Abstract Body

We have recently been developing a 33-T Cryogen-free Superconducting Magnet (33T-CSM) with REBa₂Cu₃O_7−x (REBCO-CCs, RE: rare earth and/or Y) insert coils.  In addition to a complicated critical surface of REBCO-CCs, REBCO-CCs are exposed to magnetic fields of various strengths (B) and orientations (θ) depending on the axial and radial coordinates in the coils.  Hence, it is essential to design the magnet considering the position-dependent load factor LF(T, B, θ) = (operating current Iop)/(critical current I_c(T, B, θ)).  Conversely, improving the J_c value at (B, θ) that gives the most severe load factor is an effective way to generate higher magnetic fields.

In order to explore better geometry and distribution of artificial pinning centers (APCs) to reduce the load factor, we performed (i) simulations on the magnetic field distribution in REBCO pancakes and (ii) J_c(T, B, θ) measurements on typical REBCO-CCs with different pinning properties.  The magnetic field distribution in REBCO pancakes was simulated with the Biot-Savart law using the design of the 33T-CSM (19 T generation by 64 pancakes under 14 T background field [1]) under construction at the HFLSM of Tohoku University and the 45.5-T Little Big Coil (45.5T-LBC, 14.4 T generation by 12 pancakes under 31.1 T background field [2]) at the NHMFL of Florida State University.  J_c(T, B, θ) of four types of REBCO-CCs (GdBCO without APC from SuNAM, YGdBCO with fine nano-particles provided by Dr. Izumi group of AIST [3], EuBCO with short nano-rods from Fujikura [4], and YGdBCO with long nano-rods from SuperPower) was measured by a conventional DC four-probe method down to 20 K under the magnetic field up to 24 T by using the 25T-CSM at the HFLSM of Tohoku University.

Based on (i) the simulated magnetic field distribution in REBCO coils and (ii) the critical surface evaluated by measured J_c(B, θ) curves, we successfully drew a contour map of LF(20 K, B, θ) for 33T-CSM and 45.5T-LBC (Attached figure is an example of LF(20 K, B, θ) map of Fujikura's EuBCO-CCs for 33T-CSM).  We found that the most severe load factor is expected at (B, θ) ~ (36 T, 79deg.) for 45.5T-LBC without much dependence on the geometry of APC.  This can be understood as that the intrinsic pinning is responsible for the vortex pining in 45.5T-LBC due to the large background field parallel to the ab plane of REBCO.  The most severe load factor in the case of 33T-CSM is expected at (B, θ) ~ (23 T, 62deg.), which varies slightly with APCs.  These results suggest that the improvement of J_c properties at around θ = 60deg. - 80deg. is effective to reduce the load factor of high-field magnets.  As for short nano-rods introduced in Fujikura’s EuBCO-CCs, we have numerically calculated the angular dependence of elementary pinning force and demonstrated that the peak of J_c(θ) at around B||c direction (θ = 0deg.) becomes broader when nano-rods are tilted from the c axis [5].  Such a strategy might be useful for tailoring REBCO-CCs for high-field magnets.

At the symposium, we would like to report and discuss the above results and future prospects in more detail.

References

[1] A. Badel et al., presentation at MT27 (2021) THU-OR5-301.
[2] S. Hahn et al., Nature, 570 (2019) 496.
[3] T. Okada et al., IEEE Trans. Appl. Supercond., 29 (2019) 8002705.
[4] S. Fujita et al., IEEE Trans. Appl. Supercond., 28 (2018) 6600604.
[5] T. Okada and S. Awaji, presentation at CEC-ICMC2023 (2023) M3Or4O-02 (manuscript in preparation).

Acknowledgment

This work was supported by JSPS KAKENHI Grant-in-Aid for Early-Career Scientists (No. 21K14192) and by Ensemble Grants for Early Career Researchers 2023, Tohoku University (No. 40).
Ic measurements at high magnetic fields were performed under the GIMRT Program of the Institute for Materials Research, Tohoku University (Nos. 202211-HMKGE-0401 and 202305-CMKOV-0505).