Compact sextupole permanent magnet lens: a practical approach to focusing pulsed neutrons

The focusing of pulsed neutrons can increase neutron current, reduce sample volume and enable access to smaller scattering angles. Consequently, it represents a critical challenge for next-generation spallation neutron sources. The primary difficulty stems from the inherent chromatic aberration of white neutrons. Here, a new compact nested rotating sextupole permanent magnet (Nest-Rotating-SPM) lens, with a total length of 200 mm, was developed and tested on the very small angle neutron scattering instrument at the China Spallation Neutron Source. Through synchronization of the outer sextupole lens rotation with the neutron pulse from the source, we achieved aberration-free focusing of neutrons with wavelengths between 11.0 and 15.5 Å for the first time. The implementation of water cooling and carbon fibre winding ensures both magnetic field stability and mechanical robustness of the inner sextupole. The compact design incorporating bridge sextupoles enables modular assembly of multiple lens units for focusing pulsed neutrons with wavelengths shorter than 10.0 Å, making it practically useful in a pulsed neutron instrument to enhance neutron current or access lower scattering vectors. Additional research is required to mitigate background noise.