Maintaining a Magnetic Environment for a Nab Polarimetry Measurement*

The Nab experiment at the Fundamental Neutron Physics Beamline (FnPB) at the Spallation Neutron Source (SNS) aims to make precision measurements of the electron-neutrino correlation and Fierz interference term associated with the beta decay of free neutrons. Residual polarization of the incident beam presents a potential source of systematic error in this measurement. In order to understand these effects we must measure the beam polarization using polarized $^3$He gas and an Adiabatic Fast-Passage (AFP) Spin Flipper. This measurement requires control of the magnetic environment along the beamline in order to assure adiabatic spin transport of the neutrons, and prolong the polarization lifetime of the $^3$He cells. The AFP Spin Flipper is also intended to for use in mitigating the measured polarization, to within the margins our experimental error budget permits.