Optimized superconducting coils for a high-resolution neutron spin-echo spectrometer at the European Spallation Source, ESS

The ultimate energy resolution in inelastic thermal neutron scattering spectroscopy is provided by neutron spin echo (NSE) spectrometers. The characteristic of a NSE spectrometer is that it detects neutron-velocity changes by spin-coding. It measures the intermediate scattering function I(Q, t) in terms of final beam polarisation. High resolution in that case means the extension of the Fourier time (t) range towards 1 mu s. The most important technical limitation to increase the resolution of NSE spectrometers results from the need to equalize the field integrals that determine the coding spin-precession angles along different neutron paths by combining a suitable precession field with correction coils. Here we report on the field shape optimisation and design of precession magnetic coil arrangements with minimal intrinsic field integral inhomogeneity for a wide and diverging neutron beam with a new semi-analytical approach and a numerical method. This minimizes the amount of additionally required correction. The solenoid configuration we found fulfils all the conditions required by a real NSE spectrometer.