Field distribution effect on the performances of coils wound with Ag/Bi-2223 tape

A small coil (35 mm bore) was made by winding a superconducting Ag/Bi-2223 multifilamentary tape. The coil, composed of eight pancakes, showed reduced performance in the pancakes placed at the sides with respect to central ones. This effect was attributed to the radial field at the coil ends. In order to explain this behaviour, a general method for predicting the critical current of a coil, starting from basic information on short samples, was developed. The method was used to understand the possible advantages from a reduction of radial field in the coil through a suitable iron yoke for flux return. The measured performance (in terms of critical current) of the coil with iron improved by 10%. The central field at 77 K was 1100 Gauss and 6000 Gauss at 4.2 K.