We have been developed cryogen-free superconducting magnets (CSMs) at High Field Laboratory for Superconducting Materials (HFLSM), Institute for Materials Research, Tohoku University since 1992 [1]. In particular, high temperature superconducting tapes such as Bi2223 and REBCO are adopted for the CSMs beyond 18 T. In 2015, the 25T-CSM was successfully developed [2] and is open for users with many high field CSMs at HFLSM. For high field superconducting magnets, the REBCO tape is a strong candidate, although the Bi2223 tapes are used for our 20T-CSM and 25T-CSM. To develop our high field CSMs beyond 30 T, we have performed many R&D studies with REBCO tapes. A key issue of REBCO high field superconducting magnets is to develop the robust coil against a local degradation under large electromagnetic stresses and its protection. We propose a two REBCO bundle tape winding with insulation and confirmed that more than 90% of coil critical current could be applied in the REBCO coil with a damage [3]. In addition, an improvement of mechanical stiffness is obtained in the impregnated REBCO pancake coil with thin FRP plates glued on its top and bottom, i.e., “edge impregnation” [4, 5]. In the edge impregnation with a thin FRP plate, the stress distribution is optimized and then the maximum stress can be reduced by about 10-20% [5]. Therefore, the robust coil structure consisting of two bundle winding and edge impregnation is useful for a 33T-CSM, which is a recently approved project [6, 7]. From those R&D study, we found the polyimide for insulation is not adequate from the mechanical point of view. To remove the polyimide in the coil, the REBCO coil insulated and reinforced by a MgO coated Hastelloy tape was made and tested under an electromagnetic stress [8]. Good insulation and mechanical stiffness performance could be confirmed in the REBCO coil with the MgO/Hastelloy tapes. The strategy based on REBCO tapes for high field cryogen-free superconducting magnet developments will be introduced with our future plan.

Acknowledgements: A part of R&D studies of REBCO coil were performed under a collaboration with Toshiba Energy Systems & Solutions Corporation and Fujikura Ltd.

[1] K. Watanabe and S. Awaji, JLTP 133 (2003) 17.
[2] S. Awaji, et al., SuST 30 (2017) 065001.
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[4] K. Takahashi, et al. , IEEE TAS 31 (2021) 4602305.
[5] A. Badel, et al. , IEEE TAS 31 (2021) 4700705.
[6] S. Awaji, et al., IEEE TAS 31 (2021) 4300105.
[7] S. Awaji, et al., to be presented to ASC2022.
[8] A. J. Vialle, et al., to be presented to ASC2022