Critical current measurement unit utilizing Bi-based oxide superconducting current leads and cryocoolers

摘要

A measurement unit has been developed to investigate the dependence on temperature and magnetic field of superconducting cable critical current without the use liquid helium or liquid nitrogen. A test specimen, which is tested using the four probe method, is cooled by a Gifford-McMahon (GM) type cryocooler to a temperature of 20 K to 90 K in a vacuum vessel. Transporting direct current, up to 500 A, is supplied to the specimen through the Bi-based oxide superconducting current leads. The advantages of utilizing oxide superconducting current leads are that the leads have low thermal conductivity, and also that the leads create no Joule heating effect, so the heat input to the sample is minimized. The external magnetic field of up to 3 T, which is applied to the specimen, is generated by a superconducting magnet which also uses Bi-based oxide superconducting current leads and is also cooled by a GM cryocooler. Detail design of the unit, the results of operating test and an example of the measurement result on the Bi-based oxide bulk specimen are presented in the paper.