Influence of filamentary and strand aspect ratios on AC loss in short, untwisted samples of HTSC and LTSC superconducting multifilamentary composites

Abstract

Measurements of magnetization, M, vs. magnetic field strength, H, have been made on HTSC Bi:2223/Ag and LTSC NbTi/matrix multifilamentary tapes as a function of ramp rate. The integrated loss, Q tot, was extracted from these M– H loops, which had a maximum field amplitude of 17 kOe and ramp rates, d H/d t, ranging from 20 to 700 Oe/s. All measurements were taken at 4.2 K, with H directed normal and parallel, respectively, to the broad tape face. The samples were from 0.5 to 1.5 cm in length, and untwisted. The expressions needed to properly extract effective transverse matrix resistivities, ρ ⊥, are discussed and applied. In general, these expressions indicate that transverse loss in untwisted strands grows linearly with the aspect ratio, in contrast to that in twisted strands, whose losses grow with the square of the aspect ratio. With low aspect-ratio LTSC tapes, our equations gave the expected values of ρ ⊥. However, departures from the expected ρ ⊥ emerged with further flattening of the strands as the filamentary array itself became sparse and highly aspected, and the filaments themselves became flattened. A model attributing the anomalous ρ ⊥ to the effects of these features is proposed.