Masayuki Ishizuka

Performance of a Cryogen-Free 30 T-Class Hybrid Magnet

The worlds first cryogen-free hybrid magnet, which was developed at the High Field Laboratory for Superconducting Materials in order to realize an easy-operational magnet system with no use of liquid helium and nitrogen, has achieved 22.7 T in a 52 mm room temperature bore. After this success, we started to construct a new cryogen-free 30 T-class hybrid magnet, consisting of an outer wide-bore cryogen-free 11 T superconducting magnet and an inner water-cooled 19 T resistive magnet. Up to now, the NbTi outer section coil and the Nb3Sn inner one of a wide-bore cryogen-free superconducting magnet has generated individual central fields of 5.3 T at 350 A and 5.8 T at 303 A, respectively in a 360 mm room temperature bore. The wide-bore cryogen-free superconducting magnet was energized up to 9.5 T as a total background field. In hybrid magnet mode the system was operated up to an 8.5 T background field form the cryogen-free superconducting magnet, because a cooling problem was encountered with the innermost coil bobbin during ramping the Bitter magnet. As a result, the cryogen-free hybrid magnet generated 27.5 T in a 32 mm room temperature bore

Advances in the first cryogen-free hybrid magnet

In order to solve problems of a large amount of liquid helium supply for a wide bore superconducting magnet of a hybrid magnet, we intended to construct the first cryogen-free 23 T hybrid magnet, consisting of an outer wide bore cryogen-free superconducting magnet and an inner water-cooled resistive magnet. Up to now, the wide bore cryogen-free superconducting magnet was tested to generate a total central field 6.0 T in a 360 mm room temperature bore. The worlds first cryogen-free hybrid magnet achieved 21.5 T in a 52 mm room temperature bore, combining with 15.5 T water-cooled Bitter magnet. As the result, the cryogen-free hybrid magnet no longer needs a troublesome handling time for liquid helium transfer, and the machine time is extremely enlarged. The improvement of the maximum magnetic field generation toward our design value of 23 T is being carried out continuously. Furthermore, we have started a new construction project of a cryogen-free 30 T hybrid magnet. Since the magnetic force field BdB/dz of 2274 T/sup 2//m is obtained at 21.5 T, the cryogen-free hybrid magnet can provide a large magnetic force field enough to levitate diamagnetic materials. For new processing of materials development, a YAG laser furnace was installed into the cryogen-free 23 T hybrid magnet. A container-less melting for paraffin was examined by controlling a thermocapillary convection. We succeeded in fabricating a magnetic field oriented ball in magnetic levitation.

Design of a cryocooler-cooled large bore superconducting magnet for a 30T hybrid magnet

We are now developing a 30 T hybrid magnet utilizing a cryocooler-cooled superconducting magnet wound with highly strengthened (Nb, Ti)/sub 3/Sn. Diameter of the room temperature bore of the superconducting magnet is 360 mm and it generates 11.1 T. Water cooled resistive insert magnet generates 18.9 T, thus the hybrid magnet generates a central field of 30.0 T. The (Nb, Ti)/sub 3/Sn multifilamentary wires are strengthened by Cu/NbTi composite which volume ratio in conductor is about 35%. The reinforcing Cu/NbTi composite changes to CuTi intermetallic compounds during heat treatment for reaction of (Nb, Ti)/sub 3/Sn phase formation. The (Nb, Ti)/sub 3/Sn coil with inner diameter of 400 mm will be fabricated by wind and react method with Cu/NbTi reinforced (Nb, Ti)/sub 3/Sn wires. The innermost section of (Nb, Ti)/sub 3/Sn coil is wound with a wire which diameter is 1.85 mm and next second section is wound with a wire diameter of 1.8 mm. The (Nb, Ti)/sub 3/Sn coil is operated at 303 A and generates 5.8 T. The NbTi coil is wound with NbTi wires of 2.0 mm and 1.6 mm diameters. The NbTi coil generates 5.3 T at an operating current of 350 A. The maximum hoop stress is under 220 MPa for (Nb, Ti)/sub 3/Sn coil and 200 MPa for NbTi coil.

First performance test of the cryogenfree hybrid magnet

We are now constructing a cryogenfree 23 T hybrid magnet at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. At present, an outer section coil employing NbTi multifilamentary wires for a cryogenfree superconducting magnet of the hybrid magnet was combined with an inner 15.5 T water-cooled resistive magnet, and was tested as the worlds first cryogenfree hybrid magnet. The NbTi coil with 491 mm inner diameter and 584 mm outer diameter generated 4.59 T at 198 A, and the central magnetic field of 20.0 T was generated in a 52 mm room temperature experimental bore. The magnetic force field of 2030 T/sup 2//m was obtained, and a piece of paraffin was levitated at 1200 T/sup 2//m. Using a CO/sub 2/ laser combined with the cryogenfree hybrid magnet, a containerless melting experiment in magnetic levitation was demonstrated easily for paraffin.

High Field and High Temperature Characteristics of Small Test Coil Using CVD-YBCO Tape for SMES

Magnetic field dependencies of the Ic of the IBAD/CVD-YBCO short tape sample and its small coil sample were measured in high fields, up to 18 T at 77 K. Compared with the Ic of the tape sample, the Ic of the coil sample at 0.1 muV/cm showed the same tendency in high fields. If YBCO tape is applied to a high-field coil application, the application should be operated at a temperature which is lower than 77 K. Using long CVD-YBCO tape, six stacked pancake coils were fabricated. Various current tests were conducted using one of these stacked coils. In AC current tests, thermal stability of the YBCO coil was estimated. When the peak values of AC current were 1.2 times higher than the maximum DC current in a thermal stable state, Idcmax, and the average electric field of the coil at the first triangular wave was about 10 times higher than 1 muV/cm criterion, the peak values of the built-up voltage did not tend to increase even after the 99th triangular wave current, and thermal run-away in the coil was not observed. In DC current with overlapped pulse current tests, the maximum peak current of the coil in a thermal stable state was obtained as a function of DC current and sweep time. It was 1.3 times higher than Ic and 1.4 times higher than Idcmax in a test condition. These results indicate that the YBCO coil has high potential in short-time, over-current operations at high temperatures. In cases where built-up voltages did not disappear, they began to increase just after the coil currents reverted to the initial DC currents. It was found that DC current influenced the increasing speed of built-up voltages once the pulse current had decreased to zero.