In experiments utilizing fourth-generation synchrotron radiation and Free Electron Laser (FEL) beamlines, a primary challenge for X-ray optical elements is to achieve and maintain high-intensity focused x-ray beams with near-perfect wavefront quality and high stability. These optical elements inherently demand more stringent specifications than those for other applications because of the shorter wavelength and the ultra-small emittance of the radiation beams from these sources. Coherent photons from diffraction-limited light sources further underscore the necessity for a controlled wavefront. Maintaining a uniform wavefront is crucial for phase-sensitive imaging techniques and for various coherent x-ray scattering experiments, such as tomography, coherent x-ray diffraction imaging, x-ray photon correlation spectroscopy and coherent surface scattering imaging. Therefore, x-ray optics must be manufactured close to ideal mathematical shapes, automatically align and focus beams according to experimental needs, and offer real-time correction to wavefront deformations. At the Advanced Photon Source (APS), we have demonstrated the application of a neural network model to automatically control deformable mirrors and the use of Bayesian optimization with Gaussian processes to align and stabilize focusing optical systems.
At Lawrence Berkeley National Laboratory’s Advanced Light Source, we are developing x-ray wavefront sensors to support the creation and operation of beamlines with diffraction-limited quality. Our new approach to rapid, intermittent wavefront sensing operates in reflection at glancing incidence angles and is compatible with the high-power densities of modern beamlines. For soft x-ray applications especially, the wavefront sensor can operate upstream of the exit slit in a vertically dispersed beam. This single-shot technique supports lateral shearing interferometry and Hartmann wavefront sensing; it can be adapted to speckle-based techniques as well. The reflected beam is directed to an off-axis YAG crystal that produces scintillated visible light. A small mirror reflects the light to a microscope and camera, and the measured wavefront shape information can be used as feedback to adaptive x-ray mirror elements. A compact array of gratings enables measurement across a broad range of photon energies or wavefront curvatures. We describe recent demonstrations at soft x-ray and hard x-ray wavelengths measuring an adaptive x-ray mirror, and a toroidal focusing mirror.
We have made significant progress in developing at-wavelength X-ray techniques and tools for optics characterization and beamline diagnostics at the Advanced Photon Source (APS). In the past few years, advanced techniques, such as the coded-mask-based method, are routinely used to characterize lenses, mirrors, crystals, and windows for APS and the APS upgrade projects at the 28-ID-B Instrumentation Development, Evaluation & Analysis (IDEA) Beamline and the 1-BM optics and detectors testing beamline. This paper reviews our recent achievements in developing at-wavelength metrology tools and activities in characterizing and developing advanced refractive optics for the APS upgrade beamlines. We summarize the quality evaluation results of hundreds of commercial lenses and highlight the measurement procedures and application of data in designing transfocators. We then discuss the characterization of APS-fabricated silicon compound refractive lenses (CRLs) for high-energy (>40 keV) focusing, and their potential application for the CHEX beamline (Coherent High- Energy X-ray Sector for In Situ Science). Silicon CRLs fabricated by Deep Reactive Ion Etching (DRIE) with different design parameters were evaluated at the IDEA beamline. At-wavelength metrology results show that silicon CRLs are promising options as high-energy focusing optics.
A coded-mask-based X-ray wavefront sensing technique was recently developed at the Advanced Photon Source, aiming for the ultimate phase sensitivity, spatial resolution and high speed using deep-learning-based analysis. It is a versatile tool capable of single-shot reference-free measurements and scanning mode for the best resolution and noise robustness. This work extends its application in at-wavelength metrology to achieve variable-resolution analysis when combined with a short-focal-length focusing optic. We showcase the complete characterization of beryllium refractive lenses using the coded-mask-based method in a collimated-beam setup and a divergent-beam setup with large geometric magnifications. The collimated-beam measurement provides the lens thickness error over the entire optical aperture down to a micron spatial resolution. On the other hand, wavefront sensing with the divergent beam can provide detailed local information of the sample with a few tens of nanometer spatial resolution, ideal for investigating lens defects.
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