Abstract
To fully exploit the ultimate source properties of the next-generation light sources, such as free-electron lasers (FELs) and diffraction-limited storage rings (DLSRs), the quality requirements for gratings and reflective synchrotron optics, especially mirrors, have significantly increased. These coherence-preserving optical components for high-brightness sources will feature nanoscopic shape accuracies over macroscopic length scales up to 1000 mm. To enable high efficiency in terms of photon flux, such optics will be coated with application-tailored single or multilayer coatings. Advanced thin-film fabrication of today enables the synthesis of layers on the sub-nanometre precision level over a deposition length of up to 1500 mm. Specifically dedicated metrology instrumentation of comparable accuracy has been developed to characterize such optical elements. Second-generation slope-measuring profilers like the nanometre optical component measuring machine (NOM) at the BESSY-II Optics laboratory allow the inspection of up to 1500 mm-long reflective optical components with an accuracy better than 50 nrad r.m.s. Besides measuring the shape on top of the coated mirror, it is of particular interest to characterize the internal material properties of the mirror coating, which is the domain of X-rays. Layer thickness, density and interface roughness of single and multilayer coatings are investigated by means of X-ray reflectometry. In this publication recent achievements in the field of slope measuring metrology are shown and the characterization of different types of mirror coating demonstrated. Furthermore, upcoming challenges to the inspection of ultra-precise optical components designed to be used in future FEL and DLSR beamlines are discussed.
Highlights
With the advent of highly brilliant synchrotron sources like the PETRA III storage ring, X-ray free-electron lasers (XFELs) as well as diffraction-limited storage rings (DLSRs) like the new MAX IV laboratory, the requirements for synchrotron optics in terms of precision have significantly increased
Compared with the situation two decades ago, when the first third-generation storage rings came into operation, an improvement of the mirror shape by one order of magnitude in terms of residual slope error has been achieved to date
Future plane gratings at the European XFEL will have a length of 530 mm and require a radius of curvature of >300 km and 50 nrad r.m.s. for the residual slope deviation (Vannoni et al, 2013)
Summary
With the advent of highly brilliant synchrotron sources like the PETRA III storage ring, X-ray free-electron lasers (XFELs) as well as diffraction-limited storage rings (DLSRs) like the new MAX IV laboratory, the requirements for synchrotron optics in terms of precision have significantly increased. Elliptical-cylindershaped focusing optics of 20 nrad r.m.s. residual slope deviation up to a length of 1000 mm are proposed to focus photons at the Single Particles, Clusters and Biomolecules (SPB) experimental station at the European XFEL (Mancuso et al, 2013) In contrast to such long focusing mirrors, the length of Kirkpatrick–Baez (KB) mirror substrates at sources like PETRA III or MAX IV will be in the range 100–200 mm (Kalbfleisch et al, 2010; Johansson, 2014). Such optical components of elliptical and hyperbolic shape are already available today (Matsuyama et al, 2012; Siewert et al, 2012) and allow diffraction-limited focusing of hard X-ray photons within nanometre focus size (Mimura et al, 2010). It should be noted that metrology does provide characterization of optical components, but such data can be directly used in realistic beamline modelling (Samoylova et al, 2009)
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