Spherical Gratings Research Articles

Precision demanded of a Rowland circle monochromator: Its realization

A new monochromator design for high-energy resolution requires precise angle positioning of the grating during simultaneous linear motion of the grating vessel. The results obtained show that the required precision in the tilt angle of 0.05 arcsec can be met by the prototype of this monochromator. This is mainly achieved by a compensation of unavoidable systematic—time dependent and time independent—deviations from the ideal shape of the granite block on which an air bearing system moves. The compensation can be performed using the angle positioning system of the grating. The positioning accuracy here is better than 0.1 arcsec and shows a reproducibility of about 0.01 arcsec rms. The accuracy is provided by an angle encoder directly connected to the axis of rotation to be measured. A closed loop connection of the positioning and measuring system then allows a control on the grating angle within ± 0.03 arcsec. Thus, the mechanical precision made necessary by the use of the nearly perfect geometric surfaces of present day plane and spherical mirrors and gratings has been experimentally shown to be achievable.

Optical transfer matrices for arbitrary incidence on spherical surfaces and gratings in classical mounting

This work investigates the arbitrary incidence on spherical surfaces and curved gratings in classical mounting. We show that optical processes involving sudden changes in beams' direction through any optical process can be described by direct use of half-process matrices. Arbitrary incidence involves a new basic matrix connected to distortion, variable convergence and nonlocal operation.

Design of a low-energy extended UV beamline for bending magnet radiation

The design of a beamline, covering the energy range from 5 to 200 eV, based on a double incidence spherical gratings monochromator designed for ELETTRA bending magnet radiation is described. The monochromator consists of two interchangeable sets of gratings working one at normal incidence and the other at grazing incidence, to cover the entire spectral interval. The photon energy range of interest is covered with five gratings (two at normal and three at grazing incidence) achieving a resolving power of the order of 10000 and a photon flux approximately 1012 (photons/s/0.1% bandwidth/mm2) at the sample position.

Constant Angle of Diffraction-Spherical Grating Monochromator for Undulator Radiation

A new type of spherical grating monochromator for undulator radiation is proposed. Two reflecting elements of a plane mirror and a spherical grating are used between the entrance slit and the exit slit. It has a simple scanning mechanism: the plane mirror rotates around an axis on its surface so as to change the angle of incidence, while the grating is fixed with a constant angle of diffraction. Resolution of the monochromator is estimated for a soft X-ray undulator beamline of the SPring-8 by a ray tracing calculation. The results show that the monochromator can achieve high resolution, high energy and high photon flux in the soft X-ray region.

Holographic spherical gratings: a new family of quasi-stigmatic designs for the Rowland-circle mounting

In the context of aberration-corrected holographic spherical gratings used in the Rowland-circle mounting, we have investigated the off-Rowland recording geometries with stigmatic sources that nullify the defocus and meridional coma at all wavelengths. We introduce the additional requirement that astigmatism vanishes at a given wavelength. Then we demonstrate that a family of solutions exists and has a quasi-zero sagittal coma at the wavelength adopted for the astigmatism correction. This simultaneous reduction of sagittal coma and astigmatism greatly enhances the spectral performances of the Rowland mount. We also point out that a subfamily is found when the derivative of astigmatism is equal to zero at the wavelength of correction; the performances are then extended to a wider spectral range. Finally ray traces and performances of two representative examples of the new family of gratings are presented.

Comparison of grating designs for the Lyman Far-Ultraviolet Spectroscopic Explorer spectrograph

Various grating designs have been proposed by several investigators for possible use with the Lyman/Far-Ultraviolet Spectroscopic Explorer mission. The image quality, the feasibility, and the efficiency of five designs are compared, each using a distinct type of grating: (1) a grating ruled on a deformed ellipsoidal or toroidal blank, (2) an ellipsoidal grating recorded holographically with two auxiliary spherical mirrors, (3) a spherical holographic grating recorded with two auxiliary spherical holographic gratings, (4) a spherical ruled grating with variable spacing and straight grooves, and (5) a spherical ruled grating with a groove pattern that is determined theoretically (hybrid grating). From a purely theoretical viewpoint, grating (5) provides the finest images, followed by gratings, (3), (1), (4), and (2). In view of the current technological limitations, the order of practical importance is gratings (4), (1), (2), (3), and (5).

Imaging spectroscopy of high-temperature plasma sources

The imaging spectrograph with spherical crystal and transmission grating have been used to obtain the high spectrally and spatially resolved images of plasma X-ray microsources (fast z-pinch) as well as of laser-produced plasma flares up to 5 mm in extent. The spectral resolution λ/Δλ = 2·104 have been achieved using a quartz spherical mirror with the curvature radius 500 mm. Using a transmission grating spectrometer, the spectral density of power about 105 W/A in the “water window” has been measured in experiments with a fast z-pinch large pulse storage facility.

