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, show ed reduced performance in the pancakes placed at the sides with respec t to central ones. This effect was attributed to the radial field at t he coil ends. In order to explain this behaviour, a general method for predicting the critical current of a coil, starting from basic inform ation on short samples, was developed. The method was used to understa nd the possible advantages from a reduction of radial field in the coi l through a suitable iron yoke for flux return. The measured performan ce (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.