The effect of epoxy impregnation on the behaviour of a Bi-2223/Ag coil carrying DC or ACcurrent

Abstract

We studied the behaviour of a superconducting coil consisting of three doublepancake coils made of Bi-2223/Ag multifilamentary tape with high criticalcurrent, cooled by liquid nitrogen at atmospheric pressure. One double pancakecoil located at one edge of the winding had a lower value of critical currentthan the other ones, as occurs in real coils quite often. We measured DCI–V curvesof the complete coil and of all pancake coils and the temperature increase in several parts of the winding atvarious d I/d t. The coil impregnation resulted in larger heating of the coil. The critical currents of theindividual pancakes after impregnation were reduced by 5% to 15% in comparison with thevalues measured before impregnation. The thermal runaway current was approximately twotimes larger than the coil critical currents defined by the mean value of the electric field1 µV cm−1. The values of safe operation currents are discussed. We also measured the dependence ofAC losses and the coil temperature on the coil current with a frequency of 50 Hz. Thecontribution of the resistive losses in the double pancake coil with reduced critical currentto the total AC losses of the whole coil was very small, which is very important for ACapplications of Bi-2223/Ag tapes.