New Nutators for POLI at MLZ
The POLI instrument at FRM II (Forschungs-Neutronenquelle Heinz Maier-Leibnitz) is dedicated to carrying out experiments with polarised neutrons with a minimum wavelength of 0.55 Å. Before and after scattering the neutron beam passes through a nutator, i.e. a polariser and an analyser unit, respectively. In recent years, new polariser and analyser units have been built as an upgrade of POLI in order to polarise the neutrons in situ. However, compared to the previously used devices, the polarisation direction of the neutrons has changed. With the old configuration the magnetic field was in longitudinal direction, but with the new configuration of the 3He-polariser a transversal field is provided by the new nutators. Still their purpose is to manipulate the direction of the spin polarisation of the neutrons after they have passed through the polariser and before they pass through the analyser. One fixed and two rotatable, motor-driven Halbach ring stacks comprising NdFeB permanent magnets adiabatically rotate the field into the desired transverse direction.
It will be investigated, if the new nutators show an increased field uniformity in the cryo-pad area compared to the old nutators. Both nutators have a modular design for greater adaptability to this and other applications: individual rings with permanent magnets can be added or removed to adapt the length of the device to a specific instrument. The next task is to assess the performance of the nutators in experiments at FRM II and to further develop a vacuum compatible version to extend the application range of the new device to other neutron experiments ( more on ICANS Poster 2026).
The next figure shows, on the left, a schematic diagram of the arrangement of the Halbach rings during the project’s conceptual phase. The graph in the top right-hand corner plots the vector components of the magnetic flux density along a line scan at the centre of the rings, following the beam axis. The graph at the bottom right shows the adiabaticity over the line scan mentioned earlier. Adiabaticity can be regarded as a measure of the extent to which the spins in the neutron beam are able to align with the magnetic field generated in the nutators.
Measurement-based verification of the simulation results
The nutators were assembled in the ITE workshop. All 3D printed structures were manufactured inhouse. The commissioning of the automation was processed at JCNS.
The simulations of the magnetic flux density within the nutators were confirmed by measurements taken using a Hall probe on the ITE test bench. All ring structures were measured individually and as an assembly. Calculations of adiabaticity based on the measurements have shown that it is present to a sufficient degree. The quality of the system will be demonstrated during its deployment on the POLI instrument.
Technical Callenges
The technical challenges included, amongst other things, achieving a very high angular positioning accuracy. For this reason, standard rotary tables with belt drives were used, which are additionally controlled by means of stainless steel measuring scales and optical read heads from Renishaw. The geometric positioning accuracy is 0.1°. The concept is based on commercially available NdFeB magnets. To ensure the magnetic field of the permanent magnets is disturbed as little as possible, materials with low permeability, such as aluminium, stainless steel and brass, were used in the system. The largest part of the system by volume, the structure in which the permanent magnets are housed, consists of 3D-printed PETG. As the accuracy of the 3D printers in relation to the material used first had to be determined, preliminary statistical tests were carried out to identify the best possible geometry for the mounting points of the permanent magnets.
The system is capable to rotate the magnetic field direction of the flux density up to 180° which is based on the measurement of the nutator. Currently the new Nutators have been installed at MLZ in May 2026 and the quality of the system will be demonstrated during its deployment on the POLI instrument.
Additional pictures
Technical Data
The nutators consist of the following core components:
- NdFeB permanent magnets, approx. 500 per Nutator
- PTEG support structures for the magnets
- Aluminium support structure for mounting the turntables, which are capable of rotating the Halbach rings
- Per nutator: 2 Standa rotary tables, type 8MRB240-152-59
- Per nutator: 2 Renishaw measuring encoders, type RESM20USA200, together with associated optical sensor systems
- Siemens PLC control system
Mass of nutator analyser: 44 kg
Mass of nutator polariser: 50 kg
Max. operating temperature: 35 °C
Electric motor operating voltage: 48 VDC
Rated power: 100 W
Rated speed: 300 min⁻¹
More Information about POLI at the Heinz Maier-Leibnitz Zentrum.