The High Field Laboratory for Superconducting Materials (HFLSM), Institute for Materials Research, Tohoku University has demonstrated the first practical cryocooled superconducting magnets using a GM-cryocooler and high temperature superconducting current leads. For high magnetic field research, various kinds of easy-to-operate cryocooled superconducting magnets such as wide bore, high field, and new functional magnets have been constructed. Recently, the HFLSM succeeded in developing a cryocooled 18.1 T high temperature superconducting magnet and a cryocooled 27.5 T hybrid magnet. This paper reviews cryocooled superconducting magnet technology and relevant new research areas.
A cryocooled superconducting magnet with a 360-mm room-temperature bore has been developed for hybrid magnets. Magnetic fields of 9.5 T in individual mode operation and 27.5 T in hybrid mode operation were successfully achieved when used in combination with a high-power water-cooled resistive magnet, although quenching occurred at 8.6 T during the first operation due to an unexpected rise in temperature. We discuss the thermal behavior of the innermost coil, where quenching was generated, through the analysis of temperature distribution measurement and two-dimensional thermal conductivity.
A new water-cooled magnet for a cryocooled hybrid magnet was designed by fully utilizing an 8 MW electric-power source and the cooling system installed at the High Field Laboratory for Superconducting Materials, Tohoku University. The magnet consists of four axial water-cooled Bitter coils capable of producing 22.8 T using 7.5 MW in a room-temperature bore of 16 mm. The cryocooled hybrid magnet can generate 33.8 T with a backup field of 11.0 T. The magnetic-force field, B(∂B/∂z), reaches approximately -11000 T2/m at the maximum central field, which is sufficient to levitate metals such as gold or semiconductors including silicon.
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.
Nb3Sn superconducting wires reinforced with a CuNi-NbTi compound have been developed. The critical current density of the CuNi-NbTi reinforced Nb3Sn superconducting wire with a is the same as conventional Nb3Sn wire without reinforcement, but 0.2% proof stress is twofold higher that of the conventional wire. To investigate distribution of Cu, Ni, Nb and Ti in the CuNi-NbTi wire, observation using EPMA (Electron Probe Micro Analyzer) was performed. Based on these developments, an industrial-scale CuNi-NbTi/Nb3Sn wire was successfully. The critical current density and 0.2% proof stress at room temperature of the industrial-scale wire were greater than 600 A/mm2 at 12 T and 350 MPa, respectively. The high-strength Nb3Sn wires were applied in the manufacture of a cryocooled hybrid magnet and 18 T cryocooled superconducting magnet at Tohoku University.