Design of the Holding Field and Spin-Transport Coil System for the LANL nEDM Experiment*

Permanent electric dipole moments present a promising avenue for the discovery of beyond standard model physics. The advent of experimental techniques using stored ultracold neutrons (UCNs) has placed the neutron electric dipole moment (nEDM) at the forefront of permanent electric dipole moment searches. The current experimental upper limit for the nEDM is $d_n < 1.8\times10^{-26}$ e-cm (90\% CL). The neutron EDM search to be conducted at Los Alamos National Laboratory (LANL) aims to advance the experimental measurement of the nEDM to an uncertainty of $3\times10^{-27} $ e-cm (68\% CL). Reaching the proposed uncertainty requires precise magnetic field control, specifically a highly uniform $B_0$ holding field. In the LANL-nEDM experiment, the holding field is provided by a gapped solenoid with an octagonal cross section. Efficient transport of UCN polarization from the polarizing magnet into the storage volume is also essential to accomplish the goal uncertainty. A series of modified, self-shielding cos $\theta$ coils have been designed to meet the specification for polarization transport. The two coil systems interface in a pseudo-continuous manner such that the fringe fields do not depolarize the UCNs during transport or generate non-uniformities in the cell volumes. This talk will discuss the techniques employed in the design of the $B_0$ and spin-transport coils for the LANL-nEDM experiment.