Practical VUV stigmatic configurations for spherical holographic gratings

Stigmatic configurations for spherical holographic gratings are generally explained by the properties of the harmonic conjugate points; this global approach, however, does not explain how the various aberrations generated by the blank are cancelled by the hologram. In this paper, light path function analysis is used to explain this balancing effect and to derive practical VUV configurations.

The advanced light source U8 beam line, 20–300 eV

The U8 is a beam line under construction at the Advanced Light Source (ALS). The beam line will be described along with calculations of its performance and its current status. An 8 cm period undulator is followed by two spherical collecting mirrors, an entrance slit, spherical gratings having a 15{degree} deviation angle, a moveable exit slit, and refocusing and branching mirrors. Internal water cooling is provided to the metal M1 and M2 mirrors as well as to the gratings. Calculations have been made of both the flux output and the resolution over its photon energy range of 20--300 eV. The design goal was to achieve high intensity, 10{sup 12} photons/sec, at a high resolving power of 10,000. The U8 Participating Research Team (PRT) is planning experiments involving the photoelectron spectroscopy of gaseous atoms and molecules, the spectroscopy of ions and actinide spectroscopy.

Aberration-reduced holographic spherical gratings for Rowland circle spectrographs

We used a numerical minimization method to design a Rowland holographic spherical grating that is recorded with two stigmatic laser sources. The method aims at simultaneously reducing all aberrations up to fourth order over a significant spectral range. In the context of a high spectral resolution, far-ultraviolet spectrograph, an original solution is found that implies a nonclassical recording geometry with one virtual source. This solution satisfies the requirement of a resolving power of ~ 30 000 with the unquestionable advantage of manufacturing and testing simplicity. Finally, another example, which is obtained in a different context, shows that the properties of this recording geometry probably have a general applicability.

Soft x-ray spectroscopy beam line on the NSLS X1 undulator: Optical design and first performance tests

The layout and first data on the performance of the soft x-ray undulator beam line on the x-ray ring at Brookhaven are described. The undulator (X1) has 35 periods of 8 cm, resulting in a tuning range for the fundamental of about 200–700 eV at the nominal ring energy of 2.5 GeV. The monochromator is based on the Dragon concept with spherical grating, movable exit slit but with water-cooled optical elements in order to deal with the maximum flux of ca. 400 W. This system represents one of the next generation of soft x-ray undulator beam lines which will figure prominently at new synchrotron radiation facilities such as ALS, ELETTRA and BESSY II. During the first performance tests absorption spectra of simple molecules at the C, N, and O K edges have been recorded with resolution and flux so far unattained in the soft x-ray region.

The high energy spherical grating monochromator−A new source of soft x rays at Daresbury (abstract)

In this article we report the characteristics of the new High Energy Spherical Grating Monochromator beam line on the SRS. The instrument, which has no entrance slit, was designed to provide high photon flux with small spot size, in the energy range covering the 1s binding energies of carbon, nitrogen, oxygen, and fluorine. Radiation from a bending magnet is horizontally focused onto the exit slit by a long, Pt-coated meridian cylinder (R=299 m, 2 mrad horiz. aperture, 2° glancing angle). The light is vertically diffracted and focused by one of three interchangeable spherical gratings (1050, 1500, and 1800 lines mm−1) operating in negative order. Finally, the light is refocused by an ellipsoidal mirror. The photon flux, determined with copper and carbon photocathodes, is presented for the three gratings. Useful flux is obtained in the range 250–1200 eV, with intensity maxima for each grating at 600, 700, and 800 eV of 11, 9, and 5×1010 photons s−1 per 100-mA stored beam into a band pass of 0.05%. The influence of contaminants which are present on the optical elements is discussed, together with details of beam line operating conditions which minimize the build up of such contaminants. Photoabsorption and photoemission measurements indicate a high (up to 30%) second order and some third order light content. Resolution determinations obtained from photoabsorption measurements are presented. Although features as narrow as 250 meV have been resolved, the resolving power of the instrument is found to depend strongly on stored beam current. We suggest this may be due to electron beam (i.e., source) blow-up. We critically discuss the suitability of the new facility for surface EXAFS of low Z adsorbates, in particular above the carbon, nitrogen, and oxygen 1s edges, using examples from recent studies which have been undertaken on the beamline.

