Experimental study on opening compensation for magnetic shields by current superposition

A new method is studied experimentally by which magnetic fields coming in through an opening of a magnetic shield can be compensated. The horizontal component of the earth’s field is targeted as an external magnetic field to be compensated by the method, in which a single figure-eight coil is used. A cylindrical shielding case is developed for this study using a Permalloy cylindrical case (outer diameter=68 cm, length=180 cm) with an opening at an end and Metglas 2705M amorphous ribbons, which are wound helically on the outer surface of the Permalloy case by 16 layers. A magnetic shaking field of 200 Hz is applied to only the amorphous layers by a toroidal shaking coil of 36 turns. The size of the figure-eight coil used is 74 cm in diameter with 30 turns. Residual magnetic field measured by a flux gate magnetometer in the horizontal center plane of the shielding case shows that the area where the amplitude of the residual magnetic field is within ±0.15 mG is extended by about 30 cm toward the opening.