Superconducting joint is crucial for second generation high-temperature superconducting (2G HTS) closed-loop coils that work in persistent current mode (PCM) operation. Here, we report an efficient approach for superconducting joint of YBCO coated conductors (CCs). The YBCO layer was etched to from some microchannels, which serve as oxygen diffusion paths during oxygenation annealing, to accelerate the recovery of critical current (I c) of the joined two YBCO layers. The I c of the superconducting joint is 118 A at 77 K with an optimized joining temperature and a short oxygenation annealing time (10 h), which is about 82% I c of the joined YBCO CCs. The persistent field decay of the closed-loop coil shows a joint resistance (R j) of less than 2.2×10-13 Ω at 77 K. Two YBCO films epitaxially grown along the c-axis diffuse into each other at the interface and form a dense joining, enabling superconducting current path. This superconducting joint technique can promote the achievement of the PCM operation in 2G HTS magnet applications, such as magnetic resonance imaging and nuclear magnetic resonance.

Second-generation high-temperature superconducting (2G-HTS) tapes based on REBa2Cu3O7-x (REBCO, RE: rare earth) materials enable the energy-efficient and high-power-density delivery of electricity, thereby promoting the development of clean energy generation, conversion, transmission, and storage. To overcome the weak grain-boundary connection and poor mechanical properties of these superconductors, a thin-film technology for epitaxy and biaxial textures based on flexible substrates has been developed. In recent years, high-quality 2G-HTS tapes have been produced at the kilometer scale and used in superconducting demonstration projects. This review first summarizes the development of HTS materials and briefly expounds the properties of REBCO superconducting materials. Subsequently, the structural characteristics, preparation methods, and current research progress of 2G-HTS tapes are given. In addition, the applications of REBCO tapes in constructing high-field magnets are also briefly reviewed.

Irradiation can precisely control defects in, and improve the superconducting properties of, REBa2Cu3O7-δ (REBCO, RE: rare earth) coated conductors (CCs). Here we report an effective approach for enhancing the in-field performance of GdBCO CCs. The critical current density (Jc) of GdBCO films was significantly improved through cooperative defects created by co-irradiation with O ions and protons, especially at low temperatures and high magnetic fields. Surprisingly, the in-field Jc of even highly optimized commercial CCs can be nearly doubled. The cooperative irradiation-induced defects are uniformly distributed throughout the GdBCO layer, which promotes the overall performance of the CC. Moreover, the dimensions of these irradiation-induced defects closely match the coherence length of REBCO. This simple and efficient method is a practical post-production solution to improve the in-field performance of commercial REBCO CCs.