The characterization of a sensitive room-temperature probe for use in a SQUID nondestructive evaluation system

In this work, a SQUID nondestructive evaluation (NDE) system with outstanding mobility and performance is developed using two units connected by copper wire. One of those units is a SQUID unit consisting of a high-Tc SQUID as well as a surrounding input coil, which are both cooled in liquid nitrogen and kept in a Dewar shielded by a cylindrical can with a shielding factor of 80-100 dB ranging from DC to 1 kHz. The other is a small, room-temperature probe composed of excitation coils and pickup coils. Based on the quadruple excitation field with the advantages of not only good balance of excitation noise but also a higher field gradient for enhancing the magnetic field distorted by small cracks underneath and a coplanar and differential coil with good ability to diminish the sensing of the magnetic field in the non-flaw region, the small and activated probe is made of quadruple excitation coils distributed in double D-shape differential pickup coils. The analysis shows that the SQUID NDE system using the novel probe design has advantages such as low thermal-noise introduction to the SQUID system, high transfer efficiency, efficient balancing of the excitation field as well as the dynamic noise during scanning, and high sensitivity, with a signal-to-noise ration (SNR) of 2 against the noise level of 8.5 ± 1.5 pT. This SQUID NDE system with the proposed probe is characterized by cracks with different widths at different depths, for example, fine spatial resolution up to 7 μm crack width on the surface of copper foil.