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

Abstract

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 80-100 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.

Spatial Recognition of a Superconducting Quantum Interference Devices Nondestructive Evaluation System Using a Small Room-Temperature Probe

“…The crack of 7 mm width showed the minimum detectable width with a signal-to-noise ratio of about 2. In conclusion, the spatial resolution of the crack width was up to several tens of micrometers at a depth of 3 mm 17) and reached 7 mm on the surface. Such results represented a significant improvement over other methods where cracks ranged from several millimeters to tens of micrometers on the surface.…”

“…14) By connecting the pickup coil of the probe to the input coil surrounding the SQUID sensor, the noise level of the SQUID NDE system is increased to 8:5 AE 1:5 pT/ ffiffiffiffiffiffi Hz p at 400 Hz, within the requirement of NDE application, because of the introduction of the thermal noise of the transfer coil. 17) The noise reduction ability of the pickup coil is demonstrated by the phenomenon indicating that the noise level at 400 Hz remains the same without the noise effect from the power of the stepper motor or scanning vibration. 17) Both components of the magnetic field produced by eddy currents, i.e., the out-of-phase component B 90 and the inphase component B 0 , were measured with respect to the excitation field using a lock-in amplifier.…”

“…17) The noise reduction ability of the pickup coil is demonstrated by the phenomenon indicating that the noise level at 400 Hz remains the same without the noise effect from the power of the stepper motor or scanning vibration. 17) Both components of the magnetic field produced by eddy currents, i.e., the out-of-phase component B 90 and the inphase component B 0 , were measured with respect to the excitation field using a lock-in amplifier. The variation in raw output, B 90 or B 0 , with the scanning path is denoted by a solid line in Figs.…”

“…Consequently, the noise level of the SQUID sensor surrounded by an input coil that is not yet connected to the pickup coil of the probe is around 4 pT/ ffiffiffiffiffiffi Hz p at an operating frequency of 400 Hz. 17) Here, the noise level is the same as that of the bare rf-SQUID sensor, 18) and the operating frequency is determined by the noise spectrum of the system and the considered optimal frequency. 11) In our design, the coplanar probe is composed of a double D-shaped pickup coil and a quadruple excitation coil.…”