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 demonstrate by rigorous dynamical theory calculations that four-beam diffraction (4BD) can be activated only by a unique photon energy and a unique incidence direction. Thus, 4BD may be used to precisely calibrate X-ray photon energies and beam positions. Based on the principles that the forbidden-reflection 4BD pattern, which is typically an X-shaped cross, can be generated by instant imaging using the divergent beam from a point source without rocking the crystal, we illustrate a detailed real-time high-resolution beam (and source) position monitoring scheme for monitoring two-dimensional beam positions and directions of modern synchrotron light sources, X-ray free-electron lasers and nano-focused X-ray sources. In particular, we will present our recent experiments that exactly verify this theory.
Continued advances of modern synchrotron light sources are pushing crystal-based X-ray optics, mainly monochromators and analyzers, to have unprecedented high resolution, with resolution towards sub-meV for inelastic X-ray scattering (IXS) and sub-10-meV for resonant IXS (RIXS) as examples, for spectroscopy and other applications. The Advanced Photon Source (APS) has been the center in the USA for designing, fabrication, and implementation of state-of-the-art crystal optics. In this presentation, we will introduce our recent pioneering work on: (1) Design and development of new non-dispersive nested channel-cut monochromators that are completely in-line monochromators with resolution from sub-meV to ~10 meV based on silicon, quartz and sapphire crystals in the medium-energy range, (2) development and deployment of novel flat quartz-based RIXS analyzers (combined with Motel mirrors) to achieve unprecedented resolution of ~3.9 meV at the Ir-L3 absorption edge (11.215 keV) with the unique capability of efficient polarization analyses, and (3) successful dicing and assembling of spherical quartz analyzers that also achieved sub-10-meV resolution for RIXS. We will particularly emphasize the applications of “unconventional” quartz and sapphire crystals in developing these optics. Compared with silicon or germanium that has only a few tens of Bragg reflections in the medium energy range (<16 keV), quartz and sapphire can provide hundreds to thousands of different Bragg reflections for making near-back-reflection analyzers with energies virtually at any atomic absorption edges or emission lines. The guidelines for dynamical-theory modeling, orienting, fabrication and characterization of quartz and sapphire will also be presented.
The use of channel-cut crystal monochromators has been traditionally limited to applications that can tolerate the rough surface quality from wet etching without polishing. We have previously presented and discussed the motivation for producing channel cut crystals with strain-free polished surfaces [1]. Afterwards, we have undertaken an effort to design and implement an automated machine for polishing channel-cut crystals. The initial effort led to inefficient results. Since then, we conceptualized, designed, and implemented a new version of the channel-cut polishing machine, now called C-CHiRP (Channel-Cut High Resolution Polisher), also known as CCPM V2.0. The new machine design no longer utilizes Figure-8 motion that mimics manual polishing. Instead, the polishing is achieved by a combination of rotary and linear functions of two coordinated motion systems. Here we present the new design of C-CHiRP, its capabilities and features. Multiple channel-cut crystals polished using the C-CHiRP have been deployed into several beamlines at the Advanced Photon Source (APS). We present the measurements of surface finish, flatness, as well as topography results obtained at 1-BM of APS, as compared with results typically achieved when polishing flat-surface monochromator crystals using conventional polishing processes. Limitations of the current machine design, capabilities and considerations for strain-free polishing of highly complex crystals are also discussed, together with an outlook for future developments and improvements.
The advent of X-ray Free-electron Laser (FEL) such as the Linac Coherent Light Source (LCLS) has and will continue
to enable breakthroughs and discoveries in a wide range of scientific disciplines including physics, chemistry, structural biology, and material science. It has created high demand on user beamtime that is often left unfulfilled. We report here the fabrication, characterization and X-ray measurements of ultra-thin silicon single-crystal membranes for potentially beam-sharing the LCLS beam. Using a special fabrication process, samples of (111), (110), and (100) orientations were made with thicknesses ranging from 5 to 20 μm. Both high-resolution rocking curves and white-beam topographic data were first obtained using synchrotron X-rays, demonstrating near ideal diffraction qualities. Subsequent tests using the full LCLS FEL beam revealed lattice distortions from beam-induced membrane vibrations, which were then shown to be effectively reduced by ambient air or with smaller membrane dimensions. These findings are paving a way for a practical beam-sharing implementation at LCLS in the near future.
