Magnetic field design for polarised neutrons in European Spallation Neutron Source instruments

Abstract: The goal of this project is the design of the magnetic field of the neutron analyser for the European Spallation Source instrument DREAM (Diffraction Resolved by Energy and Angle Measurements). This instrument is a neutron diffractometer and its purpose is the determination of the magnetic and chemical structure of materials. Simulations have been performed over the span of a semester using the Finite Element Method (FEM) in a software called COMSOL Multiphysics. Three different setups, with different guide fields, have been investigated and compared in order to achieve the best results. This has required to study how to produce highly uniform magnetic fields with coils. The initial setup, where the guide was composed by four rectangular coils, was found not to meet the required field characteristics concerning adiabaticity and uniformity. The second setup, which comprised also four compensation coils, fulfilled the specifications. However, a third setup based on a Halbach four-coil exceeded the required field characteristics and thus allows for potential imperfections in the manufacturing of the coils. As part of this work, different aspects of neutron physics have been investigated, including, neutron scattering, spin-filtering and spin-transport, which are the main physics principles behind DREAM.