Sagittal Focusing Research Articles

The Naval Research Laboratory’s new hard x-ray beamline at the National Synchrotron Light Source

The Naval Research Laboratory is currently rebuilding its hard x-ray beamline at the National Synchrotron Light Source (NSLS, X-23B) to increase the flux and energy range. The optical and mechanical design of the new configuration is described and its performance predicted. The new design contains a fixed-exit pair of mirrors for harmonic rejection and source collimation and a fixed-exit two-crystal monochromator with sagittally focusing second crystal. In addition, the horizontal acceptance of the line will be increased from 6 to 18 mrad. The line is expected to focus radiation to a spot 0.5 mm vertical by 1.0 mm horizontal in the energy range 2 to 30 keV. At 8.5 keV a throughput of 5.7×1012 photons/s/200 mA is expected with an E/ΔE of 5×103 with Si (111) crystals, a factor of 10 to 30 over present beamline throughput. The increased flux is due to improved mirror surfaces, decreased optical aberrations from conical sagittal focusing, and the increased horizontal acceptance.

Anticlastic curvature measurements on unribbed crystal optics for synchrotron radiation

Various methods have been proposed for measuring the distortion in perfect crystals using double-crystal methods. The majority of these methods rely on making comparisons between double-crystal rocking curve measurements under the spatial extent of an extended x-ray beam. Unless the beam is large and parallel (such as at a synchrotron bending magnet), these methods are not easily scalable to large crystals (e.g., crystal focusing elements for synchrotron beamlines) due to the mechanical inaccuracies inherent in moving the various optical components. We present a method based on a scanning source which simplifies the problems in scaling double-crystal methods to large optics. In addition, results using this method are presented on a ribless sagittal focusing Si(111) crystal demonstrating that the anticlastic deviation can be made to be less than ±1 s of arc over a 1-cm-long section parallel to the sagittal axis.

Compact monochromator for the multipurpose X3 synchrotron beamline at NSLS

A newly designed monochromator assembly for the State University of New York (SUNY) X3 beamline at the National Synchrotron Light Source (NSLS) consists of two monochromators enclosed in a single vessel, positioned at 7813 mm from the bending magnet source. It allows delivery of monochromatic radiation into two independent end stations. The first unit contains two crystals reflecting in the vertical plane, and operating in a ‘‘channel-cut’’ mode. The second of the crystals can be bent for sagittal focusing of the beam. The second monochromator unit reflects 2 mrad of the radiation fan between the A2 and B branches of the beamline at a fixed angle of 2Θ=12°, and serves the sideways experimental station A1. Four interchangeable triangular crystals, mounted on a rotary holder, provide x rays of four different energies.

A cryogenically cooled, sagittal focusing scanning monochromator for ESRF

A double crystal monochromator for fast scans over large energy ranges has been designed, constructed, and tested at ESRF. The instrument is based on a drive unit operated in air that rotates a large wheel sitting in UHV and supporting the two crystal assemblies. The link between the drive and the wheel is realized by a differentially pumped feedthrough. In order to withstand the full power of typical ESRF undulators without noticeable deformation, the first crystal is cooled by liquid nitrogen via pipes located in the hollow core of the differentially pumped feedthrough. The second crystal mount is based on an elastically deformable structure driven by piezoelectric and pneumatic actuators for precise angular adjustments. In its basic version the relative distance between the first and second crystal is kept constant (channel cut type of design), but a nearly fixed beam at the focal plane can be obtained, as the Bragg angle is changed, by the partial compensation of two different effects: the vertical displacement of the exit beam typical of channel cut designs and its divergence with respect to the incoming one due to the difference in lattice spacing between the first cooled crystal and the second one at room temperature. Horizontal focusing of the beam is achieved by sagittal bending of the second crystal. The major drawbacks of crystal focusing, the necessity of changing the radius of curvature while scanning, and the losses in throughput due to the anticlastic deformation experienced at high energy are circumvented by an appropriate choice of the crystal geometry and orientation.

Conical geometry for sagittal focusing as applied to x rays from synchrotrons

We describe a method for simultaneously focusing and monochromatizing x rays from a fan of radiation having as high as 15-mrad divergence in one dimension. This geometry is well suited to synchrotron radiation sources at magnifications of 1/5 to 2 and is efficient for x-ray energies between 3 and 40 keV (0.48 and 6.4 fJ). The method uses crystals bent to part of a cone for sagittal focusing and permits the collection of a larger divergence with less mixing of the horizontal into the vertical divergence than is possible with x-ray mirrors. We describe the geometry required for achieving the highest efficiency when a conical crystal follows a flat crystal in a nondispersive two-crystal monochromator. At a magnification of 1/3, the geometry is identical to that of a cylindrical focusing design described previously [ Nucl. Instrum. Methods172, 237 ( 1980)]. A simple theoretical calculation is shown to agree well with ray-tracing results. Minimum aberrations are observed at magnifications near 1. Applications of the conical focusing geometry to existing and future synchrotron radiation facilities is discussed.

