Influence of demagnetization coil configuration on residual field in an extremely magnetically shielded room: Model and measurements

The Berlin magnetically shielded room 2 (BMSR-2) features a magnetic residual field below 500 pT and a field gradient level less than 0.5 pT/mm, which are needed for very sensitive human biomagnetic recordings or low field NMR. Nevertheless, below 15 Hz, signals are compromised by an additional noise contribution due to vibration forced sensor movements in the field gradient. Due to extreme shielding, the residual field and its homogeneity are determined mainly by the demagnetization results of the mumetal shells. Eight different demagnetization coil configurations can be realized, each results in a characteristic field pattern. The spatial dc flux density inside BMSR-2 is measured with a movable superconducting quantum interference device system with an accuracy better than 50 pT. Residual field and field distribution of the current-driven coils fit well to an air-core coil model, if the high permeable core and the return lines outside of the shells are neglected. Finally, we homogenize the residual field by selecting a proper coil configuration. (C) 2008 American Institute of Physics.