Electromagnetic nondestructive evaluation with simple HTS SQUIDs: measurements and modelling

For ac EM NDE research, the field sensitivity of a SQUID should be around 100 pT, in a bandwidth of 5 Hz or less in the frequency range from 1 Hz to 10 kHz or more. Existing HTS SQUIDs and commercial electronics can already satisfy these demands. More-over, the small size of HTS SQUIDs and the large dynamic range of SQUID systems permit measurements even in some magnetically unshielded environments. Here, we describe non-contacting ac EM NDE with a simple, small, single layer HTS SQUID. We present results demonstrating the detection of a subsurface feature in aircraft-grade aluminium by horizontal field measurement. Although they indicate the potential usefulness of HTS SQUIDs, variation between nominally identical data has been much greater than expected. This is because of fundamental HTS SQUID problems and the fact that experimental constraints on SQUID position, induction coil geometry and stand-off led to a far from optimum system design. We have analysed the problem using FEM; here we present selected results and indicate how they may affect future designs.