Many high-field superconducting magnets wound with Nb3Sn tape are now being used in the magnetic field range of 15 T. at higher fields the current carrying capacity of V3Ga tape recently developed in Japan is much higher than that of Nb3Sn tape. The first hybrid superconducting magnet with an outer Nb3Sn section and an inner V3Ga section has been successfully operated at the National Research Institute for Metals (Tsukuba Branch). This magnet generates a field of 17.5T and stores a magnetic energy of 1.8MJ; both values far exceed those of any other high-field superconducting magnet ever constructed.
The outer Nb3Sn section generates 13.5T in a 160mm bore, and the inner V3Ga section generates an incremental 4.0T in a 31mm bore. Both sections are disc-wound and are excited by separate power supplies. The inner section can be removed permitting full use of the large bore of the outer section. The outer magnet is self-protected by shunt resistors across each module, and the inner magnet is protected by the external resistors and switches.
The magnet is cooled down from room temperature to 20K by the parallel operation of two helium refrigerators with a total refrigeration power of 750W at 20K. It takes about 9hrs to cool the magnet from room temperature to 20K, and subsequently about 30l of liquid helium is required to cool the magnet to 4.2K. About 120l of liquid helium is transferred into the cryostat. The liquid helium evaporation rate is about 4.5l/hr when the magnet is in full operation.
