Conductor development with high-Tc superconductors in the USA is reviewed. This article includes the level of funding toward this R and D as well as the sources of the funds. The institutions, who are heavily involved for this work, are also identified. Then, the highlights of recent development, as of January 1999, in each of the high-Tc superconducting materials are given with some of my private assessments of various aspects of the conductor fabrication processes.
A tightly constructed, rigid structure of the conductor winding and GFRP spacers in the coil case are essential elements for a superconducting magnet that is to operate at cryogenic temperature and sustain large magnetic forces. However, the contact pressures on the conductor, spacers and coil case imposed during the fabrication process may be greatly reduced by relaxation at room temperature and thermal contraction in the GFRP spacers during the cool-down process. We therefore studied 61 kinds of commercial and test GFRPs and established a basis for suitable GFRP spacer material to be used in superconducting magnet windings. Glass transition temperature, Tg, of the impregnating resin plays an important roll in the transverse creep deformation of GFRPs. GFRP spacers with Tg above 423K can maintain 80% of the initial pressure in a winding for two years at room temperature. This result was obtained by utilizing the transverse creep moduli of GFRPs at different temperatures and the time-temperature superposition procedure. Transverse thermal contraction from 293 to 4K decreases uniformly in all GFRPs as the resin weight content, Rc, decreases. Also, contraction is smaller than that of the 304 stainless steel used for the coil case when Rc is less than 15%. As a result, pressure decrease in the winding can be prevented during cool-down. The elastic modulus in the transverse direction of a GFRP is calculated by dividing the elastic modulus of the impregnating resin by Rc. Fractures of the GFRPs at low temperatures are primarily in shear mode under four-point bending, in-plane compression, transverse compression, or interlaminar shear loading. Moreover, the transverse-compressive fatigue limit of plain-woven GFRP at low temperature depends on the maximum compressive stress of the cyclic loading under high mean-compressive stress and the stress range under low mean-compressive stress.
Critical current density properties for YBa2Cu3O7 films which have different pinning strengths were measured as a function of magnetic field, temperature or magnetic-field angle. We found that a kink anomaly appeared in the temperature dependence of critical current densities, Jc, and that the temperature at which the anomaly appeared depended on the extrinsic pinning strength. The magnetic field and angular dependences of Jc indicate that the anomaly is due to the crossover from intrinsic to extrinsic pinning. Using a competition model of intrinsic and extrinsic pinning, a kink anomaly is explained successfully.
In order to establish a technology needed for a small-scale 100kWh SMES device, a SMES model coil was fabricated. Performance tests were carried out at the Japan Atomic Energy Research Institute (JAERI) in 1996. After that, the coil was installed in facilities of the Lawrence Livermore National Laboratory (LLNL) and tested in 1998, in collaboration between Japan and the United States. The AC loss results at LLNL were in good agreement with those at JAERI. It was reconfirmed from the results that the coupling loss of the coil consists of two components with short and long time constants. We found out from the signals of Hall probes that the current loops with long decay time constants were induced in the CIC conductor. These loops could enhance additional AC loss in the coil. To reduce the AC loss, we made small-sized CIC conductor of strands having a CuNi surface, a fabricated the small coil. The measured AC loss of the small coil was reduced to about 1/6. The CuNi surface on the strands was demonstrated to be effective to reduce AC loss in the coil.