Crystal-based x-ray optics are widely used in the synchrotron radiation field. Such optics include monochromators,
channel-cut crystals, spectral analyzers, and phase plates that are generally made with standard fabrication tools such as
grinders, ultrasonic mills, blade saws, and wire saws. However, modern synchrotron radiation instruments require more
complicated and high-precision crystal structures that cannot be fabricated by these conventional tools. Examples include
narrow channels and crystal cavities that require smooth and strain-free sidewalls or inner surfaces. Since it is extremely
difficult to polish such surfaces by conventional means, specialized cutting tools are required to make the as-cut surfaces
as smooth as possible. A possible way to obtain such smooth surfaces is to use a dicing saw as a fabrication tool - a tool
typically used in the microelectronics industry to cut or dice semiconductor and other materials. Here we present our
studies on the use of dicing saws for cutting innovative, monolithic, x-ray optic devices comprised of tall, narrow-standing,
thin crystal-plate arrays. We report cutting parameters that include the rotational speed of the cutting blade
(a.k.a. spindle speed), cutting speed (a.k.a. feed rate), number of passes for each cut depth (if required), and diamond grit
size for producing the flattest and most smooth side walls. Blade type and construction (sintered, Ni, and resin) also play
critical roles in achieving optimum results. The best experimental data obtained produced an average surface roughness
of 49 nm and a peak-to-valley flatness of 3625 nm. By achieving these results, we have been able to assist experimenters
in the synchrotron radiation field in their efforts to create functional and novel optical devices.
The use of high quality X-ray mirrors at synchrotron beamlines as low-energy bandpass, harmonic rejection and high
heat load optical elements has become routine. Nearly perfect optical surfaces generated on substrates and held in strain-free
fixtures are of paramount importance to their success. Production of these mirrors requires extensive care, yet the
effect of residual fabrication stress has not been closely studied. This paper examines the effect of surface and near-surface
residual stress on the performance of hard X-ray mirrors using topography and X-ray reflectivity techniques. The
present approach complements the information provided by standard optical metrology, giving a more comprehensive
understanding of polishing induced surface deformation on X-ray reflectivity. This information is invaluable for the
characterization of future, coherence preserving optics where scattering and evanescent sub-surface X-ray penetration
may impact beam quality.
Beryllium windows are used on many X-ray synchrotron beamlines to separate and protect the ultra-high vacuum of the
storage ring from the experimental environment. Currently, such a window is typically made of a thin, high-purity,
beryllium foil, which may or may not have been polished. It is well known that these windows affect the transmitted
beam quality. The impact ranges from non-perceptible to profound, depending on the experiment.
The degradation of the X-ray beam is of increasing importance and concern, however, and in fact a number of beamlines
now are run windowless or with a very small and thin silicon nitride window. There remain many instances where a
large and robust window is desirable or necessary, and it is for this reason that developing windows that have little or no
impact on the transmitted X-ray beam quality is important.
This presentation reports on the progress in developing single-crystal beryllium X-ray windows. Due to its high purity
and homogeneity, relative structural perfection, and high polishiblity single-crystal beryllium is an attractive window
material candidate, particularly for beamlines conducting imaging or coherence-based experiments. Development of
thin and uniform windows with less than 1 nm rms surface roughness and their preliminary characterization results are
presented.
The Topography X-ray Laboratory of the Advanced Photon Source (APS) at Argonne National Laboratory operates as a collaborative effort with APS users to produce high performance crystals for APS X-ray beamline experiments. For many years the topography laboratory has worked closely with an on-site optics shop to help ensure the production of crystals with the highest quality, most stress-free surface finish possible. It has been instrumental in evaluating and refining methods used to produce high quality crystals. Topographical analysis has shown to be an effective method to quantify and determine the distribution of stresses, to help identify methods that would mitigate the stresses and improve the Rocking curve, and to create CCD images of the crystal. This paper describes the topography process and offers methods for reducing crystal stresses in order to substantially improve the crystal optics.
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