Joint characteristics of ultrasonic welded CC bridge joints for HTS coil applications

Long lengths of second-generation REBCO coated conductor (CC) tapes must be joined together when power cables, coils, and magnets requiring kilometer-scale lengths are fabricated. Various resistive joining techniques of CC tapes, such as soldering and diffusion reaction, have been studied. The joint method should provide good adhesion, low joint resistance (R _j), no critical current (I _c) degradation, cost effectiveness, and simplicity in processing. An ultrasonic welding (UW) process was developed and used to form various types of CC joint structures, such as lap- and butt-joints, using differently stabilized REBCO CC tapes. In this study, the UW process was used to form various bridge-joints of CC tapes for high-temperature superconducting coils. A mechanical-controlled soldering method which has provided a low contact resistance was also adopted for comparison. CC joints of various structures were fabricated using UW and soldering, and the retained I _c and R _j were measured at 77 K and self-field in each. The electromechanical properties of the CC bridge-joints were evaluated using a bending test. Results showed that R _j varied in the CC joints depending on the structure type and joint area. Moreover, the additional Sn layer in Cu-stabilized CC tapes improved R _j in each joint structure because it provided a solder-like bond at the joint interface. The bridge-joints formed using UW showed comparable bending diameter tolerances for irreversible I _c degradation to those formed using mechanically controlled soldering. The multiple 4 mm wide CC bridges provided more flexibility at the joint part due to gaps between bridges and resulted to a better tolerance to tension bending than the single 12 mm wide ones. (paper)