Reduced Magnetization and Loss in Ag–Mg Sheathed Bi2212 Wires: Systematics With Sample Twist Pitch and Length

Suppression of magnetization and effective filament diameter deff with twisting was investigated for a series of recent Bi2212 strands manufactured by Oxford Superconducting Technologies. We measured magnetization as a function of field (out to 14 T), at 5.1 K, of twisted and nontwisted 37 × 18 double restack design strands. The samples were helical coils 5-6 mm in height and approximately 5 mm in diameter. The strand diameter was 0.8 mm. The magnetization of samples having twist pitches of 25.4, 12.7, and 6.35 mm were examined and compared to nontwisted samples of the same filament configuration. The critical state model was used to extract the 12-T d_eff from magnetization data for comparison. Twisting the samples reduced d_eff by a factor of 1.5-3. The d_eff was shown to increase both with L and L_p. Mathematical expressions, based upon the anisotropic continuum model, were fit to the data, and a parameter γ₂, which quantifies the electrical connectivity perpendicular to the filament axis, was extracted. The bundle-to-bundle connectivity along the radial axis was found to be approximately 0.2%. The d_eff was substantially reduced with L_p. In addition, the importance of understanding sample length dependence for quantitative measurements is discussed.