Abstract
We have successfully developed a cryocooled superconducting magnet that generates the highest magnetic field in the world, 18.1 T in a 52-mm room-temperature bore, as a conduction-cooled superconducting magnet. The magnet consists of a high-Tc superconducting (HTS) insert and low-Tc superconducting (LTS) coils. The superconducting coils are cooled conductively by a GM-JT cryocooler with a cooling capacity of 4.3 W at 4.3 K. The ramp-up time to 18 T is 60 minutes. The HTS insert is composed of 25 double-pancake coils using Ag/(Bi,Pb)2Sr2Ca2Cu3O10 (Bi2223) high-Tc superconducting tape with a stainless-steel tape reinforcement. The reinforcement co-winding reduces the effective hoop stress to 48 MPa at 18 T, which is sufficiently applicable to the Bi2223 tape. The LTS coils are subdivided into five coils. The innermost layer employs an internal-tin-processed Nb3Sn wire due to the high critical current density in a high magnetic field of 16 T. The three middle layers require high mechanical strength capable of tolerating a hoop stress of 230 MPa at 18 T; hence, bronze-processed high-strength Nb3Sn wires reinforced with a Cu-NbTi compound were employed. The outermost layer is an NbTi coil. A distinctive feature of the magnet is that the HTS insert is designed to be replaceable, allowing the magnet to be used as a 16-T backup magnet for a new insert coil. HHTS coils employing a YBa2Cu3O7 (Y123)-coated conductor, and Bi2Sr2CaCu2O8 (Bi2212) are candidates for replacement.
