Automatic Detection of Local Obstacles in a Long Length RE-123 Coated Conductor by Deep Learning Based Image Classification in Reel-to-Reel Magnetic Microscopy

We have succeeded in detecting local obstacles automatically in a 200-m-long RE-123 coated conductor (CC) by introducing deep-learning based image recognition in reel-to-reel scanning Hall probe microscopy (RTR-SHPM). Longitudinal critical current (
I_c
) homogeneity in CCs is one of the most important requirements for practical applications. Usually, such properties as
I_c
variation as a function of longitudinal coordinate is characterized by magnetization measurements adopting Hall probe array as a de facto standard characterization method for ensuring uniformity in long CCs. In this measurement, local
I_c
drop indicates the existence of current blocking obstacles. The group of authors also developed a magnetic microscopy applicable to reel-to-reel continuous measurements, RTR-SHPM, which makes it possible to visualize two-dimensional critical current density (
J_c
) in the tape plane because of its high-resolution imaging along the tape width. As a result, more elaborate defect detection has been enabled. However, in the conventional technique, the observation depends on the human eye in order for characterizing detailed features of the obstacles such as shape and size especially in case with a small
I
_c
drop, therefore, there was a limit to analyze the
J_c
mapping with such detailed information extending to thousands of images in the long tape of several 100 of meters. In this study, the image classification based on the deep learning method was introduced in our magnetic microscopy. The analytical model classifies the input image into the defect position and the normal position, respectively, together with a heat map and a score of confidence in the recognition. As a result, we have succeeded in detecting obstacles automatically from more than 4,000 of
J_c
images with a high accuracy of 98.5%. Furthermore, we revealed the existence of the obstacles which were not distinguishable by the local
I_c
criterion. This method allows us to clarify the origin of the instability of long CC wire and will have a strong impact as an evaluation technique for dramatically improving the reliability of the CCs.