High homogenious field superconducting magnets are reviewed. Field analysis which is suitable for design of high homogenious field magnets are explained in detail. Designing methods using a computer are also touched on briefly. Problems to be considered in high homogenious field magnets including operating current densities, electromagnetic forces, and magnetization effects of superconducting wires and magnet formers are shortly discussed.
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.
Experiments on energy transfer between two superconducting magnets have been carried out using an inductive energy transfer system similar to the flying capacitor system developed at the Karlsruhe Institute. In our system capacitor was grounded and diodes were used instead of thyristors, and a fraction of stored energy was transfered to the capacitor only when the relay connected parallel to the magnet was switched off. The capacitor is expected to have no constraint in size, while in the flying capacitor system the capacitor must exceed a threshold size. Consequently it is possible to shorten transfer time somewhat compared to the one in the flying capacitor system. Transfer experiments were carried out using a storage magnet with inductance of 1.2H and a load of 0.41H. The capacitance was 200μF. 80.1% of the stored energy of 221J was transfered into the load within about 0.35 seconds.