Design of demountable toroidal field coils with REBCO superconductors for a fusion reactor

Abstract

The recent development of REBCO superconducting tapes, cabling methods and joint concepts could be a revolutionary development for magnetic fusion. REBCO has significantly better performance at high magnetic fields than traditional low temperature superconductors (LTS), and can be operated at a higher temperature than LTS for reduced thermodynamic cost of cooling. Use of REBCO superconductors in the magnet systems of tokamaks allows building demountable toroidal field (TF) coils, greatly simplifying reactor construction and maintenance. A demountable TF coil system with REBCO superconductors for a fusion reactor has been conceptually designed. The coil system operates at 20 K, with a maximum magnetic field of 20 T. The magnets are divided into two coil segments and can be detached and remounted to allow the internal components of the reactor to be removed vertically as one piece. Operating at 20 T and 20 K, the stress in most of the coils is acceptable (less than 2/3 the yield strength and less than 1/2 the ultimate tensile strength of the structural materials). The strain in the superconductors is lower than the reversible degradation limit. The electrical resistance in each conductor joint is 10 n [Omega]. The total heat generation in the reactor superconducting TF magnets is approximately 1.9 MW, of which about 25 % is nuclear heating and 75 % joint heating. 71 MW of electricity are required for cooling the coils at 20 K, about 7 % of the electric energy the reactor generates. The expected time to warm-up the magnets from the operation temperature to room temperature is 7 days, and approximately the same for cool-down back to the operation temperature. The analysis of the conceptual magnet design is encouraging, as no insuperable problems have been identified. This conceptual design can be used as a starting point for a full engineering design of demountable fusion reactors magnets.