Inductance Matching in Axial SQUID Gradiometer and the Influence on Its Performance

The use of external pickup coils in a gradiometer based Superconducting Quantum Interference Device (SQUID) enhances the sensitivity of gradiometer. These pickup coils convert the coupled magnetic flux into a current that is transmitted to the input coil, establishing a secondary coupling with the SQUID and thus enabling the measurement of weak signals. The combination of inductance between the pickup antenna and input coil critically affects the magnetic field to voltage conversion coefficient and noise level of the entire gradiometer module. Through theoretical derivation, simulation, and practical testing, this study demonstrates that the optimal magnetic field to voltage conversion coefficient and noise levels are achieved when the inductance ratio of the detection to input coils is between 0.9 and 1.1. Moreover, for optimal inductance matching, the twisted pair lengths of the entire gradiometer are presented for axial second-order gradiometers of different radii and baselines in this paper. This guides the practical winding of sensor modules, and a gradiometer with 5 cm baseline and 9 mm radius was validated by experiment.