Low-resistance joining technology of REBCO wires is a key technology in the fundamental in applications of REBCO wires. Sound bonding is one of the promising methods since it is possible to create a diffusion bonding of Cu stabilizing layers to each other only by the acoustic energy from the horn. This allows us to realize not only a low-resistance bonding but also simple and excellent in energy efficiency. In order to realize the optimum bonding conditions, however, it is necessary to clarify the relationship between the process conditions such as the weighting in the process and the input energy, and the bonding characteristics such as the resistivity of the bonding interface and the soundness of the superconducting layer. Previous studies on ultrasonic welding have reported that soft metal can be used to reduce the joint resistivity by increasing the effective contact area of the bonding interface and by increasing the ultrasonic energetics [1, 2]. In this study, we monitored and control such process parameters as the time variation of the horn weighting and the energy consumed in the horn during the bonding process by using a 15 kHz of acoustic wave, and the relation with the characteristics of the obtained joint was studied. The joint resistivity of 30 nW cm2 was realized at 77 K, s.f. in the process-time 0.5 s without using an intermediate metal. It was also confirmed that there is no degradation of the superconducting layer in the joint area by the observation of magnetization current distribution in the joint area as well as tape strands based on magnetic microscopy.

[1] Hyung-Seop Shin et al., Superconductor Science and Technology, Vol.29, 015005 (2016)
[2] Ito et al., Journal of Physics Conference Series, 14th EUCAS, 012065 (2020)

This work was supported by JSPS KAKENHI Grant Number JP19H05617.