Magnetic Field Control and Monitoring for the ACME III Electron Electric Dipole Moment Search

Improved control and monitoring over magnetic fields ($B$) directly reduces systematic errors in many precision spectroscopic measurements. In the ACME III search for the electron electric dipole moment (eEDM), This was achieved by mitigating the ambient magnetic field using shields and by applying uniform magnetic fields created by coils.

We successfully diminished the ambient field by over $10^4$ times using a three-layer mu-metal shield. Our advanced self-shielding coil system was capable of generating a uniform magnetic field necessary for eEDM measurements, while ensuring the mu-metal shields remained non-magnetized across more than a thousand field reversals. Combining both shields and coils, we attained magnetic field control with a precision of 10 µG and maintained gradients below 1 µG/cm within the spin-precession volume of 100 × 4.2 × 4.2 cm³. Furthermore, the coils are capable of offering a flexible combination of magnetic field gradients, aiding in the examination of potential systematic errors.

The monitoring of magnetic fields is achieved with an array of magnetometers. We also use the magnetically sensitive Q state of ThO as an in-situ co-magnetometer. With these upgrades, we aim to minimize systematic errors for ACME III eEDM measurement to a level below the statistical accuracy of $1\times 10^{-31}e\dot cm$. The newly constructed ACME III beamline including these upgrades will be presented.