Analysis of eddy current AC loss for untwisted, multifilamentary superconducting composites with various aspect ratios

AC loss was measured for several types of untwisted multifilamentary superconducting tapes using a VSM (vibrating sample magnetometer) at 4.2 K in fields both face-on (FO) and edge-on (EO) to the wide sides of aspected samples. The strand types investigated were NbTi/Cu-Mn (5000 filaments), NbTi/Cu (5000 filaments), and Bi2223/Ag. The samples were typically 1-1.5 cm long, untwisted, with a wide variety of outer aspect ratios, a/b. The measured losses were compared to those predicted by an expression linear in a/b. From this expression, the transverse resistivity of the matrix, ρ⊥, was extracted and compared to that determined from directly measured RRR values, where _ρ⊥=((1+λ)/(1-λ))^±1 _ρbulk=G_ρbulk. While the measured losses were generally in agreement with the calculations for strand types with larger numbers of filaments, significant deviations were found for strand types with smaller numbers of filaments. Strands with lower numbers of filaments always had lower than expected losses, leading to aρ_⊥,eff ≫Gρ_bulk, which was interpreted in terms of a reduced or “effective” eddy current volume, V_f. This effect, while quite strong for these untwisted samples, is expected to be less pronounced in twisted samples, with some exceptions as noted