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
Recently, much developing work on large superconducting magnet for MHD power generation has been in progress, in which the arrival of liquid helium cooling system capable of stable and low-cost operation is one of the essential matter. For this, we have developed a closed cycle cooling system for 45kG saddle shaped magnet of 12 tons. The system is composed of 28l/hr helium liquefier, liquid helium storage, 20°K helium refrigerator with cooling capacity of 400 watt at 20°K, helium recovery system with 80Nm3/hr at 130 atm and instrumentation, which has following basic functions; to precooling the magnet to less than 20°K, to liquefy helium continuously and supply liquid helium, and to recover and reprocess any evaporated helium gas from the cryostat.
The helium refrigeration system for the 45kG saddle shaped superconducting magnet of 12 tons was installed in the Electrotechnical Laboratory, Then we have succeeded in continuous long-run operation of the system in Oct. 1969. The system is helium closed loop and proto-type of a practical cooling system for a large superconducting magnet. This completely system was certified such as short cooling down time of a heavy magnet, supply of large quantity of liquid helium and trouble free operation with recovery gas. Operation results are as follows: (1) Cooling down time to 15°K was 50 hours. (2) Total amount of supplied liquid helium during the test run was 6, 000 liters. (3) Helium gas was recovered and reused without any losses.
As a part a national project of Japan Agency of Industrial Science & Technology of the MHD power generation, a large superconducting magnet has been built and tested. The magnet has an overall length of 77.4cm, a bore diameter of 10.0cm, an outside diameter of 78.4cm and a weight of 1, 600kg, and composes of three separate pancake coils. The outer two coils and the inner coil were wound with composite superconductors of 1.0cm width by 0.14cm thickness and 1.5cm by 0.20cm, respectively. The conductor is a composite consisting of ten copper-clad Ti-Nb-Ta superconducting wires soldered on a flat side of a copper strip. The test results indicated that the magnet system was capable of stable operation at magnetic field of 75kG. The maximum current supplied to the coils in series was 800A at this operation.
In recent years, knowledge of mechanical properties of industrial materials at cryogenic temperatures is essential for designing cryogenic appliances such as superconducting magnets. With this consideration, the authors tested tensile and fatigue properties of materials of cryogenic use at cryogenic temperatures. The brief results of the work are as follows: (1) Aluminum and 18-8 stainless steel retain considerable ductility at cryogenic temperatures. (2) In general, tensile and yield strength of materials increases and elongation decreases with temperature decrease. But elongation of copper increases at cryogenic temperatures, as an exceptional case. (3) In the cases of 18-8 stainless steel sheet and brass sheet, some differences in mechanical properties have been observed between longitudinal and transverse directions with respect to rolling direction. (4) As for fatigue tests at -269°C, fatigue strength of SUS 27 is somewhat higher than that of SUS 28. Fatigue strength of SUS 27 is about 45kg/mm2 at 106 times repetition of load.
In this paper a detailed description is given of an integrated system of a Rectifier Type Flux Pump. Solving a most significant problem occured during a current exchange from one cryotron to the other, a maximum current of about 1, 300Amp is obtaind and produced a field strength of about 30kG in a small superconducting coil of L=1.7mH. Current charge, discharge and persistent mode operation is very easily performed by changing a phase shift of the control unit. There considered to be no basic problems in the small magnet systems with Flux Pump. But in applying Flux Pumps to magnet systems storing many megajoules of energy, several problems had to be solved which were not directly related to the Flux Pump itself. One and essencial of these is “Reliability” of the Systems.