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Quench Protection Study of a Large Scale REBCO Magnet With Additional Copper Tapes

REBCO coated conductors (CCs) are expected to be applied to high field magnets due to their high longitudinal tensile strength and excellent critical current characteristics in high fields. For large scale magnets with large stored energy, quench protection is a key issue. Although various quench protection methods have been proposed for REBCO magnets, the most traditional and straight forward approach is to increase the copper stabilizer and reduce the current density. This approach leads to a significant increase in conductor length to generate a required magnetic field in case a small bore diameter magnet, while the conductor increase would not be serious in case a large bore diameter one. Based on this approach, we have successfully fabricated a practical cryogen-free superconducting magnet (CSM) with a large bore of 200 mm-diameter and a central field of 5 T using REBCO CCs laminated with 0.3 mm-thick copper tapes. For further study, we designed a MJ-class 10 T-CSM with the same inner-diameter as the 5 T-CSM and investigated the feasibility of the magnet through quench tests in this study. We conducted quench tests on short samples of copper-plated REBCO CCs with copper tape soldered on different surfaces, and quench tests on a small coil assuming an energy recovery method with external resistance. In the short sample tests, it was found that the temperature rise was suppressed regardless of the position of the copper tape lamination, and the normal zone propagation velocity was improved when the copper tape was laminated to the substrate side. In the small coil tests, the decay time constant after shutdown was varied to find the conditions that can protect the magnet with high stored energy. We confirmed that quench protection of 10 T-CSM was feasible under practical conditions.