Pushing coated conductor critical currents beyond 1 kA per cm width: Stacks of YBCO layers

For a number of superconducting power applications, a high value of the engineering critical current density (J_e) for the wire is crucial. The superconducting layer in the coated conductor is typically a small portion of the overall cross-section, so increasing the superconductor fraction will directly result in an increase of J_e. However, as the thickness of the superconductor is increased,J_c eventually drops. We describe a way to increase J_e by making a stack of superconducting layers using sequential Ion-Beam Assisted Deposition (IBAD)/Superconductor deposition. Reactive Co-Evaporation by Cyclic Deposition and Reaction (RCE-CDR) is used for superconductor. An IBAD-textured layer resets the crystalline structure after each superconducting layer and we use IBAD-MgO for this purpose. However, IBAD-MgO texturing requires an extremely smooth starting surface (about 1 nm root mean square roughness), whereas the YBCO layer is typically 10-100 times rougher. We employ the Solution Deposition Planarization (SDP) process to planarize the rough surface of YBCO. The SDP layer is insulating and it provides for an easy way to separate the superconducting layers electrically. We discuss unique features of the stacking structure that allow for high I_c , low ac-losses in applied fields, as well as high J_e.