EDDY-CURRENT EFFECTS IN TWISTED AND WOUND SSC STRANDS

Although the effect of strand twist upon eddy-current losses in multifilamentary superconducting composites is well understood, the combined effects of twist and subsequent winding are not as well documented. An analysis has been made of the effective pitch (with respect to an applied field transverse to the winding axis) of a pair of filaments that have been first twisted and then helically wound. In the case of very different values of twist pitch and winding pitch, the effective pitch is simply the smaller of these. However, as the value of winding pitch approaches that of the twist pitch, the effective pitch may become quite large. Results of this analysis are compared with magnetization measurements of a series of helically wound strands with varied twist pitch. These measurements show large eddy-current losses in a twisted strand with twist pitch comparable to winding pitch; more generally it is shown that eddy-current loss depends upon the superposition of twist and winding. The general shape of the magnetization as a function of applied field M(H) loop due to the combined effects of filamentary hysteresis and eddy-currents has been mathematically modeled.