Construction and commissioning of the magnetic field system for the NPDGamma experiment at the SNS and results of the spin transport calculations

The NPDGamma experiment measures the parity-violating (PV) angular asymmetry of the gamma rays emitted in the capture of the polarized cold neutrons on protons. The neutrons from the Spallation Neutron Source (SNS) are polarized by a super mirror polarizer with a 350 G internal field. The polarizer fringe field is decreased using an additional magnet. After the polarizer the neutrons are guided to the hydrogen target by a uniform, stable, and vertical magnetic field of 9.5 Gauss. The direction of the neutron spin is reversed on a pulse- by-pulse basis by a resonant spin rotator located in front of the target. Downstream from the spin rotator the relative field gradient in the vertical direction has to be smaller than 3E-4 cm$^{-1}$. The field has to be vertical and aligned with the vertical axis of the detector with a precision better than 0.1 degrees to avoid the mixing of the left-right and up-down angular asymmetry. Measurements of the NPDGamma magnetic field and the simulation of the neutron spin transport will be presented.