Structural optimization of the superconducting outsert of a 45-T hybrid magnet

The National High Magnetic Field Laboratory (NHMFL), Tallahassee, Florida, in collaboration with the Francis Bitter National Magnet Laboratory, MIT, is constructing a 45 T hybrid magnet system using cable-in-conduit conductor (CICC) technology for the superconducting coils. In this technology, superconducting wires are cabled and placed in a steel conduit that acts as the principal load-bearing member of the solenoid. In this paper, a structural optimization of the CICC is performed with the aim of reducing the coil mass required to generate the same field strength. The constraints for the optimization are the upper limits values (allowable limits) for the three state variables: the average and peak values of equivalent von Mises stress, and hoop strain. Different geometric and material options were analyzed for optimum performance. Global-local finite element technique was used to conduct the stress analysis. First, a global analysis, using effective material properties, was performed; followed by a local structural analysis. This procedure was iteratively applied until one of the state variables just reached its allowable limit, thus yielding the optimized design. Significant reductions were achieved in mass and stored energy of the magnet as a result of the optimization.