A passively shielded HTS magnet for polarized neutron scattering
T. Huang, D. Pooke, M. Fee, V. Chamritski
Over the last sixty years neutron scattering has been an indispensable tool for probing the microscopic structure and dynamics over a broad range of length and time scales in condensed matter. The use of spin-polarized neutrons in neutron-scattering experiments provides fundamental information on the magnetic properties. One of the key issues of polarized neutron scattering experiments is to maintain the polarization of a neutron beam on its way through the large magnetic fringe fields produced by a high field superconducting magnet.
Up to now, most Low Temperature Superconducting (LTS) magnets for neutron scattering use active shielding coils to reduce the fringe fields around the neutron spin flippers and an asymmetric mode to guide polarized neutrons through the region of the zero-field node. By exploiting the use of iron in the magnetic circuits, High Temperature Superconducting (HTS) magnets would be easier to maintain the neutrons’ polarization. Recently a passively shielded 3T HTS magnet for polarized neutron scattering experiments was designed, constructed and tested by HTS-110. This split-pair magnet provides a maximum horizontal magnetic field of 3 Tesla while the fringe field is less than 10 Gauss at 0.5 m from the magnetic center in the magnet axial direction and the fringe field is less than 1 Gauss at 1 m from the magnetic center in the magnet radial direction. Furthermore the zero-field nodes are outside the magnet cryostat. The magnet has a vertical room temperature bore of 80 mm in diameter for sample access and 4 horizontal bores with an opening angle of 32° for neutron access, allowing high flexibility without any material in the beam to cause scattering background. This magnet exemplifies that the combined requirements of magnetic fields, sample access, optical access and neutron depolarization for polarized neutron experiments can be satisfied by passively shielded, symmetric split-pair HTS magnets.