A superconducting joint technology used for high-temperature superconductors (HTS) is the key for enabling persistent operation of HTS magnets. In the present work, we have succeeded in developing a superconducting joint between REBCO-coated conductors (CCs) using a joint strap with a microcrystalline GdBCO precursor intermediate layer. Heat treatment and oxygen annealing, with a total processing time of less than 1 d, grows a biaxially-textured intermediate layer to connect the GdBCO layers in the CCs. Microstructure observation of a part of the joint cross-section with SEM and TEM showed that the intermediate layer and the GdBCO layers in the conductors were atomically connected. An electron backscatter diffraction result showed that both the c- and a-axis misorientations among the GdBCO layers of the joined conductor and the GdBCO layer of the joint strap were about less than 5°. This intermediate grown superconducting joint gives a critical current of >100 A at 77 K in a self-field. A critical current of a joint at 4.2 K in a self-field is seven times higher than that at 77 K. The persistent field decay of a small double pancake coil, terminated with this joint, showed a joint resistance in the range of <3 × 10−12 Ω to <5 × 10−13 Ω at 77 K in a self-field over three days, with an operating current of ∼10 A (∼14% of the calculated coil critical current). The results show a promising prospect of the joint to be used for persistent magnets such as NMR and MRI.

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This paper describes the first persistent-mode medium magnetic field (400 MHz; 9.39 T) nuclear magnetic resonance (NMR) magnet which uses superconducting joints between high-temperature superconductors (HTSs). As the ultimate goal, we aim to develop a high-resolution 1.3 GHz (30.5 T) NMR magnet operated in the persistent-mode. The magnet requires superconducting joints between HTSs and those between an HTS and a low-temperature superconductor (LTS). Towards this goal, we have been developing persistent-mode HTS inner coils to be operated in a 400 MHz (9.39 T) NMR magnet and here we present the first prototype inner coil wound with a single piece (RE = rare earth)Ba2Cu3O7−x (REBCO) conductor. The coil and a REBCO persistent current switch are connected with intermediate grown superconducting joints with high critical currents in external magnetic fields. To evaluate the performance of the joints in an ultimately stable and homogeneous magnetic field, the coil is operated in the persistent-mode, generating 0.1 T, in a 9.3 T background magnetic field of a persistent-mode LTS outer coil. The magnetic field drift over two years of the 400 MHz LTS/REBCO NMR magnet is as small as ∼1 ppm, giving high-resolution NMR spectra. The magnetic field drift rate over the second year was 0.03 × 10−3 ppm h−1, which is more than three orders of magnitude smaller than that required for an NMR magnet, demonstrating that the superconducting joints function satisfactorily in a high-resolution NMR system. The corresponding joint resistance is inferred to be <10−14 Ω.

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