Here we review the research project on the superconducting magnets for MHD power generators. This is the one of the main subjects of the developmental research of MHD power generators, which started in 1966 as the National Research and Development Programs sponsored by the Agency of Industrial Science and Technology, Ministry of International Trade and Industry. Some of the results accomplished so far are as follows: (1) MHD power generation by means of the saddle shaped superconducting magnet with magnetic field of 24kG. (2) Generation of central magnetic field of 47kG by the saddle shaped superconducting magnet with room temperature diameter of 25cm. (3) Generation of central magnetic field of 75kG by the pancake shaped superconducting magnet with bore diameter of 10cm.
The research works on superconducting magnet for MHD electrical power generator were begun in 1966 at Electrotechnical Laboratory. The works have been done by three groups, namely (1) development group of superconducting magnets, (2) cryogenic engineering group and (3) research group of superconducting material. (1) The magnet group has constructed and tested a saddle shaped magnet for about 1kW MHD generator. It has the following characteristics: Bore diameter: 29cm, Outer diameter: 76cm, Length: 76cm, Critical current: 455Amp. Maximum central flux density: 2.4T. Fully stabilized to 510Amp. (2) The cryogenic group has been developing expansion engines for large refrigerators. The design characteristics of the turbine are as follows: Expansion ratio: 4, Efficiency: 65%, Helium gas flow rate: 106.6g/sec. Number of revolution: 345, 000rpm, Bearing: lubricated by helium gas, (3) The material group has been researching the stabilized superconducting wires. The members are investigating how to cool the superconductor in a magnet, how to compose stabilized wires and preparation of new superconducting materials.
A large saddle shaped superconducting magnet was built and tested. The conductor used was a composite consisting of 0.7cm wide by 0.16cm thick copper strip in which ten Nb-40Zr-10Ti (HISUPER X-alloy) wires were embedded in two rows. The physical characteristics of the magnet were as follows: winding inside diameter, 38cm; winding outside diameter, 88cm; overall length, 180cm. A cryostat to contain the coil had a room temperature space (25cm in diameter) for MHD plasma duct. Outside diameter and depth of the liquid helium vessel were 150cm and 330cm, respectively. As a result of the test running the following characteristics of the magnet and cryostat were assured. Test Results Design Values Central field 47kG 45kG Field uniformity 95% >85% Current decay time-constant (persistent mode) >1, 300 hours >720 hours Boil-off rate of liquid helium (with magnet not energized) 10 1/h 11 1/h