AC Losses and Mechanical Properties of Multifilamentary High Temperature Superconductor Tapes and Wires
With the ability to operate at high magnetic fields and high temperatures, and with significant progress in critical current density, RE-Ba-Cu-O (REBCO, RE = rare earth) coated conductors (CC) have an immense potential for use in various coil and cable applications. However, the tape form of these CC creates additional problems. The excessively high AC losses and the limited flexibility are major challenges impeding the development and commercialization of these tapes. Recently, there have been efforts to convert flat REBCO tapes into a round wire. The diameter of these wires has been reduced using thinner substrate tapes and by positioning the copper stabilizer mainly on the REBCO film side. In this dissertation, using a combination of experimental and analytical results, the copper thickness on the REBCO side has been optimized to maximize the critical current retention of tapes with various substrate thicknesses at small bend diameters. The presented analytical method accounts for the neutral axis shift caused by the progressive plastic deformations. Using these results, an optimal design for ultra-small diameter symmetric tape round (STAR) wires is also proposed. In addition, an alternate approach to enhance REBCO CC bending properties by using two tapes joined face-to-face is presented. In this structure, the two REBCO layers are positioned closer to the neutral axis. Two methods to fabricate such structures were implemented and their bending performance characterized. To reduce the AC losses in REBCO tapes, a fully-scaled reel-to-reel filamentization process allowing the production of long length multifilamentary tapes has been developed. The process uses laser ablation followed by oxygenation and selective electroplating. The AC losses of reel-to-reel produced multifilamentary tapes was significantly reduced by preventing copper growth on their back side. To address the coupling losses over long length, a new transposition pattern was also proposed and implemented. Finally, the striation process was modified to allow the integration of the multifilamentary tapes into the STAR wires. STAR wires with various number of filaments per tape were fabricated and their AC losses characterized. Results showed exceptional AC losses reduction over a broad range of frequencies and fields compared to a normal REBCO tape.