Masaki Shimada

Experimental Study of the Active Compensation to a Full-Size Separate-Shell Magnetic Shield

Shielding efficiency of a full-size separate-shell magnetic shield equipped with active compensation coils is presented. Its size is 135 cm in height, 100 cm in width, and 240 cm in depth. With respect to active compensation two control schemes, i.e., feedback and feedforward, are compared. The shielding efficiency is estimated from the measurements of the attenuation of environmental magnetic fields in two axial directions (horizontal and vertical). In the case of feedback, a reference sensor that measures the incoming magnetic fields is positioned inside the shield, whereas in the case of feedforward the reference sensor is positioned outside the shell. In both axial directions, the environmental magnetic noise can be reduced to less than 500 pT: the noise floor measured by a fluxgate magnetometer is approximately 10 pT/¿Hz in the frequency range from 10 to 100 Hz.

Desktop Shielding System

A new active shielding system, which can be operated on a desktop, is developed with two identical separate magnetic shells: cap shell and cup shell. Shells are axis-symmetric and made of fiber-reinforced plastic (FRP) layers and 20 layers of magnetic sheets sandwiched with the FRP layers. A ring coil for the active compensation is attached to both of the shells so that the axial component of the magnetic field can be shielded at the center space between two shells. This shielding system may be useful in such a case where a single component of the magnetic field from samples needs to be measured for detecting important information. Furthermore, due to the opening at all along the shell edge, samples can be continuously conveyed to the measurement point in the shield. The shielding performance has been investigated numerically for various design parameters and confirmed experimentally. The shielding factor (SF) achieved at low frequency is very high as it reaches 3162 at 1 Hz and it is as high as 100 at 100 Hz.