Two-Dimensional Coil System Compensating X, Y, Z-Components of Leakage Field from an Aperture of Magnetic Shields and its Application to a Large-Scale Cylindrical Shield

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A new method was developed by which magnetic fields coming in through an opening of a magnetic shield can be compensated. The earth's magnetic field and ac magnetic field were targeted as external magnetic fields to be compensated by the method. A large-scale cylindrical shielding case was developed for this study using a Permalloy cylindrical case (outer diameter =68cm, length=180cm) with an opening at an end and Metglas 2705M amorphous ribbons, which were wound helically on the outer surface of the Permalloy case by 16 layrs. Magnetic shaking field was applied to only the amorphous layrs by a toroidal shaking coil of 36-turns. The cylindrical shielding case was laid on the floor with its axis perpendicular to the direction of the earth's magnetic field. The earth's magnetic field was attenuated to below 0.1 mG (10 nT) at the central region of the shield. The distribution of residual magnetic field was nearly flat due to the cylindrical shape. The horizontal component of the earth's field (0.32 G) as well as vertical one (0.35 G) was compensated at the open end of the cylindrical shield using a flat figure-eight coil (74 cm in diameter) controlled by a feedback system corresponding to each component. Effectively shielded area where residual magnetic field was less than 0.15 mG was extended by about 30 cm (approximately by a radius) toward the open end. Compensating current for the horizontal component of the earth's field was 2.3 A-turn. Magnetic field applied axially by a large square coil was also compensated using a ring coil of the same diameter.By incorporating a newly developed compensating technique we successfully constructed a prototype magnetic shield of human size which is light-weight, high-performance and low-cost