Design and Test Results of 18.1 T Cryocooled Superconducting Magnet With Bi2223 Insert

An 18 T cryocooled superconducting magnet (18T-CSM) with a 52-mm room temperature bore has been successfully constructed. The magnet consists of an outer NbTi coil, which generates 6.2 T, four Nb₃Sn coils, which generate 9.3 T and a Bi2223 high-Tc insert coil, which generates 2.5 T. The magnet system is cooled by a GM/JT cryocooler and two single-stage GM cryocoolers. The initial cool down takes 11.5 days. The ramp time up to 18 T is 60 minutes. The innermost layer of the Nb₃Sn coils employs an internal-tin processed Nb₃Sn wire due to its high critical current density in high magnetic fields. High mechanical strength is required for other three layers because of the large hoop stress of 234 MPa at 18 T. With this view, bronze-processed high-strength Nb₃Sn wires reinforced with Cu-NbTi intermetallic compound are employed. The Bi2223 high-Tc insert coil is composed of 25 double pancake coils using Ag-alloy sheathed Bi2223 superconducting tape with stainless steel tape reinforcement. The reinforcement co-winding reduces the hoop stress down to 48 MPa, which is sufficiently applicable to the Bi2223 tape. The magnet is successfully operated up to 18.1 T.