Generation of extremely strong magnetic fields in open space by using metal-ring-reinforced 60 mm ∅ Sm-Ba-Cu-O superconducting bulk

Based on detailed analysis on the maximum tensile stress acting in the 60 mm diameter Sm123 bulk superconductor due to trapped fields and the compensation effect due to a stainless-steel ring reinforcement, we have proposed the new "stress-controlling magnetization" method, in which a bulk superconductor is magnetized not to a full extent but to such a level that super current does not circulate in its central area. This is very effective because the bulk remains within its fracture limit while increasing its total trapped flux upon decreasing temperature as low as possible. In addition, we could optimize a thickness of the stainless-steel ring to prevent the bulk from fracturing at a given temperature. In this way, we could produce the highest field of 7.2 and 5.2 T on the surface of the bulk superconductor and on the top surface of the vacuum cylinder 3 mm above the surface of the bulk, i.e., in the working space at room temperature, respectively.