Ellipsoidal mirrors are promising focusing devices for soft x-rays. Ellipsoidal mirrors were fabricated by our group using nickel electroforming, for which a precise master mandrel is required because the surface accuracy of the reflective surface is limited by the mandrel. The aim of this study was to develop a processing method for correcting the surface figure error of the mandrel with high spatial resolution. This method is based on loose abrasive machining with organic abrasive slurry and a small rotating tool. We call this method “Organic Abrasive Machining” (OAM). The organic particle materials are acrylic resin or urethane resin, which have excellent characteristics that include such as high dispersion, low hardness, and high washability. In the presentation, we will report the basic performance of the newly developed OAM system, including such as a stationary spot profile, surface roughness, and long-term stability of the removal rate during figure corrections. Spatial wavelength of 100 m on a rod lens was achieved, which means that the OAM could be applied for deterministic figure correction of the surface waviness of ellipsoidal mirrors.
Ellipsoidal mirrors are ideal focusing optics for soft x-rays because of advantages that include high numerical aperture, high efficiency, and no chromatic aberrations. Shape accuracy of nanometer order is required on the internal surface of a mirror with a diameter of around 10 mm. Because of the difficulty of processing the internal surface, ellipsoidal mirrors are fabricated by replication of the shapes of master mandrels. In previous studies, a fabrication process was developed for x-ray ellipsoidal mirrors involving mandrel fabrication and nickel electroforming. 40-mm-long ellipsoidal mirrors were fabricated and a focused beam with full width at half maximum (FWHM) of 240 nm was obtained. For better focusing performance and expansion of the applicable energy range, we designed and fabricated a 120-mm-long ellipsoidal mirror from the master mandrel with a shape accuracy of 3.8 nm (root mean square). A focusing experiment was also performed at the synchrotron radiation facility, SPring-8 (BL25SU). A focused beam with FWHM of 1 μm was obtained.