Development of fabrication process for aspherical neutron focusing mirror using numerically controlled local wet etching with low-pressure polishing

The aspherical supermirror is among the most useful optics to focus a neutron beam with a wide wavelength range. The improvement of surface roughness and enlargement of mirror size are essential for increasing the focusing gain. A highly efficient and high-precision fabrication process for the substrate of a large aspherical mirror combining conventional precision grinding, numerically controlled local wet etching (NC-LWE) figuring and low-pressure polishing was developed. Using this new fabrication process, a 400 mm-long plano-elliptical neutron focusing mirror substrate was successfully fabricated with a figure error of 0.39 μm p-v and an rms surface roughness of less than 0.2 nm.