Excitation Current Optimization of Fluxgate Magnetometers for Active Magnetic Shielding of SQUID-Based Magnetocardiography System

Abstract

In order to achieve stable operation for a developed high-Tc SQUID-based magnetocardiography system, a two-stage active magnetic shielding technique is implemented. This technique is based on a combination of fluxgate and high-Tc SQUID magnetometers feedback loops. While two YBCO rf SQUIDs in a gradiometric configuration are used at liquid Nitrogen temperature as the main sensors for the heart signal detection and low-amplitude noise signals cancellation in one stage, a fluxgate is used to cancel large far-field environmental noise in the other stage feedback loop. The fluxgate sensors working in the fundamental mode under the optimized bias conditions have white noise levels less than 10 pT/ √Hz. The noise of fluxgates is measured to be dominantly due to the magnetic origin and generates within amorphous core of the sensor. It is found that both the white noise and the 1/f noise of the sensors used in the setup decrease as the dc bias current is increased when the ac bias current is minimized. We investigated the influence of the core excitation current of the fluxgates on the performance of the SQUID sensors. Whereas the white noise level of the SQUIDs remains unchanged, their 1/f noise increases as the dc bias current of the fluxgate increases. It is demonstrated that there is a general rule to choose the excitation condition for the optimized sensitivity and noise of the fluxgates while minimizing their negative effects on the SQUIDs. The complete principal configuration of the developed active shielding system is reported in this paper