Abstract
Active compensation is studied for a multi-shell shield with an inner passive shell. We made a miniature shield consisting of four concentric magnetic shells, which has the same structure as our previously developed shielding system for a demonstration of MEG alpha-rhythm measurement. FINEMET rather than Permalloy was chosen as the material for the inner passive shell. We measured the amplitude and phase of the incoming magnetic field to the shielded area. It was found that the inner passive shell can help to suppress the phase difference between the observed field and the external field. A model for explaining this phenomenon is presented. We tried a simple feed-forward active compensation system for the shield, which consists of three canceling ring coils and a single monitoring flux-gate magnetometer. The axial shielding factor at 10 Hz, 10 μT was 300, and with active compensation a value of 12,000 was achieved.
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