Ali M. Khounsary

Short focal length Kirkpatrick-Baez mirrors for a hard x-ray nanoprobe

We describe progress in the fabrication of short-focal-length total-external-reflection Kirkpatrick-Baez x-ray mirrors with ultralow figure errors. The short focal length optics produce nanoscale beams (<100nm) on conventional (∼64m long) beamlines at third generation synchrotron sources. The total-external reflection optics are inherently achromatic and efficiently focus a white (polychromatic) or a tunable monochromatic spectrum of x rays. The ability to focus independent of wavelength allows novel new experimental capabilities. Mirrors have been fabricated both by computer assisted profiling (differential polishing) and by profile coating (coating through a mask onto ultra-smooth surfaces). A doubly focused 85×95nm2 hard x-ray nanobeam has been obtained on the UNICAT beamline 34-ID at the Advanced Photon Source. The performance of the mirrors, techniques for characterizing the spot size, and factors limiting focusing performance are discussed.

Reconstruction of an astigmatic hard X-ray beam and alignment of K-B mirrors from ptychographic coherent diffraction data.

We have used coherent X-ray diffraction experiments to characterize both the 1-D and 2-D foci produced by nanofocusing Kirkpatrick-Baez (K-B) mirrors, and we find agreement. Algorithms related to ptychography were used to obtain a 3-D reconstruction of a focused hard X-ray beam waist, using data measured when the mirrors were not optimally aligned. Considerable astigmatism was evident in the reconstructed complex wavefield. Comparing the reconstructed wavefield for a single mirror with a geometrical projection of the wavefront errors expected from optical metrology data allowed us to diagnose a 40 μrad misalignment in the incident angle of the first mirror, which had occurred during the experiment. Good agreement between the reconstructed wavefront obtained from the X-ray data and off-line metrology data obtained with visible light demonstrates the usefulness of the technique as a metrology and alignment tool for nanofocusing X-ray optics.

A beamline for 1–4 keV microscopy and coherence experiments at the Advanced Photon Source

The third‐generation Advanced Photon Source will open up dramatic new opportunities for experiments requiring coherent x‐rays, such as scanning x‐ray microscopy, interferometry, and coherent scattering. We are building a beamline at the Advanced Photon Source to exploit the potential of coherent x‐ray applications in the 1–4 keV energy region. A high brightness 5.5‐cm‐period undulator supplies the coherent x‐rays. The beamline uses horizontally deflecting grazing‐incidence optical elements to preserve the coherence of the undulator beam. The optics have multilayer coatings for operation at energies above 1.5 keV. This paper discusses the beamline design and its expected performance.

Performance of a high-resolution x-ray microprobe at the Advanced Photon Source

We have developed a x-ray microprobe in the energy region from 6 to 20 keV using undulator radiation and zone-plate optics for microfocusing-based techniques and applications at a beamline at the Advanced Photon Source (APS). The performance of the beamline was shown to meet our design objectives, including preservation of the source brilliance and coherence, selectable transverse coherence length and energy bandwidth, high angular stability, and harmonic suppression of the beam. These objectives were achieved by careful thermal management and use of a novel mirror and crystal monochromator cooling geometry. All beamline optical components are water cooled, and the x-ray beam in the experiment station is stable in beam intensity, energy, and position over many days with no active feedback. Using a double-crystal Si(111) monochromator, we have obtained a focal spot size (FWHM) of 0.15 μm(v)×1.0 μm(h), and a photon flux of 4×109 photons/sec at the focal spot, and thus a photon flux density gain of 15,000. A...

A novel monochromator for high heat load synchrotron x-ray radiation

The high heat load associated with the powerful and concentrated x‐ray beams generated by insertion devices (IDs) at a number of present and many of the future (planned or under construction) synchrotron radiation facilities poses a formidable engineering challenge for the design of monochromators and other optical devices. Successful utilization of the intense x‐ray beams from insertion devices depends critically on the development, design, and availability of optical elements that provide acceptable performance under high heat load. Present monochromators can handle, at best, heat load levels that are an order of magnitude lower than those generated by these IDs. The monochromator described here, and referred to as the ‘‘inclined’’ monochromator, can provide a solution to the high heat load problem. The inclined monochromator is different in a number of aspects from other conventional monochromator designs. Its primary differentiating characteristic is in the orientation of the diffracting planes. In th...

Thermal contact resistance across a copper-silicon interface.

