Effect of ferromagnetic rings on critical current and AC losses of a coil made from Bi-2223/Ag tape

Laminated ferromagnetic rings positioned at the ends of coils wound with anisotropic Bi-2223/Ag tapes are supposed to reduce the radial component of magnetic field at the coil ends. As a result the critical current of the coil should increase and AC losses should be reduced. To test the effects of ferromagnetic laminated rings on the critical current and AC losses we used a test coil consisting of three double pancake coils made from multifilamentary Bi-2223/Ag tape, which were connected in series. We measured DC current-voltage curves and AC losses of the coil with ferromagnetic laminated rings attached to the coil ends and compared the coil behavior with that measured without the rings. Hall Probes were used to measure magnetic field in the vicinity of the winding and the results are compared with those obtained by calculations. The influence of the distance between the coil and the rings was also studied. The additional losses in the rings are compared to the loss reduction in the superconducting coil winding.