Progress in the use of SQUIDs for Electromagnetic Nondestructive Evaluation

The superconducting quantum interference device (SQUID) is attractive for electromagnetic nondestructive evaluation: sub-nT sensitivity and wide bandwidth permit high lift-offs or small excitation signals and deep subsurface or thin skin measurements from DC to MHz frequencies. However, cryogenic operation is a major drawback: though LHe is no longer needed, there are still problems to be solved in high temperature superconductor SQUID fabrication to allow reliable operation in LN2. Also, the system must be configured differentially so that small signals are not swamped by environmental fields. Here we report progress in a number of areas: practical systems engineering is improving cryogenics and leading to better instrumentation; simulation tools are being developed, based on the finite element and volume integral methods, and results are being compared with experimental measurements; and integration of thin film SQUIDs and pick-up coils is being studied, to reduce costs and increase signal to noise ratio.