Superconducting sector magnets are to be the main components of the superconducting ring cyclotron for the RIKEN RI beam factory. A full-scale prototype sector magnet is being developed and constructed. One of the critical points of the design is how to support the huge magnetic force exerted on the superconducting main coil. A cold-pole arrangement is adopted as the solution to this problem. Unbalanced magnetic forces were measured using 1/6-scaled-Cu/Fe magnets. A cryogenic stabilizing method is applied to make the main and trim coils quench-free. The whole structure, magnetic force and its support structure, quench protection, eddy current loss in the cold-pole, and helium cooling system are described.
