INTEGRATED LOW-TEMPERATURE SUPERCONDUCTOR SQUID GRADIOMETERS FOR NONDESTRUCTIVE EVALUATION

We describe an eddy current nondestructive evaluation system using a l ow-temperature superconductor magnetic field sensor in an electromagne tically unshielded environment. The sensor comprises a niobium de supe rconducting quantum interference device (SQUID) integrated with a firs t-order gradiometric pickup coil to reject spatially uniform interfere nce fields but remain sensitive to flaw induced fields. We demonstrate its use in locating and mapping subsurface flaws in a multilayered al uminum structure using an eddy current excitation field at a frequency of 70 Hz, with no magnetic shielding around the specimen or cryostat.