Adaptive compensation of magnetic fields inside an open cylindrical magnetic shield

For the development of a magnetic field compensation system suited for the open end of the cylindrical magnetic shell, magnetic field distributions were characterized on the axis of the magnetic shell for the externally impressed low-frequency AC magnetic field and for the compensating magnetic field generated by a set of ring coils. Based on this observation, a system for actively compensating the magnetic field leakage through the opening is proposed. A cylindrical magnetic shield of desk-top size was constructed by combining an axial shell made of 2 in wide Metglas 2705M amorphous tapes encircling the outer surface of a cylindrical body in its axial direction, with a magnetic shaking coil, and then a proposed compensation system was built on top of the structure. The axial shielding factor was improved with this active compensation system by 55 times for a 2 Hz impressed magnetic field, 20 times for a 10 Hz impressed field, ten times for a 30 Hz impressed field, and five times for a 50 Hz impressed field. An interesting phenomenon observed during the characterization of the magnetic fields is that the magnetic field in the space inside the axial magnetic shell leads in phase with respect to that of the impressed magnetic field, unlike the case with a helical shell where the magnetic field is lagging in phase. A model for explaining this phenomenon is also given.