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

Abstract

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 µs. 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.

Neutron phase spin echo

“…[3][4][5]. Nowadays, NSE beam lines are part of the instrument suite of almost every major neutron scattering facility world-wide as they provide excellent energy resolutions and often deliver unique and complementary information compared to other scattering techniques [6][7][8][9][10][11][12][13][14]. Moreover, NSE and related neutron spin precession techniques, e.g.…”

Semi-analytical calculations of intrinsic field magnetic field inhomogeneities for a Neutron Spin Echo spectrometer at the ESS

“…Equations (8), (10) are valid for any shape of a neutron beam and not only the radial symmetry implicitly assumed in Ref. [11]. Using Eq.…”

“…(12), and Eq. (26), respectively, provided in the paper of Pasini et al [11], with the difference that they are valid for any shape of a neutron beam. The contribution of one arm to the reduction of spin-echo amplitude is then given by:…”

“…The upgraded IN15 [3] as well as a proposal of a NSE spectrometer at the ESS [11] use coils with reduced intrinsic inhomogeneities based on a optimal field shape (OFS) solution suggested by Zeyen [17]. However, for comparable field integral inhomogeneities, the OFS solution requires the use of longer precession regions than simple solenoids.…”

“…The magnetic field integral inhomogeneities arise from two sources: i) inhomogeneities of the magnetic field which lead to different precession angles for two parallel trajectories of the same length, and ii) beam divergence, which leads to trajectories with different lengths and thus with different precession angles even in a ideally homogeneous magnetic field. The separation of the effects of these two sources was considered by Zeyen [17] and, recently in more detail, by Pasini et al [11].…”