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 /spl plusmn/ 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.