Electronic gradiometry for NDE in an unshielded environment with stationary and moving HTS SQUIDs

Difficulties in the fabrication of multilayer high-temperature superconductor (HTS) devices have led to recent interest in the use of simpler HTS SQUID magnetometers in electronic gradiometers. One application of such systems is electromagnetic non-destructive evaluation. We have developed a prototype two-SQUID system and we present recent results in this paper. We first demonstrate the level of interference suppression by comparing magnetometer and gradiometer signals. Then we present several results taken conventionally with the HTS SQUIDs stationary above moving specimens and, for the first time, with the SQUIDs unshielded in motion above stationary specimens. The specimens comprise a pair of wires in a return current loop as a calibration source, and an aircraft-grade aluminium plate with fine slits mimicking fatigue cracks, first exposed and then covered with an additional aluminium sheet to simulate internal flaws. These results are an important, though by no means final, step towards practical non-destructive evaluation of real test subjects with HTS SQUIDs