EFFECT OF STRAIN ON AC POWER LOSS OF BI-2223 AG SUPERCONDUCTING TAPES/

The ac power losses of monofilament and multifilament Bi-2223/Ag compo site tapes were investigated to determine the effect of mechanical str ain on the loss and to identify the loss mechanisms. Measurement of th e self-field losses was performed at 77 K and 60 Hz as a function of a c current amplitude (0-100 A) for tapes in their as-prepared or virgin state and after being subjected to applied strain by temperature cycl ing or bending to small radii of curvature (R = 1-50 mm). For good qua lity virgin tapes the experimental data are well described by the Norr is equation P similar to I-m(n) for the dependence of the power loss P on the amplitude I-m of the ac transport current, with n = 3 for mono filament and n = 4 for multifilament tapes. Applied strain causes the loss to increase by several orders of magnitude and the exponent n to decrease below the Norris values. At strains very much greater than th e irreversible strain limit, the loss becomes purely ohmic (i.e., (IR) -R-2 loss) and n = 2. Examination by SEM of the transverse cross-secti on of tapes reveals a variety of strain-induced structural defects inc luding transverse cracks that sever the filaments and extended regions where the superconductor core has delaminated from the silver sheath. (C) 1998 Published by Elsevier Science B.V, All rights reserved.