Superconducting and mechanical properties of impregnated REBCO pancake coils under large hoop stress

We performed hoop stress tests of REBCO multilayer pancake coils impregnated by epoxy resin. The mechanical deformation and electric field-current properties were measured under the large hoop stress. The maximum hoop stress of about 530 MPa per Hastelloy substrate, calculated from the $BJR$ relation, was applied in the background magnetic field of 8 T. Because the hoop stress level is much smaller than the mechanical tolerance of the GdBCO tape, the coil performance was limited by the angular dependence of critical current in this test. Furthermore, the hoop stress test under the large electromagnetic stresses was also carried out for the other (Y, Gd)BCO coil. The (Y, Gd)BCO epoxy impregnated coil was operated without any degradation even in the huge hoop stress over 1300 MPa. The mechanical deformation of the coil is analysed on the basis of the measured strains. We confirmed the large strain about 0.3%-0.5% because of the hoop stress over 1300 MPa at I _op = 460 A and B = 13.5 T but no degradation of the coils. However, it is suggested that the coil deformation is very complicated under the large electromagnetic stress.