Shielding factor enhancement method for Bi-stage active shield in SQUID-based Magnetocardiography system

We proposed a simple method to enhance shielding factor of our previously proposed bi-stage active shield system employed in a SQUID-based magnetocardiography system. The additional proposed design is optimized for canceling the power-line magnetic interference field to provide a calmer magnetic environment for the bi-stage active shield. A 50 Hz cancellation coil is placed around the bi-stage shielding system which includes inner and outer coils designed for compensating low-frequency (0-0.1Hz) and high-frequency (0.1-100Hz) environmental magnetic noise, respectively. In this configuration, a SQUID magnetometer is located at the center of these coils. Considering that the power-line interference is far-field, a coil-based sensor was placed on top of the liquid nitrogen Dewar which contains the SQUID magnetometer. The coil-based sensor is aligned with the SQUID sensing washer area vertically. Using optimized controlling circuits, the sensor measures the power-line magnetic interference field and feeds the measured signal to the 50 Hz cancellation coil. Although this configuration is optimized to reduce the power-line interferences, it also causes lower magnetic noise level at low-frequency range which is interpreted to be caused by decreasing flux trapping probability in the SQUID magnetometer. The designed 50Hz cancellation system has shielding factors of about 23 dB at 50Hz and 19 dB at 1Hz.