Magnetic shield design for atom interferometry over a 100m baseline

MAGIS-100 is a next-generation atom interferometer detector with a 100-meter baseline, currently under construction at Fermilab, that is designed to search for ultralight dark matter and to serve as a prototype for a future gravitational wave detector. Magnetic shielding over the entire baseline length is crucial to suppress unwanted phase shifts from the background magnetic field. Due to the segmented nature of the detector, our magnetic shield design consists of 17 identical sections, each with an octagonal cross section. Optimized overlap of multiple mu-metal sheets minimizes field inhomogeneity within each shield segment. Field penetration at the gaps between the segments is addressed by a combination of additional mu-metal couplers as well as active compensation coils. This design is compatible with the modular assembly procedure required by MAGIS-100, and accommodates structural, vacuum, and cable penetrations through the shield without unacceptable degradation of shielding performance.