Here, we report the design, manufacturing, and characterization of x-ray optical components for the cavity-based x-ray free-electron laser (CBXFEL) cavity, in the framework of the CBXFEL R&D collaborative project of Argonne National Laboratory, SLAC National Accelerator Laboratory, and SPring-8. The optical components include high-reflectivity diamond crystal mirrors, reflecting and output coupling diamond drumhead crystal with thin membranes, focusing beryllium refractive lenses, and channel-cut Si crystal monochromators. All the designed optical components have been fully characterized at the Advanced Photon Source to demonstrate their desired performance for the CBXFEL cavity.
We present an analytic method for the calibration of X-ray fluorescence spectra collected using cylindrically bent crystal analyzers in any arrangement with respect to the sample and detector. Cylindrically bent analyzers are often used in the von Hamos geometry at X-ray Free Electron Lasers to image and disperse fluorescence from a point source to an easily calibrated line. When not in the von Hamos configuration, cylindrically bent analyzers produce spatio-spectral patterns that cannot be calibrated using existing methods. Our formula allows us to rapidly fit and optimize geometric parameters for fluorescence data and calibrate the resulting spectra.
Optical components in optics hutches of a hard x-ray undulator beamline of BL05XU at SPring-8 was restructured for providing a high flux beam at 1% bandwidth in the x-ray energy range from 5 to 100 keV. The so-called pink beam by a double-multilayer monochromator or total reflection mirrors pair with a prism made of glassy carbon as a harmonic separator are prepared in this beamline. The total reflection mirrors have three stripes; rhodium and platinum coated surface and silicon uncoated surface. Additionally, a silicon single crystal monochromator and a silicon channel cut crystal monochromator with liquid nitrogen cooling system are planned to be installed. The installation of these optical components started at January 2020. The commissioning of some components using undulator radiation will be started at April 2020.
Focusing x-ray free-electron lasers (XFEL) allows us to study nonlinear optics within the xray region. Recently, we challenged the focusing XFELs to below 10 nm. However, the conventional multilayer Kirkpatrick-Baez(KB) mirrors require too strict alignment accuracy of the incident angle. To solve this problem, we propose advanced KB (AKB) mirrors, based on Wolter type III geometry. Because the configuration satisfies the Abbe sine condition, AKB mirrors enables a tolerance of incident angle error 1000 times greater than conventional KB mirrors. The remaining problem is how such mirrors are to be fabricated, because required shape accuracy is below 1 nm and the small radius of curvature on the mirrors makes high accuracy shape measurement difficult. In this work, we performed a mirror fabrication procedure based on a combination of a grating interferometer and a differential deposition. Experiment at BL29XUL of SPring-8 demonstrated AKB mirrors with an accuracy of λ/4 fabricated.
Tight XFEL focusing is very important for significantly enhancing photon flux density, which is highly demanded by users exploring nonlinear X-ray optics. However, focusing XFEL down to 10 nm or less is so difficult from the viewpoints of both optical fabrication and optical alignment. The former can be overcome using techniques of wavefront sensing and fine shape correction. For the latter, techniques for directly measuring beam size on the focus without an influence of vibration of nanobeam are required. We have developed a technique for determining the size of nanobeam on the focus using an intensity interferometer, based on the Hanbury Brown and Twiss effect, of X-ray fluorescence emitted from a thin film inserted into the focus. The spatial coherence of X-ray fluorescence observed far from the focus depends on the distance from the focus and emission region of X-ray fluorescence. Therefore, the measured coherence can determine the size of X-ray nanobeam. This method has advantages that vibration of nanobeam does not affect the result and the setup is so simple.
A demonstration experiment was performed using a 100 nm focusing system based on total reflection KB mirrors at SACLA. X-ray fluorescence (8 keV) emitted from a thin Cu film by irradiation of focused XFEL pulses (12 keV) was detected shot-by-shot with a dual MPCCD. Analyses of approximately 1000 images based on the autocorrelation revealed that the beam size obtained with this method is in good agreement with one obtained with the wire scan method.
Focusing X-ray free-electron lasers (XFELs) is very important for producing ultra-intense X-ray nanoprobes. We have developed a system based on multilayer Kirkpatrick–Baez (KB) mirrors to focus XFELs to 10 nm or less at the SPring-8 Angstrom Compact free-electron LAser (SACLA) facility. The mirror optics in the system are designed with a large NA of greater than 0.01 to produce a diffraction-limited size of 6 nm at 9 keV. We constructed a precise X-ray grating interferometer based on the Talbot effect, and succeeded in fabricating near-perfect focusing mirrors with wavefront aberrations of λ/4.
However, strict error tolerances for mirror alignment can prevent sub-10 nm focusing. Errors of perpendicularity, incident angle, and astigmatism cause aberration on the focusing wavefront and characteristically change the beam shape. In particular, the required accuracy of the incident angle is 500 nrad. Due to shot-by-shot variations in the XFEL beam position and vibration of the optics, a single-shot diagnosis of beam shape is essential to align the mirrors quickly and accurately. By improving the method proposed by Sikorski et al. at the Stanford Linear Accelerator Center (SLAC), National Accelerator Laboratory, we propose a nanobeam diagnosis method based on the speckle pattern observed under coherent scattering. Computer simulation revealed that speckle size and beam size are inversely proportional. Platinum particles with a diameter of 2 nm were prepared and irradiated with X-rays to obtain a speckle pattern. Our experimental results demonstrate the successful estimation of beam shape and the alignment of all mirrors with the required accuracies.
This article reports the progress in the beamlines at the SPring-8 Angstrom Compact free electron LAser (SACLA). The beamline optical and diagnostics systems have been upgraded to further accelerate the scientific applications of X-ray free-electron lasers (XFELs). End-station instruments have also been developed to provide user-friendly experimental platforms which allow efficient data collection. Along with the upgrades of beamlines and experimental stations, we have established reliable and efficient procedures of the beamline operation.
We proposed a split and delay optics setup with Si(220) crystals combined with Kirkpatric-Baez mirror optics for x-ray
pump-x-ray probe experiments at x-ray free-electron laser facilities. A prototype of the split-delay optics and its
alignment procedure were tested at BL29XUL of SPring-8. The horizontal focal profile, measured via double-beam
operation, showed good spatial overlap between the split beams with an FWHM of 100 nm, near the diffraction limit at
10 keV. High throughputs of the split-delay optics of 12% (upper) and 7.4% (lower) were obtained. The throughputs can
be improved to 30% and 20% by optimizing the upper and lower central energy, respectively.
We have developed a new method to fabricate ultrathin silicon single crystals, which can be used as spectral beam
splitters for the hard x-ray regime, based on a reactive dry etching process using plasma at atmospheric pressure. The
high crystalline perfection of the crystals was verified by both topographic and high-resolution rocking curve
measurements using coherent x-rays at the 1-km-long beamline, BL29XUL of SPring-8. The development of thin
crystals enables the construction of a split-delay unit and the provision of a dedicated branch for photon diagnostics. By
using a 20-μm-thick Si(111) crystal in the symmetric Bragg geometry as a component of a Si(111) double-crystal
monochromator, an arrival-time monitor using a destructive manner has been upgraded to a non-destructive method at
SPring-8 Angstrom Compact free-electron LAser. Using the splitting crystals in a helium atmosphere can prevent
oxidation, which can introduce a lattice distortion.
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