In this article the authors describe the present situation and future developments of the hybrid magnets at the High Magnetic Field Laboratory in Grenoble. They specify the condition under which the 30T, 50mm-diameter now in service is operated, and describe the projects for hybrid magnets which could be constructed in the years to come. A magnet generating over 40T should be operational within three years; it would be built economically, and the magnetic field from the superconducting element would contribute less than 10T. A considerably more powerful magnet generating a continuous magnetic field of 50T is under consideration for later.
Multifilamentary Nb3Sn composite conductors with submicron filament diameters have been developed by the internal tin diffusion method. The heat treatment condition to optimize the critical current density was studied in detail. The different heat treatment was performed in two stages according to the arrangement of the copper clad tin cores in strands. The first stage is for the homogenization heat treatment (HHT) of tin in the copper matrix. In the second stage, the reaction heat treatment (RHT) of Nb3Sn was applied. With a suitable combination of HHT and RHT, Nb3Sn with a critical current density high enough for a.c. use was formed at a reaction temperature as low as 350°C.
Discussed are the link conditions which appeares in analyzing a multi-stage refrigerator or a Cooke-Yarborough engine employing a computer code THERMOACOUSTICA based on thermoacoustic theory. At a link point, intensive quantities such as temperature and pressure are common to all branches and flow quantities such as energy flow, work flow, heat flow and volume displacement of fluid are continuous. These link conditions are discussed in detail. The link conditions are general, since they are applicable to links which appear in thermoacoustic devices equipped with a displacer piston.
Coupling loss properties were studied theoretically and experimentally for three superconducting conductors of the Rutherford cable type which were developed for a 1kWh/1MW-SMES. The strand of the conductor has a CuNi/Cu mixed matrix and a Cu core; its surface is not insulated. The experimental results showed that the inter-strand coupling loss of the conductor is much smaller than the intra-strand coupling loss when the compression stress corresponding to the designed value of the electromagnetic force for the SMES was applied to the sample conductor. We also measured the crossover resistance between strands of the conductor by means of the four probe method, and estimated the contact surface resistance by taking account of the result of an analog simulation with an electric circuit model. By using the contact surface resistance obtained, we could theoretically explain the experimental result of the loss. As for the intra-strand coupling loss, the effect of the existence of a Cu core and a CuNi cladding around it on the loss was also discussed.