Design of a superconducting 20 MJ induction heating coil

A pancake-wound, low-loss, superconducting, induction heating coil designed to demonstrate the feasibility of superconducting poloidal system for the Tokamak reactors is described. The coil is designed to store 20 MJ at 50 kA. The superconductor material is NbTi for a 7.5 tesla maximum field. The coil is designed to survive at least 100,000 cycles of full bipolar half cycle sinusoidal operation from +7.5 tesla to -7.5 tesla fields in one second. The coil is natural convection immersion cooled at 4.5 k in liquid helium bath. The total energy loss during a full bipolar pulse is approximately 0.3 percent of the stored energy. The key design features of the coil include modular construction, no superconductor joints at I.D. transition, independent cooling of individual pancakes, and scalability to ETF size ohmic heating system. The results of structural and thermal analyses are presented.