A superconducting joint for GdBa₂Cu₃O _7-δ-coated conductors

Second-generation (2G) GdBCO-coated conductors (CCs) are promising for superconducting magnet applications because of their high critical current (I_c) density, low dependency of the _c on the external magnetic field, good mechanical properties and reasonable cost, which offer opportunities to develop ultra-high-field magnets. However, they have not been used in hightemperature superconducting (HTS) applications with persistent current mode (PCM) operation such as nuclear magnetic resonance/magnetic resonance imaging magnets owing to unavailability of fabrication techniques for proper joining and contacts. Here we report a resistance-free joint, termed a 'superconducting joint', for 2G GdBCO CCs that forms a direct connection to establish a superconducting closed loop for PCM operation. The I_c value of the joined CCs is identical to that of the parent conductors in a liquid nitrogen bath (77 K). Moreover, the initially induced magnetic field of a model GdBCO coil containing a superconducting joint is maintained without decreasing, indicating the complete absence of electrical resistance. Thus, this fabrication method is a unique practical solution for lengthening the 2G HTS CCs and, more importantly, achieving PCM operation in 2G HTS magnet applications, including ultra-high-field nuclear magnetic resonance/magnetic resonance imaging magnets generating more than 1 GHz.