Mobile cryocooler-based SQUID NDE system utilizing active magnetic shielding

A mobile cryocooler-based SQUID NDE system was developed for realization of NDE of fixed targets that can´t be moved or rotated. In order to move the SQUID in an ambient field, an active magnetic shielding technique employing a compensation coil and feedback circuit including a band elimination filter was introduced in a SQUID NDE system. The HTS SQUID gradiometer was cooled by a coaxial pulse tube cryocooler to 74 K ± 0.04 K. The flux noise caused by moving the SQUID at 32 mm/s was well suppressed. Unlocking of the flux-locked loop (FLL) circuit or a significant increase in noise did not occur during the motion. Detection of hidden slots in fixed single and double-layered carbon/carbon composites in ambient field was demonstrated by moving the HTS SQUID gradiometer.