A strategy for the reduction of ac losses in YBCO coated conductors

YBCO coated conductors (CCs) are being developed with extremely high dc critical currents, which should allow their extensive use in power applications. However, the CCs manufactured at present have unacceptably high energy losses in ac magnetic fields applied perpendicular to the CC face. This magnetic ac loss can be significantly reduced if the superconductor is divided into parallel electromagnetically decoupled filaments. Simply cutting the YBCO into filaments does not achieve this, as the filaments remain coupled. In this paper we present a new technique for obtaining decoupled filaments. This is achieved by cutting the superconductor into parallel filaments plus introducing periodic transverse cross-cuts. The cross-cuts allow the magnetic flux to penetrate between the filaments and thus decouples them. The continuity of the current path is maintained by means of normal metal 'bridges' across the cross-cuts. We present data demonstrating that at power frequencies five cross-cuts every metre are sufficient to decouple the filaments. We also show that it is straightforward to produce normal metal bridges whose Ohmic losses due to the transport current are sufficiently low that the overall losses of a CC carrying ac or dc transport currents in applied ac magnetic fields are reduced to manageable levels. In a specific example, the losses of a CC in a 20 mT, 60 Hz magnetic field carrying 100 A ac transport current were reduced by nearly two orders of magnitude. Numerical simulations based on the finite-element method have been also performed and confirm some of the experimental results. We also indicate how this technique may be further improved.