Application of high temperature superconductor in National Institute for Fusion Science

In the National Institute for Fusion Science (NIFS), the following three research items are being advanced as application of high temperature superconductor (HTS) to an experimental fusion plasma system, through collaboration with universities and laboratories. The first is an application of HTS in the large current feedthrough to a superconducting coil cooled with superfluid helium in order to reconcile low heat load and stable large current transport capability. A 20 kA current feedthrough from 4.4 K to 1.8 K has been being developed using YBCO bulk superconductors manufactured by the QMG process. The second is the development of a 30 kA HTS current lead. The Bi2212 bulk material fabricated by the diffusion process is considered as a candidate material for HTS current leads. The third is the development of the superconducting magnetic-levitation coil for Mini-RT wound with Bi2223 tape conductor. A magnetic-levitation coil operated in persistent-current mode has to levitate for 8 hours without a feed of refrigerant. Present status of these HTS application researches in NIFS is reported

Development of high temperature superconducting current feeders for a large-scale superconducting experimental fusion system

The National Institute for Fusion Science (NIFS), in collaboration with universities and laboratories in Japan, the Forschungszentrum Karlsruhe (FZK) and the Max-Planck Institut fur Plasma Physik (IPP) in Germany, is planning to develop high temperature superconducting (HTS) current feeders for large-scale superconducting coils. Two programs are being progressed: one is a current feedthrough for superfluid helium (He II) cooled superconducting coils; the other is current leads for experimental fusion system. The paper describes the present status of the two programs, including joint experiments of a 20 kA Bi-2223 current lead developed by FZK and development of a prototype YBCO bulk current feedthrough for He II cooled superconducting coils. In addition, the test results of Bi-2212 tubes fabricated by diffusion process, applicable to HTS current leads, are described Bi_2Sr_2 CaCu_2O_

Normal-superconducting transition induced by high current densities in YBa2Cu3O7-d melt-textured samples and thin films: Similarities and differences

Current-voltage characteristics of top seeded melt-textured YBa2Cu3O7-d are presented. The samples were cut out of centimetric monoliths. Films characteristics were also measured on microbridges patterned on thin films grown by dc sputtering. For both types of samples, a quasi-discontinuity or quenching was observed for a current density J* several times the critical current density Jc. Though films and bulks much differ in their magnitude of both Jc and J*, a proposal is made as to a common intrinsic origin of the quenching phenomenon. The unique temperature dependence observed for the ratio J*/Jc, as well as the explanation of the pre-quenching regime in terms of a single dissipation model lend support to our proposal.Comment: 10 pages, 10 figures, submitted to Physical Review