For modern synchrotron light sources, the push toward diffraction-limited and coherence-preserved beams demands accurate metrology on X-ray optics. Moreover, it is important to perform in-situ characterization and optimization of X-ray mirrors since their ultimate performance is critically dependent on the working conditions. Therefore, it is highly desirable to develop a portable metrology device, which can be easily implemented on a range of beamlines for in-situ metrology. An X-ray speckle-based portable device for in-situ metrology of synchrotron X-ray mirrors has been developed at Diamond Light Source. Ultra-high angular sensitivity is achieved by scanning the speckle generator in the X-ray beam. In addition to the compact setup and ease of implementation, a user-friendly graphical user interface has been developed to ensure that characterization and alignment of X-ray mirrors is simple and fast. The functionality and feasibility of this device is presented with representative examples.
To achieve high resolution and sensitivity on the nanometer scale, further development of X-ray optics is required. Although ex-situ metrology provides valuable information about X-ray optics, the ultimate performance of X-ray optics is critically dependent on the exact nature of the working conditions. Therefore, it is equally important to perform in-situ metrology at the optics’ operating wavelength (‘at-wavelength’ metrology) to optimize the performance of X-ray optics and correct and minimize the collective distortions of the upstream beamline optics, e.g. monochromator, windows, etc. Speckle-based technique has been implemented and further improved at Diamond Light Source. We have demonstrated that the angular sensitivity for measuring the slope error of an optical surface can reach an accuracy of two nanoradians. The recent development of the speckle-based at-wavelength metrology techniques will be presented. Representative examples of the applications of the speckle-based technique will also be given – including optimization of X-ray mirrors and characterization of compound refraction lenses. Such a high-precision metrology technique will be extremely beneficial for the manufacture and in-situ alignment/optimization of X-ray mirrors for next-generation synchrotron beamlines.
Deformable, piezo bimorph mirrors are often used to expand X-ray beams to a continuous range of sizes. However,
optical polishing errors present on all X-ray mirrors introduce striations into the reflected beam. To counteract them, reentrant
surface modifications with alternating concave and convex curvature have been proposed and applied to mirrors
of fixed shape or bimorph mirrors. For the latter, a new method of constructing re-entrant surface modifications on
segments of unequal length is described. This allows the re-entrant modification required for a desired beam size at the
focal point to be matched to the bimorph mirror’s polishing errors, thus reducing the voltage variations. Optical
profilometry using the Diamond-NOM showed that a 5-segment and a 7-segment modification could be suitably applied
to a deformable bimorph mirror. X-ray tests showed that striations caused by the 5-segment modification in the beam at
the focus are concentrated at the beam edges, while the beam center is left clear. This is in contrast to simple defocusing,
in which a strong side shoulder appears. The 7-segment modification produces a pattern of evenly spaced striations. The
intensity spikes seen with the re-entrant modifications are caused chiefly by the finite curvature of the mirror at the
turning points. The question of whether deformable bimorph mirrors with different piezo response functions could
sharpen the curvature changes will be investigated. Optimal modifications of continuous curvature, which could more
realistically be applied, will be sought.
Lightweight Asymmetry and Magnetism Probe project (LAMP) was proposed in China to observe the polarized radiation around 250 eV emitted by soft X-ray celestial sources, like puslars, active galactic, black hole binaries, etc. To produce the high efficiency soft X-ray polarizer for LAMP, we are developing Co/C, Cr/C, CoCr/C X-ray multilayers using magnetron sputtering and reactive sputtering with nitrogen. Hard X-ray grazing incidence reflectometry (GIXR) and soft X-ray reflectance measurements were used to study the interface and microstructure of different multilayers. In comparison to the multilayers deposited by normal magnetron sputtering, reactively-sputtered multilayers show higher reflectivity around 250 eV and lower interfacial roughness. As a result, Co/C, Cr/C, CoCr/C multilayers with smaller period can be made with respect to the multilayer fabricated using non-reactive sputtering. The X-ray performance of reactive sputtered Co/C and CoCr/C multilayers can be further improved by optimizing the deposition process.
Modern, third-generation synchrotron radiation sources provide coherent and extremely bright beams of X-ray radiation.
The successful exploitation of such beams depends to a significant extent on imperfections and misalignment of the
optics employed on the beamlines. This issue becomes even more critical with the increasing use of active optics, and the
desire to achieve diffraction-limited and coherence-preserving X-ray beams. In recent years, significant progress has
been made to improve optic testing and optimization techniques, especially those using X-rays for so-called atwavelength
metrology. These in-situ and at-wavelength metrology methods can be used not only to optimize the
performance of X-ray optics, but also to correct and minimize the collective distortions of upstream beamline optics,
including monochromators, and transmission windows. An overview of at-wavelength metrology techniques
implemented at Diamond Light Source is presented, including grating interferometry and X-ray near-field speckle based
techniques. Representative examples of the application of these techniques are also given, including in-situ and atwavelength
calibration and optimization of: active, piezo bimorph mirrors; Kirkpatrick-Baez (KB) mirrors; and
refractive optics such as compound refractive lenses.
Grazing incidence mirrors are now a standard optic for focusing X-ray beams. Both bimorph and mechanically bendable mirrors are widely used at Diamond Light Source because they permit a wide choice of focal lengths. They can also be deliberately set out of focus to enlarge the X-ray beam, and indeed many beamline teams now wish to generate uniform beam spots of variable size. However, progress has been slowed by the appearance of fine structure in these defocused beams. Measurements showing the relationship between the medium-frequency polishing error and this structure over a variety of beam sizes will be presented. A theoretical model for the simulations of defocused beams from general mirrors will then be developed. Not only the figure error and its first derivative the slope error, but also the second derivative, the curvature error, must be considered. In conclusion, possible ways to reduce the defocused beam structure by varying the actuators' configuration and settings will be discussed.