mu-metal magnetic cavities for polarization and maintenance of polarization of He-3 gas

Low gradient magnetic holding fields are required for maintaining the polarization of polarized He-3, as diffusion though non-ideal gradients can cause total relaxation rates much greater than the intrinsic total He-3 relaxation rate of a particular He-3 cell in optimal conditions. For neutron scattering applications we often rely on mu-metal cavities to provide a degree of shielding from the many other sources of magnetic fields and gradient experienced on a typical neutron instrument. The JCNS utilizes two concepts for such cavities, one based on inexpensive plastic-bonded magnets to provide magnetic flux, and the other based on field coils wound on the sides of the mu-metal cavity. 2 different sized of permanent magnet cavities and three different geometries of coil-based cavities have been produced. Both types of boxes will be presented with magnetic design as well as mechanical construction details along with the achieved performance of the constructed devices.