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Ic agrees well with ICYS Annual Report 2023A superconducting (SC) magnet is an electromagnet made from coils of SC wires. Until now, conventional low-temperature superconductors (LTSs) have been used for SC magnets. To generate higher magnetic fields and operate SC magnets at higher temperatures, research and development of high-temperature superconductors (HTSs) have progressed. (Bi,Pb)2Sr2Ca2Cu3Oy (Bi-2223) HTS was discovered at NRIM in 19881). Ag-sheathed multifilamentary Bi-2223 tape shows high critical current (Ic). The tapes were used for a 24 T magnet developed at NIMS2). A Bi-2223 magnet operated at higher temperatures was also developed at NIMS3).SC joint is indispensable for the fabrication of SC magnets operating in the persistent current mode. Such a persistent-mode magnet enables the energy-saving systems without a power supply. The SC joint for the LTS wires is well established. In the last decade, significant progress has been made in the development of the SC joint between the HTS tapes4).We have successfully developed practical SC joints between Bi-2223 tapes by synthesizing a polycrystalline Bi-2223 intermediate layer5). The synthesis comprises a slurry process, uniaxial pressing at room temperature, and heat treatments. Lab-scale samples showed a practical level of Ic of about 300 A and low joint resistance (Rj) of less than 10−14 Ω at 4 K and 1 T6).The application of the SC joints to a Bi-2223 magnet, it is important to understand the Rj and Ic characteristics of the SC joint in detail. To evaluate the characteristics, we measured the decay of current flowing in a closed loop with the SC joint (current decay measurements). We clarify the time dependence of Rj and field angular dependence of Ic of the SC joint.We fabricated Bi-2223 closed-loop samples with the SC joint by connecting both ends of a tape6). Using the joint resistance evaluation system developed by NIMS, the current decay measurements were performed.Fig. 1 shows the time (t) dependence of circuit resistance (R) of a sample7). At t > 102 s, R decreases significantly with increasing t. We interpreted R–t by considering flux creep and current sharing at the joint. The calculated R–t agreed well with the experimental results. At t > 104 s, the calculation implies that Ij/Iloop (joint current/loop current) ≈ 1 and current sharing is negligible, indicating that R is equivalent to Rj. It is suggested that a low Rj can be achieved after a sufficiently long period. We obtain the approximation of an analytical solution for Rj, which suggests that Rj is almost inversely proportional to time.Fig. 2 shows the field angular dependence of Ic at 4 K8). The experimentally obtained Ic increased as the angle (θ) of the field (B) increased. The Ic probably showed a broad peak at 90°, similar to the angular dependence of Ic for Bi-2223 tapes.The angular dependence of Ic was calculated using a model that Research Digest 1. Outline of Research2. Research ActivitiesYasuaki TAKEDAFig. 1. [adapted from 7)] Time dependence of circuit resistance R in current decay measurements at 4.2 K and 1 T. R–t and Ij/Iloop–t curves obtained by calculation are also displayed.Fig. 2. [adapted from 9)] Angular dependence of calculated and experimentally obtained Ic at 4 K and 0.15–0.28 T.References1) H. Maeda et al., Jpn. J. Appl. Phys. 27, L209 (1988).2) K. Hashi et al., J. Magn. Reson. 256, 30 (2015).3) G. Nishijima et al., Rev. Sci. Instrum. 84, 015113 (2013).4) Y. Takeda et al., Supercond. Sci. Technol. 35, 043002 (2022).5) Y. Takeda et al., Appl. Phys. Express 12, 023003 (2019).6) Y. Takeda et al., Supercond. Sci. Technol. 35, 02LT02 (2022).7) Y. Takeda et al., Appl. Phys. Express 16, 093002 (2023).8) Y. Takeda et al., Supercond. Sci. Technol. 36, 125010 (2023).9) Y. Takeda et al., IEEE Trans. Appl. Supercond. 33, 6400305 (2024).describes the angular dependence of Ic in a Bi-2223 tape9). This model uses a Gaussian distribution of an angle of the c-axis grain misalignment. The calculated the experimentally obtained values at low angles. We believe that the Ic is appropriately described including at high angles. The microstructural observation suggested that the misalignment angle used in the calculation was a reasonable value.24Resistance and Critical Current Characteristics ofBi-2223 High-Temperature Superconducting JointEvaluated by Current Decay Measurements