Broadband Low Aberration Composite Magnet Sextupole Lens

Neutron focusing optic elements are used to enhance the intensity of neutron beams or magnify neutron images. As an alternative to focusing supermirror, magnetic refractive neutron lenses are made possible due to the fact that neutrons with their intrinsic magnetic moment are subjected to a force which is proportional to the gradient of the magnetic field under adiabatic spin transport condition. There are two technical approaches in the construction of magnetic lenses. Sextupole lenses have been developed over the past twenty years, mostly driven by KURRI (Kyoto, Japan). A periodic magnetic compound refractive lens (MCRL) has the advantage of a simpler construction but offers less focusing power. We believe the competition between reflective and refractive optics still continues, giving consideration to the drawbacks inherent to supermirror. Sextupole refractive lens may still be the best choice, especially for cold neutrons. In order to improve the imaging quality of the refractive lens and mitigate the inherent chromatic dispersion of magnetic lens, Raytum Photonics proposes these innovations/efforts towards a broadband low aberration composite magnet sextupole lens: (1) Improved design of sextupole components to minimize spherical aberrations: Our analysis shows that the spherical aberration dominates for the refractive neutron lens. Raytum Photonics will combine the technique of finite element approach and ray-tracing to establish numerical model of magnetic refractive lens to simulate the lens aberration. With this software tool, we will be able to optimize the components of the sextupole lens for ideal magnetic field distribution. The goal of an improved Sextupole lens is to double the enhancement factor of the beam flux from the SOA of 43 to over 100 with aperture size no less than 25mm. We will further improve our existing tooling and assembling process for high strength, large size and intricate magnet assemblies towards higher precision, higher throughput and lower cost; and (2) Mitigating of chromatic aberration for broadband operation: We propose to adapt electro-permanent magnets in the construction of Sextupole lens. When the long sextupole lens consists of a number of sections made from electropermanent magnets, its focusing power can be varied by turning on/off the Sextupole sections. The variable focusing length for a given wavelength corresponds to changes in the center wavelength. The sextupole lens then becomes wavelength reconfigurable. Broadband operation becomes possible for cold neutrons when the switch of the electromagnets synchronizes with the pulsed neutron beam. We propose to demonstrate the wavelength range between 10 Å to 20 Å at phase 1 and provide a viable path towards a 2~10 Å system.