An experimental setup to measure the thermal contact conductance across a silicon-copper (Si-Cu) interface is described, and the results obtained are presented. The resulting thermal contact resistance data are used in estimating the thermo-mechanical and optical performance of optical substrates cooled by interfaced copper cooling blocks. Several factors influence the heat transfer across solid interfaces. These include the material properties, interface pressure, flatness and roughness of the contacting surfaces, temperature, and interstitial material, if any. Results presented show the variation of thermal contact conductance as a function of applied interface pressure for a Cu-Si interface. Various interstitial materials investigated include indium foil, silver foil and a liquid eutectic (Ga-In-Sn). As expected, thermal contact resistance decreases as interface pressure increases, except in the case of the eutectic, in which it was nearly constant. The softer the interstitial material, the lower the thermal contact resistance. Liquid metal provides the lowest thermal contact resistance across the Cu-Si interface, followed by the indium foil, and then the silver foil.

Diffraction efficiency and diffraction bandwidth of thermal-gradient and composition-gradient crystals

Measurements were made at the Advanced Photon Source at Argonne National Laboratory on the diffraction efficiency and diffraction bandwidth of a thermal-gradient crystal (Si) and a composition-gradient crystal (Si-Ge) to which a thermal gradient was applied. Gradient crystals are crystals in which the spacing between crystalline planes varies with the position in the crystal. This change in the crystal plane spacing is obtained by applying a thermal gradient to a single crystal or by growing a two-component crystal in which the ratio of the two components changes with position in the crystal. Measurements were made at two energies, 92.6 and 153 keV. Both crystals were 1-cm cubes. Laue diffraction (transmission diffraction) was used in all experiments. The thermal gradient was applied perpendicular to the [111] diffraction planes of the pure silicon crystal and perpendicular the [400] diffraction planes in the composition-gradient crystal (Si-Ge). The thermal gradient applied to the crystals was quite unif...

Design, fabrication, and evaluation of an internally cooled silicon carbide mirror

In this article, the design, fabrication, prepolish coating, and polishing of a reaction-bonded (RB) internally cooled silicon carbide (SiC) mirror is described. The mirror was developed from a mold of SiC powder in a near-net shape and then infused with silicon vapor to make a dense mirror substrate. The mirror surface was then rough polished, coated with a thin layer of SiC, and polished to a final fine finish. The design and manufacturing of this mirror—intended to be used as a multilayer substrate on a high-heat-load undulator beamline—are described, and data on the surface figure and finish are provided. This type of mirror can provide an attractive alternative to internally cooled silicon mirrors. Because the substrate is made in one piece, it avoids the frit or metal bonding that is usually necessary with silicon substrates. Advantages of RB SiC mirrors include lower cost and higher reliability.

Potential of a beryllium x-ray lens

The use of refractive lenses for focusing x-ray beams has been the subject of publications since the early 1980s. Detailed calculations have been made for different shapes for the refractive lens: cylindrical, spherical, parabolic, and for a Fresnel-type refractive lens. The main drawback to the use of a single refractive lens to focus x-rays is that the index of refraction (n equals 1 - (delta) ) is very close to 1, which results in a lens with a very long focal length. Recently Snigerov and others have suggested and experimentally demonstrated, using cylindrical-shaped lenses, that this problem of long focal lengths can be overcome by using many lenses in series. Each lens refracts the photon through a small angle, but the sum of these sequential changes in direction can be moderately large. This increase in effective refraction angle reduces the focal length of the lens to a few meters or less and makes the multi-element lens a much more useful instrument for focusing x-rays. This paper, annualizes the expected performance of a lens consisting of a series of aligned hollow spheres in a beryllium substrate. The use of hollow spheres rather than hollow cylinders produces focusing of the x rays into a small focal spot in contrast to the single-directional focusing of the hollow cylinders, which produces a line focus. The use of beryllium as the substrate results in lower photo cross sections for both scattering and absorption relative to the value of the refractive index as compared to higher-Z materials and results in higher transmission values than for lenses with thin webs between the lens elements without distorting the surfaces of the neighbor lens element. This plus berylliums low density, keep the absorption and scattering in the web at a minimum. The calculations suggest that one will be able to make Be lenses with short focal lengths (1 to 2 m) with useable transmissions (10 to 30%). Two multi-element lenses have been constructed: one with 20 1-mm-diameter hollow spheres in an aluminum substrate, and one with 50 hollow spheres, 1 mm in diameter, in a beryllium substrate. Some construction details and calculations of the expected performance, are given for these two multi-element lenses.

Diamond monochromator for high heat flux synchrotron x-ray beams

Single crystal silicon has been the material of choice for x-ray monochromators for the past several decades. However, the need for suitable monochromators to handle the high heat load of the next generation synchrotron x-ray beams on the one hand and the rapid and on-going advances in synthetic diamond technology on the other make a compelling case for the consideration of a diamond monochromator system. In this paper, we consider various aspects, advantages and disadvantages, and promises and pitfalls of such a system and evaluate the comparative performance of a diamond monochromator subjected to the high heat load of the most powerful x-ray beam that will become available in the next few years. The results of experiments performed to evaluate the diffraction properties of a currently available synthetic single crystal diamond are also presented. Fabrication of a diamond-based monochromator is within present technical means.