The goal of the dual KB mirror system for the CXI beamline of the LCLS-HE upgrade is to realize both micro-focusing and nano-focusing functions over a photon energy range of 7-21 keV. This system consists of two pairs of bendable mirrors with the length of 1 m. The dynamical range of the mirror bending is up to 104 due to the fine adjustment of the focusing length to 1 mm over a 10 m range. The prefigured shape and the width profile of each mirror are optimized to have good performance for both functions.
The Linac Coherent Light Source (LCLS) is being upgraded to the high repetition rate (up to 1 MHz) mode using cryogenic modules. As the key optical element at the X-ray Pump-probe (XPP) beamline, the goal of the large offset double-crystal monochromator (LODCM) for the High Energy (HE) upgrade is to maintain its x-ray beam multiplexing capability at higher average beam power (up to 200W) for the whole hard x-ray operating range of 6-25 keV. The upgraded LODCM system will use an upstream diamond transmission grating to achieve high power beam multiplexing. It will enable the 0th order 'transmission’ from the grating to pass through the XPP hutch. The +1st order beam, which contains about 20% power of the incident beam of the grating, will be monochromatized at the 1st crystal position, then directed to XPP experiments at the 2nd crystal position. Both crystal positions will provide 111 and 220 Si crystals. The 1 st crystals need to be cooled by Liquide Nitrogen to minimize their thermal deformation under heat load. The second crystals will be controlled close to ambient temperature. The temperature difference between the two crystals leads to a lattice constant mismatch. The corresponding difference in Bragg angles is utilized to compensate the angle between the 1 st order beam and 0th order beam (initial beam propagation axis) from the grating splitter, making it possible to maintain the propagation direction of the monochromatized beam exiting the LODCM parallel to the 0th order incoming beam.
The x-ray free electron laser facility at SLAC National Accelerator Laboratory (named as LCLS) will be upgraded to LCLS-II in the near future. The high repetition rate light source makes the x-ray optics or components exposed to trillions of pulses over years of operation. Material fatigue properties of x-ray optics are essentially important for their lift-time prediction, optics optimization and opto-mechanical design. In this work, the fatigue properties of typical x-ray optics materials such as single-crystal silicon are experimentally measured by using laser pulses. The laser source can have an average power of 50 W at wavelength of 1.03 μm and repetition rate of 0.928 MHz with pulse duration of ~230 fs. The SHG crystal is used to generate 515 nm laser beam for the test to get an equivalent absorption length to soft x-rays. The maximum single-pulse energy is more than 16 μJ. The numbers of pulses that the optics can survive are measured for different pulse energies (fluences). The definition of the damage of x-ray optics is the significant reduction of reflectivity, which is premonitory of damage, and much more stringent than the ablation threshold.
The laser-induced plasma sources give instantaneous 4π divergent x-ray beams. The x-ray source size and pulse duration depend on the properties of the high-energy laser. Copper is the typical target material giving characteristic photon energies around 8.4keV. Different shapes of bent crystals are widely used as imaging and monochromatizing optics. Focusing and collimating are normally functioned by polycapillary x-ray lenses. Especially, the laterally-graded multilayers are applied as x-ray mirrors, which can reflect hard x-rays with big grazing angles, moderate energy resolution and high reflectivities. To get larger acceptance angel, a higher gradient of bilayer thicknesses from 2nm to 3.6nm within 80mm length scale is designed. And the alignment precision of 10μm is required to make it good performance. The reflected monochromatic x-rays can enhance the traverse coherence for the phase contrast imaging. And multi-frames of the same object can be obtained instantaneously by multi-reflections of the mirrors. The laterally-graded multilayer x-ray mirrors are also used for the pinhole imaging of a Z-pinch target, which benefits from the flat reflection surface and monochromatic imaging.
Under synchrotron radiation white beam exposure, strong mechanical stress can build up in multilayer optics, caused by
the thermal mismatch between layer material and substrate material. To study the stability and performance of multilayer
optics under heat load, Pd, Cr, and B4C single layers of thicknesses in the nanometer range and [Pd/B4C] multilayers
were prepared in the sputter-depositing facility of the ESRF Multilayer Laboratory. Curvature changes versus
temperature were measured using a Shack-Hartmann wave front sensor. Films coated on 200 μm thin Si wafers induced
significant curvature changes over a temperature range from 60°C to 200°C. A combined parameter K including
Young’s modulus and thermal expansion coefficient (CTE) was defined to describe the thermal deformation properties
of the thin-film layer. The investigation shows that all three materials in thin film cause less thermal expansion than
expected from material properties for bulk material in the literature. In particular, the thermal expansion of B4C films
appears to be close to that of the Si substrate.
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