Significant reduction of AC losses in Ag sheathed square Bi,Pb(2223) wires with twisted filaments

The large aspect ratio of standard Bi,Pb(2223) tapes results in high losses in perpendicular ac field, at least an order of magnitude higher than the loss level acceptable for practical application. Previous investigations, including oxide barriers, resulted in a more than 10 fold increase in matrix resistivity, have resulted only moderate reduction of ac losses. In the present work, we seek to explore an alternative approach of loss reduction by reducing conductor aspect ratios, with unchanged sheath, and without barriers. Square wires, packed with 36 Bi,Pb(2223) filaments were deformed by 2-axis rolling to a final width of 1 mm and then twisted at different pitch lengths. After thermal processing, J_c values above 10⁸ A·m^-2 were obtained. AC loss measurements showed that the overall loss per Ampere-meter is reduced to less than a fifth compared to that of standard tapes in perpendicular field. In addition, it was also found that for a moderate twist pitch of 20 mm, the coupling field of the present wires at 50 Hz was more that 50 mT, while the peak frequency for the coupling current losses was markedly higher than 100 Hz, a remarkable result obtained without barriers.