Magnetic field stabilization for magnetically shielded volumes by external field coils

New results on an active stabilization system for improving the performance of a four-layer ferromagnetic shield are presented. The system is based on measuring the magnetic field outside (but close to) the shield by a fluxgate sensor. The output of this sensor is used to feed a current through an external coil system surrounding the shield, thus stabilizing the ambient external field in one direction at a predefined value. Here we report on the construction and the characterization of the system in the magnetic test facility at PSI with special emphasis on a potential future application in a measurement of the neutron electric dipole moment (nEDM). Because of the long integration times in the nEDM experiment (about 100 s or more) only slow disturbances have to be corrected for. With stabilization, suppression factors around 200 have been observed for stationary, localized sources and for vehicles moving on a road at 10 m distance. The ambient magnetic noise is reduced by a factor 10-20 for integration times of 100-1000 s.