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-T _c 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 80-100dB ranging from DC to 1kHz. 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 1.5pT. 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 7νm crack width on the surface of copper foil.