ELECTROMAGNETIC NONDESTRUCTIVE EVALUATION - MOVING HTS SQUIDS, INDUCING FIELD NULLING AND DUAL-FREQUENCY MEASUREMENTS

We have previously shown that simple, single layer HTS SQUIDs can be u sed effectively in electromagnetic nondestructive evaluation (NDE) usi ng eddy current techniques in a magnetically unshielded environment, H TS SQUID systems for NDE applications are expected to be small and por table allowing non-stationary measurements to be carried out in the Ea rth's field above a stationary sample, Here we present application-ori ented results showing the ability of our HTS electronic gradiometer to cope with the movement of the sensors above a series of simulated fla ws in aircraft grade aluminum samples. To permit tile detection of fin e surface and subsurface structures we have applied active field nulli ng to the two SQUIDs to increase the effective signal to noise ratio. The excitation signal is applied via a non-superconducting coil to pro vide a lower field environment for each device. We also present result s using a dual frequency eddy current technique to allow depth profili ng of flaws in multilayer structures.