First-generation SQUID-based nondestructive testing system

The paper is an overview of the results of an investigation of a first-generation SQUID NDT system working on the principle of the detection of magnetic inhomogeneities. The system incorporates a superconducting field generation coil-to apply a static magnetic field perpendicular to the test subject-and a differential two-coil planar pick-up coil connected to a SQUID, acting as the detector. These components are operated in a liquid He cryostat in a normal (unscreened) laboratory environment. Test subjects are inspected by passing them beneath the cryostat during computer-controlled data acquisition. Typical experimental results are presented, and postprocessing techniques are outlined. Theoretical descriptions of the static field distortion technique have also been developed, based on the numerical solution of integral equations which model flaws as combinations of magnetic dipoles. The equations are presented, and results are compared with those obtained experimentally. Despite significant limitations it is concluded that potential applications for SQUID NDT exist and further investigation is worthwhile.