SQUID SYSTEMS FOR NON-DESTRUCTIVE TESTING BY AC FIELD MAPPING

Non-destructive testing by magnetic field measurement of current flow distortion by flaws is growing in importance, with AC signals used for noise reduction and to exploit the skin effect in ferromagnetic materials. The signals can be directly injected, or induced by a non-contacting coil. In this paper, we describe a low temperature 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. We 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. We suggest that even relatively poor HTS SQUID performance will be acceptable if these advantages can be exploited, and that AC field measurement is not a particularly demanding SQUID application in any case.