Development of a superconducting magnet for high magnetic force application

A laboratory size superconducting magnet to generate a magnetic force field of 1500 T² in a 40 mm diameter room temperature bore was developed. The magnet consists of Nb₃Sn and NbTi superconductors. In order to generate such a high magnetic force field, a superconducting magnet is combined with main coils and a reverse coil. The main coils generate a central field higher than 15 T. The reverse coil generates a reverse field to achieve a high magnetic field gradient of about 150 T/m in a region at a magnetic field of about 10 T. To avoid huge axial magnetic forces between the main coils and the reverse coil, the main coils are combined with three sections set asymmetrically. All coils are connected in series and also connected to a persistent current switch. Thus the magnet can be operated in persistent mode, which means that the magnet can maintain its field without supplying current. The field decay is less than 50 ppm/h. The sample can be kept in a micro gravity environment for more than one month. After shifting to persistent mode operation, the current lead can be removed from the cryostat, which allows the helium boil off rate to be reduced to less than 50 cc/h and the helium-refilling interval to be increased to more than one month. This magnet will enable epoch-making progress in the field of micro gravity application.