In the soft x-ray region, the demand for focusing x-rays into a spot of nanometer order size with high efficiency has been increasing. Ellipsoidal and Wolter mirrors, which are representative of ideal reflective focusing optics for soft x-rays, have optical advantages such as achromaticity, large acceptance, high efficiency and high numerical aperture, which are suitable to fully utilize the next-generation synchrotron light sources. Recently, the fabrication process of soft x-ray mirrors of replication type using nickel electroforming was developed, and several focusing experiments with ellipsoidal and Wolter mirrors were reported. The experimental environment of these mirrors, however, was limited due to the magnetism of the nickel body. We are currently developing the diamagnetic mirror fabrication process using copper electroforming technique to expand its application. In order to prevent oxidization of the replicated surface, this study demonstrates electroforming of copper mirror that has the reflective surface of gold. The surface roughness replicated from a flat substrate was 0.321 nm in root-mean-square in 0.1 mm × 0.1 mm area. The circularity of the gold inner surface of a copper electroformed mirror was evaluated at 26 nm in peak-to-valley.
Electroforming replication is an essential technique for fabricating full-shell, grazing-incidence mirrors for use in space, laboratories, and synchrotron experiments. For X-ray astronomy, a nickel electroforming replication process was developed and is used to produce lightweight and high-resolution X-ray mirrors. In addition, the electroforming process for fabricating X-ray mirrors for use in synchrotron experiments has undergone remarkable development over the past decade. We expect that the use of the ground-based electroforming replication process for the production of optics for Xray astronomy will lead to further improvements in the performance of X-ray telescopes. This paper describes our ongoing development efforts in the nickel-electroforming replication process, including the results of a pilot study.
High-end precision optics are generally manufactured through traditional processes, which produce unique shapes and smooth surfaces, and the modern figure correction process. Particularly for soft x-ray regions, the demand for accurate free-form optics with complex and steep shapes has been increasing recently. Wolter mirrors and ring-focusing mirrors are representative of these types of optics, which are expected to be the next-generation devices for soft x-ray microscopy. To fabricate these mirrors with sufficient accuracy to achieve ideal optical performances, both traditional and modern processes must be optimized ad hoc. In this study, we apply the high-precision lens fabrication process to a monolithic Wolter mirror and a ring-focusing mirror. The surface measurement results of the mirrors suggest that it is possible to fabricate single-nanometer accurate soft x-ray mirrors with steep shapes in complex geometries.
Focusing x-rays is a key technology for x-ray microscopic techniques. In a soft-x-ray region, focusing systems with achromaticity and a high numerical aperture have long been desired as a substitute for Fresnel zone plates. Ellipsoidal mirrors are promising focusing optics for such systems. However, two technical problems have to be overcome to allow practical application of these mirrors: their low efficiency due to their hollow shapes and strict requirements for their alignment. A novel focusing system using two reflective mirrors was proposed for this purpose. The downstream mirror is a quasi-Wolter mirror with a hollow shape similar to an ellipsoidal mirror and has a high numerical aperture of more than 0.1. The tolerance of the setting angle error of the quasi-Wolter mirror is significantly large compared to that of the ellipsoidal mirror because a quasi-Wolter mirror reflects the incident rays twice. The upstream mirror is a ring-focusing mirror, which produces an x-ray beam with a ring-shaped intensity profile, ensuring the entire beam reflects onto the quasi-Wolter mirror and reaches the focus of the system. The proposed system is ideal for soft-x-ray focusing. The design procedure and formulas are described in the present study. A prototype of the system is designed for BL25SU-A of SPring- 8. The ideal focusing spot size is estimated by numerical simulation to be 10 nm at 300 eV. The influence of alignment errors of the two mirrors is also simulated and summarized.
Since a replication-type of the Wolter mirror is obtained as the negative shape of its mandrel via shape replication represented by electroforming, a high precision mandrel fabrication process is essential for nano-focusing with the mirror at synchrotron radiation facilities. In particular, three-dimensional shape measurement technique for the mandrel is required. In this study, we developed the high precision three-dimensional shape measurement system dedicated for the Wolter mandrels. First, the shape error distributions of the ellipsoidal surface and the hyperboloid surface were measured independently. The geometrical relation between the surfaces was constrained by the longitudinal profiles which include the intersection measured by a profilometer. The diameter was also measured and finally the three-dimensional shape distribution was obtained. Applying this system, we fabricated a high precision Wolter mandrel.
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.
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