Proximity Effect Magnetization and Energy Loss in Multifilamentary Composites: Influence of Strand Design and Sample Geometry

Flux trapping and cycling energy losses were studied by vibrating sample magnetometry in fine multifilamentary Nb-Ti superconductive strands for which proximity effect coupling between the filaments is significant. Measurements were made to determine the influence of helical twist about the strand axis as well as sample length for strands experiencing varying levels of proximity effect coupling. The proximity effect strength was varied by investigating strands with a range of filament diameters, as well as by the addition of magnetic impurities to the interfilamentary medium (the matrix) to suppress the proximity effect. Critical currents and fields for the matrix were extracted from the measurements. The reduction of cycling loss1 and magnetization2 previously found was confirmed. Additionally, these measurements were extended to strands where little twist was applied, and the magnetization and cyclic loss were found to saturate. Bean-like models for anisotropic media introduced by Carr1 and later Harada2 were further developed to calculate magnetization and penetration fields in these strands over a large range of twist pitch values. A calculation of magnetic hysteresis loops was also made for short strand samples. These models provide a good qualitative understanding of the observed behavior and lead to useful predictions for applications.