Fabrication and Electromechanical Study of Symmetric Tape Round (STAR) REBCO Wires

High temperature superconducting (HTS) materials are essential for applications that require to operate at magnetic fields higher than 20 T. In commonly used HTS materials, Rare-Earth1Ba2Cu3O7-x (REBCO) coated conductor is an attractive candidate because of its exceptional mechanical strength and high current carrying capability. Due to the planar geometry, the REBCO tapes are usually reformatted into wire forms. For specific compact accelerator applications such as Canted Cosine θ (CCT) coils, the existing REBCO wires cannot provide required Je values at small bending diameters. Therefore, flexible REBCO wires that maintain high Je values at small bending diameters are needed. To improve the bending performance of REBCO tapes, the key is to reduce the distance between the REBCO layer and neutral axis. It can be achieved by reducing the thickness of Hastelloy substrate and depositing copper onto the REBCO side. The REBCO tapes that consist of a thin substrate and a copper layer with specific thickness on the REBCO side are called “symmetric tapes”, as nearly the same bending strain is expected at each side of the REBCO layer. The fabrication process and optimization of symmetric tapes were discussed. With 22 μm substrates, symmetric tapes maintained 98% of the original Ic values at a 0.8 mm bending diameter. On the basis of symmetric tapes, STAR wires were developed. A custom wire winding machine was built to produce long length STAR wires with constant quality. Parameters of STAR wire, including the wire former size, width of tape, and number of layers, were investigated in detail. The bending property and current carrying capabilities of STAR wires at 77 K and 4.2 K were analyzed. At 4.2 K in a 31.2 T background field, a Je value of 299 A/mm2, which was the highest reported value, was achieved. Other factors that influence the performance of STAR wires, such as the terminal structure and interlayer resistance, were studied as well. The results exhibit the potential and feasibility of using STAR wires in compact accelerator applications at 4.2 K.