YBCO and Bi2223 Coils for High Field LTS/HTS NMR Magnets: HTS-HTS Joint Resistivity.

This paper presents our latest experimental results on high-temperature superconducting (HTS) splice joints for HTS insert coils made of YBCO and Bi2223, that comprise a 1.3 GHz low-temperature superconducting/HTS nuclear magnetic resonance magnet currently under development at Francis Bitter Magnet Laboratory. HTS splice joint resistivity at 77 K in these insert coils must be reproducible and < 100 nΩ cm2. Several YBCO tape to YBCO tape (YBCO-YBCO) splice·joint samples were fabricated, and their resistivity and I c were measured at 77 K. First, we describe the joint splicing setup and discuss the parameters that affect joint resistivity: pressure over joint surface, solder, and YBCO spool batch. Second, we report results on YBCO-YBCO joints at 77 K in zero field. Measurements have shown that spool batch and solder are primary sources of a wide range of variation in YBCO-YBCO joint resistivity. By controlling these parameters, we expect to reproducibly achieve HTS-HTS resistive joints of resistance < 100 nΩ cm2.