HTS SQUID system with Joule-Thomson cryocooler for eddy current nondestructive evaluation of aircraft structures

For the detection of deep-lying flaws in aircraft structures, an eddy current system in conjunction with a planar SQUID gradiometer is being developed. The need for a mobile system imposes additional requirements on cooling regarding mobility, operation independent of spatial orientation, and handling. We present results on the operation of HTS SQUIDs with the commercial Joule-Thomson-cryocooler KC 100 ("cryotiger") by APD and report on cooler characteristics and system performance. Magnetometers and gradiometers of both rf and dc type have been integrated with the cryocooler. Noise spectra both in unshielded and shielded environments have been shown to be orientation independent. Compared to stationary operation, the system sensitivity is reduced by additional noise generated by the movement. With an improved integration scheme, this motion-related noise is nearly eliminated. The system was equipped with a differential eddy current excitation and a synchronized digital lock-in detection. Two-dimensional scans are presented that illustrate the system's capability to find fatigue cracks and corrosion pits hidden below several layers of aluminum.