Non-Destructive Evaluation of Deep-Lying Defects in Multilayer Conductors Using HTS SQUID Gradiometer

We examined the ability to detect deep-lying defects in multilayer stacks of aluminum plates using an eddy-current testing technique with HTS planar-type SQUID gradiometers. In addition to a previous gradiometer with a baseline of 1 mm, a new gradiometer with a baseline of 8.5 mm and a larger effective area fabricated by HTS multilayer and ramp-edge junction technologies was used. The frequency and depth dependences of the SQUID signal were compared between the two gradiometers. By employing the longer-baseline gradiometer and an excitation frequency of 35 Hz, a slit-like defect located at about 38 mm in depth could be clearly observed with a signal-to-noise ratio larger than 10.