Fast ramp superconductor for ohmic heating coils

Abstract

The present study was conducted to consider practical 10,000 ampere conductor designs to meet the operating constraints for the ohmic heating coils of TNS and experimental Tokamak reactors. The conductor must simultaneously meet the requirements for mechanical support, cryostabilization, high overall winding current density, low mechanical and electrical losses, and mechanical and electrical integrity for cyclic pulsed operation for -7 T to +7 T in one second. A suggested winding is a set of nested tubes, each made up of a stack of pancake-wound bobbins. Each pancake is co-wound of a flat open superconductor braid, steel tape, and Kapton insulation. The strands of the braid consist of sectored copper regions separated by copper-nickel and surrounding a mixed-matrix copper, copper-nickel, and NbTi multifilament core. Strands 1.5 mm in diameter provide conservative cryostabilization at overall winding current densities adequate for the OH winding of an EPR-1 or TNS sized coil (approx.1500 amperes/cm/sup 2/). Eddy current and coupling losses are at acceptable levels, and hysteresis losses can be reduced within acceptable limits with 10 ..mu.. diameter filaments, providing the winding is graded, tube to tube. The basic conductor and winding concept can be extended to provide conductors of higher currents. A 2000 ampere model conductor has been fabricated from 0.94 mm diameter strands and tested in a reinforced pancake configuration. The measured maximum recovery current corresponds to a current density of 3500 amperes/cm/sup 2/ over the entire coil cross section