Yukio Mikami

15 T Cryocooled Nb3Sn Superconducting Magnet with a 52 mm Room Temperature Bore

We succeeded in demonstrating a 15.1 T cryocooled Nb3Sn superconducting magnet with a 52 mm room temperature bore. Two operating currents of 157 A and 90 A for the divided section coils were utilized for the first time in a cryocooled superconducting magnet system. It is found that the current leads are no longer the dominant heat loads because of the use of high-temperature superconductors. In order to realize a high-field cryocooled superconducting magnet comparable to the usual superconducting magnet immersed in liquid helium at 4.2 K, we maintained the coil temperature at a value below 5.0 K during the field sweep in a high magnetic field region.

Liquid helium-free 15 T superconducting magnet at 4 K

We have successfully demonstrated a 15.1 T liquid helium-free superconducting magnet with a room-temperature bore of 52 mm using a Nb3 Sn/NbTi hybrid coil, Bi2223 current leads and two Gifford-McMahon cryocoolers. The magnet has 830 mm outside diameter, 1221 mm height and 720 kg weight. The magnet was cooled to 3.6 K in 114 h. A central magnetic field of 15.1 T was achieved in 38 min. The temperature of the coil increased to 5.7 K due to ac losses during the excitation, but it decreased to 4.0 K before reaching 15.1 T. The temperature of the coil remained at a constant value of 3.8 K over the 24 h of operation. The demonstration indicated the usefulness of a liquid helium-free superconducting magnet to generate high magnetic fields up to 15 T.

Performance Test of Cryogen-Free Bi-2223 HTS Dipole Magnet for Beam Line Switching

We have developed a Bi-2223 high-temperature superconducting dipole magnet for beam line switching at the cyclotron facility of the Research Center for Nuclear Physics, Osaka University. Exit beam lines are periodically switched by increasing and decreasing the magnetic field between 0 and 1.6 T with a switching time of 10 s. The magnet is equipped with two sets of Bi-2223 coil assemblies, which are conduction cooled by two 10-K Gifford-McMahon cryocoolers. We evaluated the superconducting property of the Bi-2223 coil assemblies in liquid nitrogen before installation in the cryostat. There were no degradation in wire performance during the assembly process. Magnetic field, strains, and temperatures of the coil assembly were investigated for performance verification after the magnet fabrication. The magnetic field and the temperatures meet specifications, and the deformation of the coil assembly is considered to be successfully suppressed by reinforcing structure. Furthermore, from a viewpoint of temperature, it is indicated that the magnet can make switching time and cycle time faster than that of the specified operation.

Development of a pre-bent react-and-wind CuNb/Nb3Sn multilayer superconducting coil

A superconducting multilayer coil using CuNb/Nb3Sn wire was fabricated by the pre-bent react-and-wind (R&W) method. The pre-bent R&W method is a combination of the pre-bending treatment and the conventional R&W process. A 0.8% pre-bending strain was applied by ten fixed pulleys. The coil was tested in magnetic fields of 14, 12, 11 and 10?T at 4.2?K. The coil quench current achieved 100% load ratio of the short sample critical current in the 14?T back-up field. The quench current of the outer section coil achieved 100% of the short sample critical current in the 12?T back-up field. The coil quench currents in various magnetic fields are discussed from a hoop stress and mechanical disturbance point of view.

Improvement of a Large Bore Cryogen-Free Superconducting Magnet for a Hybrid Magnet

A 360-mm room-temperature bore cryogen-free superconducting magnet (CSM), consisting of Nb3Sn coils and NbTi coils, for a hybrid magnet (HM) has generated the maximum magnetic field of 9.5 T. However, the magnetic field of the CSM has been limited to 8.5 T in the hybrid magnet mode because of a cooling problem. As a result, the hybrid magnet composed of a 19-T water-cooled resistive magnet (WM) had the utmost field generation of 27.5 T. Therefore, we improved the CSM to generate higher magnetic fields. For the improvement of the cooling problem, Nb3Sn coils were replaced, and thermal conduction was improved between coils and a 4K-GM cryocooler. Furthermore, support structures with a tensile strength over 80 kN and a spring support were adopted against the magnetic force to support the self-weight of coils and to absorb stress caused by thermal contraction difference between each coil. After the improvement, the CSM generated 9.5 T within 1 h and the maximum magnetic field of 9.7 T in a 360-mm room-temperature bore. The HM succeeded in generating 28 T in a 32-mm room-temperature bore with the CSM operated at 9.0 T.

Reliability of Bi-2223/AgAu Current Leads for Cryocooled Superconducting Magnet

A current lead for a cryocooled superconducting magnet (CSM) was designed and fabricated using a high-temperature superconductor (HTS) tape, which could easily facilitate modification of the transport current capacity. The current lead was constructed of two terminal blocks, a support tube, and five Bi2223/AgAu HTS tapes. The Bi2223/AgAu HTS tapes with a critical current of 100 A at 77 K in self-fields were used for the current leads. The critical current value of the current lead was 390 A at 77 K in self-fields. The initial critical current at 77 K was maintained after four thermal cycles. The transport current of 170 A was continuously applied at 75 K, 0.27 T by conduction cooling. The voltage between two terminal blocks was 0.28 mV, even after 930 cycles of electromagnetic force (Lorentz force). The heat leakage through the current lead was 0.21 W from 55 K stage to 4.5 K stage. The experimental results showed that the current lead for CSM had sufficient strength against thermal stress and Lorentz force. The current lead has been operated in a CSM to demonstrate a stable excitation and reliability after thermal cycles from room temperature to 4 K. During the excitation and thermal cycle test, overall voltage of the current lead was maintained at an operational condition of 155 A, 55 K, and 0.26 T.