Theory and mitigation of motional eddy current in high-field eddy current shielding

Eddy current shielding by a Faraday cage is an effective way to shield alternating-current (AC) magnetic fields in scientific instrumentation. In a strong static magnetic field, however, the eddy current in the conductive shield is subject to the Lorentz force which causes the shield to vibrate. In addition to mechanical issues, such vibration induces motional eddy current in the shield that can dominate the original, electromagnetic eddy current to undermine the conductor's shielding capability. In this work we investigate a method to control motional eddy current by making cut-out patterns in the conductor that follow the electromagnetic eddy current image. This effectively limits the surface current of the plate to a single mode and prevents proliferation of uncontrolled motion-induced surface currents that disrupts eddy current shielding. Implications of our results for improved shielding of gradient fields in high-field MRI are discussed.