Magnetic field apparatus for the neutron electric dipole moment search at Oak Ridge National Lab*

A search for a neutron electric dipole moment (nEDM) will be carried out at the Oak Ridge National Laboratory's Spallation Neutron Source (SNS) with a sensitivity goal of < 3 × 10^-28 e-cm. Polarized ultracold neutrons will precess in a constant magnetic holding field, B₀, inside a ∼0.4 m long liquid helium cell, which will be doped with a minute amount of He-3 to measure the precession frequency. Magnetic field gradients in the cell must be reduced below 3 ppm/cm relative to B₀ in order to mitigate the false nEDM signal due to the geometric phase effect and to increase the neutron and He-3 polarization lifetime. Superconducting magnetic coils ∼1.5-2 m in diameter will be operated inside a nearly-hermetic, superconducting lead shield to produce B₀ and other required fields. I will discuss design efforts to meet the stringent magnetic requirement in ways that also meet the challenges of cryogenic operation, in particular the significant thermal contraction. I will also show ongoing work to prepare and validate the experiment's main aluminum cryostat for use.