On the sagittal focusing of synchrotron radiation with a double crystal monochromator

A method to avoid the anticlastic bending of the second crystal in a two-crystal monochromator for synchrotron radiation is proposed. It is analyticall

A high resolution monochromator covering wide ultraviolet spectral ranges with a single grating

A new configuration for a wide ultraviolet range high resolution monochromator has been analysed. It consists of a first off-axis paraboloidal mirror collimating the radiation beam from the source, followed by a couple of plane optical elements, a mirror and a grating, constituting the dispersive system, and by a second off-axis paraboloidal mirror focusing a selected portion of the dispersed radiation beam on a fixed exit slit. Since in the extreme ultraviolet region it is necessary to use the optical elements in grazing incidence to have a good reflectivity, the two paraboloidal mirrors can be used in the so-called sagittal focusing to minimize the slope error induced aberrations in the dispersive plane. As an example, the monochromator performances when applied to a synchrotron radiation source in the range 250-1250 eV (9.9-49.6 AA) are shown.

Wavefront Aberration Function

Abstract In this paper we obtain equations for the two aperture second derivatives of the wavefront aberration function in terms of the position of the local sagittal and tangential foci. The derivatives presented here are valid for meridional rays. The local foci positions for any tangential aperture zone are calculated by tracing one ray that corresponds to that zone and using Coddington's equations. The second derivative equations can be incorporated in the usual procedure of aberration computation, increasing from two to four the number of equations available for each tangential ray traced.

Vertically and horizontally focusing beam line BL-3A of the Photon Factory

An x-ray beam line BL-3A at the Photon Factory has been designed and constructed to have optical components of a collimating paraboloidal mirror, a sagittal-focusing double-crystal monochromator, and a focusing paraboloidal mirror. The evaluation at the (+,−,−) setting of Si (111) showed that the vertical angular divergence was reduced from 13.4 to 9.5 (arcsec) by the collimating mirror. The vertical focusing with two mirrors gave the beam size of 0.2 mm (1/50). The horizontal beam size was reduced to 1/60 by sagittal focusing.

A simple sagittal focusing crystal which utilizes a bimetallic strip

A sagittal focusing monochromator which utilizes a bimetallic strip as its active focusing member has been designed and has undergone preliminary testing. The crystal bender is very easy to use as only setting the temperature of the bimetallic strip is necessary to adjust the focus. The mechanism utilizes bending rods mounted on live centers which prevent twisting of the crystal. A finite element analysis has been done on a new ribbed crystal which, based on the analysis, would have very small aberrations.

Double-crystal monochromator for high-power and high-energy synchrotron radiation

The performance of the fixed-exit-beam sagittal-focusing double-crystal monochromator installed in the elliptical-multipole wiggler beam line at the 6.5-GeV accumulation ring for the TRISTAN project is reported. The first crystal of the monochromator is directly water cooled. Because of its large size, the crystal deformation made by the cooling water pressure and the mounting method of the crystal is found to be a serious problem. The second bent crystal for sagittal focusing and the adjusting mechanisms must be also cooled to avoid the heating by Compton scattered x rays from the first crystal. It is important for the double-crystal monochromator installed in the high-power and high-energy beam line to cool various components in addition to the first crystal.

Soft x-ray monochromator configurations for the ELETTRA undulators: A stigmatic SX700 (invited)

This report presents a modification of an SX700 plane grating monochromator, which can be made to work stigmatically by introducing an entrance slit. It will then provide the highest possible photon density in the exit slit with resolving powers that can be very high because it is not any longer source-size limited. Instead it can be adjusted according to the slit settings. The deterioration of the resolving power because of slope errors in the ellipsoid can be avoided by orienting this mirror for sagittal focusing in the grating dispersion plane. In this configuration the instrument can be matched almost ideally to undulator sources.

Diffraction and diffuse scattering measurements in material science: Improvement brought to a six-circle goniometer on a synchrotron beam line

The six-circle diffractometer D23 set up in LURE-DCI (bending magnet) at Orsay has been improved since its first presentation in 1987 [M. Bessiere, G. Bessenay, J. Frouin, M. Jouvin, and S. Lefebvre, Nucl. Instrum. Methods A 261, 591 (1987)]. We present a new design for the sagittal focusing monochromator, a two-mirror device for harmonics rejection and detectors for adjustments. All these modifications are now operational and can be used together with a crystal analyzer for powder studies.

