Experimental study of loss mechanisms of AgAu/PbBi-2223 tapes with twisted filaments under perpendicular AC magnetic fields at power frequencies

AC losses under perpendicular AC fields have been measured at 77 K and power frequencies for multifilamentary AgAu (10 wt.%)/Bi-2223 tapes with filaments twisted at different pitches. Using simultaneous measurements of the first and higher harmonics of the voltage induced in the pick-up coil, the main loss contributions (superconductor and coupling current losses) have been obtained separately. At power frequencies, twisting produces the desired uncoupling of the filaments at fields lower than the coupling field, which has also been determined experimentally. In the uncoupled-filament regime, the superconductor losses are reduced strongly with respect to the untwisted tapes. The reduction of the total loss with twisting is also observed. However, due to the important contribution of the coupling current losses for this field orientation, a very small pitch (< 5 mm) is necessary for a considerably lower loss than that of untwisted tapes. The dependence of the coupling field and coupling current losses on the twist pitch has been analysed and compared with the theoretical predictions. (C) 2000 Elsevier Science B.V. All rights reserved.