Investigation on Current Distribution and Joint Resistance-Overlap Length Relationship for Non-Superconducting Joints

Low-resistance joint technology is crucial for high-temperature superconducting (HTS) closed-loop coils that work in persistent current mode. The non-uniform current distribution (NUCD) phenomenon in HTS joint has a significant influence on joint resistance, which may lead to a nonlinear relationship between joint resistance
R_j
and overlap length
l
(
R_j
–
l
curve) instead of following the laws of resistance. In this paper, an arched structure joint made by YBCO tapes has been designed to provide a direct evidence for the NUCD phenomenon inside solder joint. It is found that the current transports mainly at the end of the overlap area and is much different from the regular conductors. The
V
/
I
-
I
method is proposed to measure
R_j
of solder joints (less than 2 nΩ) with various overlap lengths in order to obtain more precise data. A complete
R_j
–
l
curve (
l
= 5–100 cm) has been successfully illustrated and a nonlinear saturation relationship between
R_j
and
l
is presented. The joint resistance tends to approach to a lower limit value about 1.5–1.6 nΩ when the overlap length exceeds 20 cm.