting tokamak requirements must be built and tested before an optimal choice can be made. This is being done through the LCP, in which three U.S. industrial teams are designing and will build one coil each to a common set of specifications. Coil specifications and test conditions were chosen to insure maximum relevance to fusion program needs. Each test coil will have a 2.5 × 3.5 m D-shape bore, will contain about 7 MA-turns, and must operate at a peak field of 8 T while subjected to pulsed fields up to 0.14 T in a test stand that can accommodate up to 6 coils in a compact toroidal array. Coils by General Dynamics/ Convair and General Electric will use different NbTi conductors cooled by pool-boiling helium. The Westinghouse coil will use Nb3Sn cooled by a forced flow of supercritical helium. These coils will be delivered in 1980 and 1981 for testing in the Large Coil Test Facility at Oak Ridge in a compact toroidal array with three coils from outside the U.S. These will be produced by EURATOM, Japan, and Switzerland for testing under an International Energy Agency agreement.