Design and Preliminary Tests of a 20-T REBCO Insert Coil for a 35-T All Superconducting Magnet

This paper presents the design and preliminary test results of a 20 T REBCO insert coil for a 35 T all superconducting magnet. The 20 T insert coil consists of two coils nested in series. The internal coil (coil 1) is composed of a stack of 20 REBCO double pancake (DP) coils, and the no-insulation (NI) winding approach was applied to increase its current density. The outer coil (coil 2) consists of 28 REBCO DP coils, metal-as-insulation (MI) technology was used to improve the overall mechanical strength and reduce the charging-delay time constant. The electromagnetic and mechanical analysis considering the screening current (SC) effect have been discussed in detail. The maximum hoop stress and hoop strain of the insert coil with the SC effect under operating conditions are 1010 MPa and 0.658%, respectively, but some favorable conditions have not been considered. The tests at 77 K and all assembly work have been completed. The preliminary test results of the insert coil at 4.2 K in self-field show that the large resistances of DP 3 and DP 4 in coil 1 caused the local temperature to rise, which led to the early quench of the whole insert coil. The maximum applied current was 229 A, and the actual central magnetic field with the SC effect was 18.4 T.