The Australian diffractometer at the Photon Factory

Outlined are design features of a versatile high-resolution two-axis diffractometer that is being constructed for operation at the Photon Factory as an Australian national facility. The instrument features optional use of multiple-imaging plates on a translating cassette to allow rapid recording of an almost complete range of data covering both the high-angle and small-angle scattering regime or alternatively the use of electronic detectors. The instrument will be capable of operation in various modes including the following: (i) high-resolution powder diffraction with single-channel counter and crystal analyzer, (ii) high-resolution, high-speed powder diffraction in the Debye–Scherrer mode with imaging plates as recording medium, either stationary or translating (for time-dependent studies), (iii) small-angle x-ray scattering with imaging plates as recording medium, (iv) protein crystallography in screenless Weissenberg mode, and (v) two- or three-axis single-crystal diffractometry. The salient features of the instrument are the use of a double-crystal sagittal focusing monochromator as primary monochromator together with the optional use of a condensing–collimating channel-cut (CCCC) monochromator or other channel-cut monochromator as secondary monochromator. The use of a CCCC monochromator enables fine tuning of beam position on sample, harmonic suppression, beam-condensation, and variation of wavelength bandpass. Further features include the use of high-precision incremental encoders on both axes, together with the capability of operating the whole diffractometer, including secondary monochromator and detectors, in vacuum of order 10−3 Torr in order to reduce absorption and parasitic scattering, and the use of a large camera radius (approximately 0.57 m) for the imaging plate cassette in order to increase angular resolution and signal to noise.

X-ray focusing monochromator for an X-ray scanning microscopy and microtomography station

The first results concerning an X-ray focusing monochromator in the energy range 5–35 keV for an X-ray scanning microscopy and microtomography station at the VEPP-3 storage ring are presented. Sagittal focusing with the use of Si(111) crystal in the Von Hamos scheme is investigated and discussed.

Deviation of second-order focal curves in common plane-symmetric spectrometer mounts

In common plane-symmetric grating spectrometer designs, the primary tangential and sagittal foci are not coincident except at a few discrete wavelengths. A quantity is defined to represent this deviation in focal curves. Expressions for this quantity are derived for common spectrometer mounts, and cases in which it may be minimized are discussed.

Anastigmat flat field polychromators

Various aplanatic flat field spectrometers fitted with spherical or aspherical holographic gratings are described. Basic conditions for stationary anastigmatism are first expressed, the object and image being located on the sagittal focus. Optical performances of some coma corrected configurations are finally described.

Ray tracing results for a doubly focusing independent crystal X-ray monochromator

The need to monochromatize and focus X-rays emitted into appreciable horizontal apertures by storage ring sources (bending magnets and wigglers) leads to the design of focusing monochromators. In order to optimize their design, it is important to understand the combined effect of focusing and diffraction of the flux available at the sample. We analyze here the combined effect of focusing and diffraction for a double-crystal monochromator which is being designed for the Superconducting Wiggler beamline (SCOW) on the ADONE storage ring in Frascati. A double-separated crystal system has been chosen; we study the possibility of obtaining focalization in two steps, by dynamically bending the crystals in order to obtain both tangential and sagittal focusing. Non-focusing and semi-focusing systems have also been considered. The calculations have been performed with the ray tracing program SHADOW. We estimate performances for various focusing geometries and consider the effect of anticlastic curvature.

High-resolution soft-X-ray monochromators of new design

We have applied the principle of sagittal focusing to obtain high-resolution soft-X-ray monochromators for application to undulators, as planned for the new generation of synchrotron radiation facilities. Grazing incidence optical configurations (like Czerny-Turner and those utilizing a grating with variable line spacing) are particularly attractive. These configurations have the advantage of using fixed entrance and exit slits. We have analyzed such configurations using sagittal focusing and have shown that the resolution obtainable is comparable with that of a spherical grating monochromator of similar size (which, by contrast, needs moving entrance and exit slits).

Operation of a five-pole superconducting wiggler in the DCI positron ring and design of the beamline (invited)

A five-pole superconducting wiggler has been installed in the DCI positron ring and operated without disturbing the machine characteristics at full energy (1.85 GeV) and maximum current (300 mA). Three beamlines have been built which feed six beam ports. The first two-crystal monochromator to be used for Compton scattering has been commissioned although the sagittal focusing has not yet been tested.