Development of Electromagnetic Nondestructive Testing for Inspecting Mechanical Lap Joint of High-Temperature Superconducting Tapes

To facilitate construction and maintenance of nuclear fusion reactors, a remountable (demountable) high-temperature superconducting magnet was proposed. The magnet is assembled from several segments with mechanical joints. The electrical resistance of joint (joint resistance) at cryogenic temperature has great influence on the performance of the magnet, a nondestructive testing technique at room temperature is highly demanded to predict joint resistance. In this study, the electrical characteristics of the joint were focused on to develop an electromagnetic NDT technique for inspecting the joint. To identify the factors relating to the joint resistance and the correlation between the electrical characteristics, the cross sections of the joint were observed and the electrical resistance and impedance of the joint were measured. The result indicated a small air gap existing in the joint that could possibly be the cause for increment in joint resistance, though the degree of influence of such gap could not be evaluated over measured electrical resistance and impedance result. Causal analyses were conducted by numerical models, which indicated arranging the current perpendicular to the joint surface is effective for inspecting the joint. Then additional numerical analyses were conducted to evaluate the feasibility of applying the direct measurement and eddy current testing for inspection. The result showed the possibility of detecting particular air gap, optimization of the measurement condition is required.