Low Noise Active Shield for SQUID-Based Magnetocardiography Systems

A light weight and low-cost active magnetic shield system is proposed for a single channel SQUID-based magnetocardiography system equipped for operating at liquid nitrogen temperature without passive magnetic shield. The developed active shield, using coils around the sensors, is a two-stage magnetic shielding system, each stage of which has been designed to compensate different frequency and dynamic ranges. The sensor part contains two high-Tc rf-SQUID magnetometers, capable of working stably under earth magnetic field, placed in an axial gradiometric arrangement with a baseline of 10 cm. The active shield setup is driven using two correlated proportional-integral-derivative closed loop systems. In the designed system, the coils are in the vicinity of the SQUIDs, resulting in coupling the coils to the SQUIDs as well as to the resonators. Hence, the impedance of the coils along with the phase of the feedback current is affected based on the configuration. In this paper, investigation of the shielding performance of our bi-stage magnetic shield as the active compensation, fed by the optimal feedback control system with all the above considerations, is reported. Using the designed shielding system, the disturbing magnetic signals could be attenuated such that the pico-Tesla signals could be obtained.