Optical Metrology for X-ray Optics at the Hefei Advanced Light Facility (HALF)

Abstract The development of fourth-generation diffraction-limited synchrotron radiation sources, such as the Hefei Advanced Light Facility (HALF), places increasingly stringent demands on the fabrication and metrology of the X-ray optical components. The beamlines require mirrors and gratings with atomic-level surface quality, typically with root-mean-square (RMS) figure errors below 1 nm and surface roughness below 0.3 nm RMS over apertures of up to one meter. They also involve complex surface geometries including elliptical, cylindrical, and toroidal shapes. To support the development and acceptance testing of beamline optics for HALF, an optical metrology laboratory (OML) has been established at the National Synchrotron Radiation Laboratory (NSRL). The laboratory combines a thermally and mechanically stable environment with complementary instruments for slope, figure, and roughness metrology, including vertical and horizontal long-trace profilers, stitching interferometry systems, and a white-light profiler. The current performance includes a slope-measurement repeatability of approximately 13 nrad for the long-trace profiler and subnanometer repeatability for Fizeau-stitching interferometry with a lateral sampling interval of 0.046 mm. These capabilities cover the range of optical geometries required for HALF, from plane and cylindrical mirrors to strongly curved elliptical optics with radii of curvature below 10 m. This paper describes the metrology requirements for HALF X-ray optics, the environmental design of the OML, the performance of its main instruments, and the planned expansion of its capabilities for future diffraction-limited beamlines.