The figure below shows the principle of our SQUID-based eddy current NDE system.  A circular coil, usually with a diameter of a few mm, generates eddy currents in the sample to be tested.  Inhomogeneities, such as cracks or inclusions of material having a conductivity different from that of the sample lead to a distortion of the eddy current flow, and thus to a change in the eddy current field, which we detect by scanning the sample with a SQUID.  In order to minimize the excitation field at the location of the SQUID, our system uses an electrical compensation scheme in which the field of the circular exciation coil is compensated electronically at the location of the SQUID by feeding part of the excitation current through the modulation coil used for flux locking the SQUID — see below. 

A low magnetic field noise is required of the sensor measuring the eddy-current field. On the other hand, the sample itself produces thermal noise so that the field noise of the sensor usually need not be smaller than about 100 fT in a bandwidth of 1 hertz.  The obtainable signal-to-noise ratio is directly proportional to the excitation field.  We use an excitation field of up to 1 mT peak-to-peak; the current in the excitation coil then is about 2 A peak-to-peak.

Although the excitation field at the location of the SQUID is minimized by the
compensation coil, a dynamic range of about 20 to 50 flux quanta is still needed at the excitation frequency to prevent unlocking of the SQUID by scanning across larger defects in the sample.  The slew rate required of the flux-locked loop then is about 3 flux quanta per microsecond.  In order to minimize temperature drift and be less susceptable to rf interference, our system employs a conventional flux-modulated flux-locked loop with a modulation frequency of 4 MHz.  The voltage noise of the electronics is about 100 pV in a bandwidth of 1 hertz, and the system achieves a dynamic range of about 15 flux quanta at 100 kHz.  The niobium dc SQUID is used as a magnetometer with a field noise of about 50 fT in 1 hertz bandwidth.  If the excitation frequency is higher than a few kHz, the system can be operated in a conventional laboratory environment.  If the full sensitivity of the system should be utilized at lower excitation frequencies, a gradiometric SQUID can be used instead of a magnetometer.

The images below show the eddy current field above two 3-mm-thick niobium sheets which contained small amounts of foreign material with slightly higher conductivity (seen as bright spots in the image).  This measurement was performed with our SQUID NDE system at an eddy current frequency of 20 kHz, at which the skin depth was about the thickness of the sheet.