Beam line 4: A dedicated surface science facility at Daresbury Laboratory

We describe a beam line currently under construction at the Daresbury Laboratory which forms part of a surface science research facility for the Interdisciplinary Research Centre in Surface Science. The beam line has three branches, two of which are described here. The first branch covers the high-energy range 640 eV≤hν≤10 keV, being equipped with a double-crystal monochromator and a novel multicoated premirror system. The second branch line is optimized for the energy range 15≤hν≤250 eV, using cylindrical focusing mirrors, a spherical diffraction grating and an ellipsoidal refocusing mirror to achieve high resolution with a small spot size.

Varied line-space gratings and applications (invited)

This paper presents a straightforward analytical and numerical method for the design of a specific type of varied line-space grating system. The mathematical development will assume plane or nearly plane spherical gratings that are illuminated by convergent light, which covers many interesting cases for synchrotron radiation. The gratings discussed will have straight grooves whose spacing varies across the principal plane of the grating. Focal relationships and formulas for the optimal grating-pole-to-exit-slit distance and grating radius previously presented by other authors will be derived with a symbolic algebra system. It is intended to provide the optical designer with the tools necessary to design such a system properly. Finally, some possible advantages and disadvantages for application to synchrotron radiation beamlines will be discussed.

The Berkeley EUV spectrometer for ORFEUS

A novel EUV spectrometer is presented for the ORFEUS-SPAS mission. It uses a set of four varied line-space spherical diffraction gratings to obtain high-resolution spectra of point sources at wavelengths between 390 and 1200 A. The spectra are recorded with two detector units, each containing curved-surface microchannel plates and a delay-line anode-readout system. An independent optical system detects the image of the source in the entrance aperture and tracks the source as it drifts during an observation, enabling a reconstruction of the spectra postflight. The overall system performance is discussed and illustrated by synthetic spectra.

Rigorous and approximated stigmatism for gratings ruled on quadrics

Many classical configurations lead to perfect stigmatism with spherical holographic gratings. Stigmatism is also reached by using quadrics. The author presents the properties of a first set of stigmatic quadrics, which work in Wadsworth mounting, and give those of a second set, suitable for Wadsworth and Rowland mountings.

Imaging properties of holographic gratings

Optical properties of spherical or aspherical holographic gratings are investigated from the imaging point of view. Basic conditions for aplanetism are first expressed, the object image and recording point sources being located on circles centered on the grating tangent. The performances (resolution, anamorphism) of some imaging configurations are described.

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.

An ultrahigh-vacuum multiple grating chamber and scan drive with improved grating change

We describe a new grating chamber and scan drive which has been designed, built, and tested by the Physical Sciences Laboratory of the University of Wisconsin for the new high flux, high resolution spectroscopy branch line of the TOK hybrid wiggler/undulator on the NSLS VUV ring. The chamber will contain spherical gratings to be used in the Spherical Grating Monochromator (SGM) configuration introduced by Chen and Sette [1]. The grating chamber houses five 180 mm × 35 mm × 30 mm gratings capable of scanning a range of 12° (−4° to +8° with respect to the incoming beam direction) for VUV, and soft X-ray diffraction. The gratings can be switched and precisely indexed while under ultrahigh vacuum (UHV) at any scan angle and are mechanically isolated from the vacuum chamber to prevent inaccuracies due to chamber distortions. The gratings can separately be adjusted for height, yaw, patch, and roll, with the latter three performed in vacuo. The scan drive provides a resolution of 0.03 arc sec with linearity over the 12° range of ≅1.5 arc sec and absolute reproducibility of 1 arc sec.

VUV-SX spherical grating monochromator beam lines at BEPC and HESYRL

Two VUV-SX spherical grating monochromator (SGM) beam lines have been designed and constructed for laboratories in China; one for the Beijing Electron-Positron Collider (BEPC) and one for the Hefei Synchrotron Radiation Laboratory (HESYRL). Each of them will cover a wide spectral range, approximately 10–1000 eV, with four or five spherical laminar gratings. Tuning of the wavelength in the monochromators is achieved by rotation of the grating and translation of a pair of stepping-motor-driven slits to follow the Rowland circle to improve the monochromator's performance. The prefocusing mirrors are different on the BEPC and HESYRL beam lines. To accept 20 mrad of the synchrotron radiation horizontally from the 800-MeV HESYRL ring, a combined Kirkpatrick-Baez-type mirror set has been developed. The 1240-mm-long horizontal prefocusing mirror consists of four 310 mm × 70 mm × 40 mm gold-coated Zerodur spherical mirrors, each of which is independently adjustable in three dimensions. In the BEPC SGM beam line, a toroidal nickel-coated aluminum prefocusing mirror intercepts up to 4.5 mrad at a distance of 12 m from the source. A heat-pipe cooling system has also been designed for this mirror to accommodate the higher power from the 2.2–2.8 GeV beam stored in BEPC in its dedicated SR mode. The optical ray tracing results, the mechanics, and control systems are described in this paper.