SQUID systems for non-destructive testing by AC field mapping

The authors describe a low-temperature liquid-helium superconducting quantum interference device (SQUID) system with high spatial resolution and a wider bandwidth than usual for an all-metal cryostat and which also provides access for a mechanism to balance the pick-up coils. They discuss the effects of these properties on AC field measurement and present experimental results from small slits which mimic growing fatigue cracks. Related work on high-temperature superconducting devices indicates that they will offer important advantages over low-temperature SQUIDs, particularly in terms of cryogenic design which has been so restrictive in low-temperature systems. The authors suggest that even relatively poor high-temperature-superconductor SQUID performance will be acceptable if these advantages can be exploited.<>