Varied Line Spacing Research Articles

Spectral Analysis in EUV Range for Study of Core Impurity Behavior in HL-2A

Extreme ultraviolet (EUV) spectroscopy has been developed for impurity diagnostics in HL-2A tokamak. The EUV spectrometer consists of an entrance slit, a holographic varied-line-space (VLS) grating, a back-illuminated charge-coupled device (CCD) and a laser light source for optical alignment. Spectral lines in wavelength region of 20-500 Å observed from HL-2A plasmas were analyzed to study the impurity behavior. Spectral and temporal resolutions used for the analysis were 0.19 Å at CV (2×33.73 Å) and 6 ms, respectively. It was found that carbon, oxygen and iron impurities were usually dominant in the HL-2A plasma. They almost disappeared when the siliconization was carried out. Although the EUV spectra were entirely replaced by the silicon emissions just after the siliconization, the emissions were considerably decreased with accumulation of discharges. Aluminum and neon were externally introduced into the HL-2A plasma based on laser blow-off (LBO) and supersonic molecular beam injection (SMBI) techniques for a trial of the impurity transport study, respectively. The preliminary result is presented for time behavior of EUV spectral lines.

Construction of the Scanning Transmission X-ray Microscope Beamline at UVSOR

Construction of a scanning transmission x-ray microscope (STXM) beamline is in progress at UVSOR (Okazaki, Japan). To obtain high brilliance with high resolving power for the incident x-rays, the combination of an in-vacuum undulator and a Monk-Gillieson mounting monochromator with a varied line spacing plane grating was adopted. Resolving power, spectrum and size of the incident x-rays of the beamline were discussed through simulations Then the photon flux at a sample of ~108 photons/s at the resolving power of 5,000 in the photon energy range from 100 to 700 eV is expected. As the first results, spectrum of calcium in chalk is shown.

Spatially-resolved flat-field soft X-ray spectrometer on experimental advanced superconducting tokamak

A space- and time-resolved flat-field soft X-ray spectrometer with the wavelength range of 1–13nm has been developed to study impurity behavior on the Experimental Advanced Superconducting Tokamak (EAST). Using an entrance slit, a varied line spacing grating (2400grooves/mm at the grating center), and a charged coupled device (CCD) system, time evolution of profiles of impurity line emissions were recorded. The spectral resolution of the spectrometer is 0.006nm at 5nm when the width of entrance slit is set at 0.03mm. The best spatial resolution obtained is 24.5mm with the height of slit at 1.0mm. The spectrometer is placed 8000mm away from the plasma center and the observed spatial range covers 0–450mm from the equatorial plane of EAST. The first experimental results were obtained from the recent EAST campaign. The system was shown to be capable of observing spectral lines from both intrinsic low-Z impurities (C, O, et al.) and highly ionized medium- and high-Z impurities (Fe, Cr, Ni, Cu, et al.). Spectral lines from the full wavelength range (1–13nm) can be obtained by moving the position of the CCD. Spectra with the wavelength intervals of 1–2nm show strong metal lines for H-mode discharges. Time evolutions of C VI (3.373nm) and O VIII (1.897nm) lines are presented and detail analysis is performed combining electron density intensity, Dα and soft X-ray and extreme ultraviolet (XUV) radiation intensities. Evolutions of profiles of C VI (3.373nm) and O VIII (1.897nm) at core plasma were also shown, indicating that the spectrometer can be applied for impurity transport studies,

First results from a next-generation off-plane X-ray diffraction grating

Future NASA X-ray spectroscopy missions will require high throughput, high resolution grating spectrometers. Off-plane reflection gratings are capable of meeting the performance requirements needed to realize the scientific goals of these missions. We have identified a novel grating fabrication method that utilizes common lithographic and microfabrication techniques to produce the high fidelity groove profile necessary to achieve this performance. Application of this process has produced an initial pre-master that exhibits a radial (variable line spacing along the groove dimension), high density (>6000 grooves/mm), laminar profile. This pre-master has been tested for diffraction efficiency at the BESSY II synchrotron light facility and diffracts up to 55% of incident light into usable spectral orders. Furthermore, tests of spectral resolving power show that these gratings are capable of obtaining resolutions well above 1300 ($\lambda/\Delta\lambda$) with limitations due to the test apparatus, not the gratings. Obtaining these results has provided confidence that this fabrication process is capable of producing off-plane reflection gratings for the next generation of X-ray observatories.

Upgrade of Saga-university beamline in SAGA-LS

Saga-university beamline has been upgraded by installing a new planar-typ e undulator for advanced researches on nano-surfaces and interfaces in the soft X-ray region. The magnetic field of the undulator along the electron beam trajectory was measured at the magnetic gap width between 30 and 150 mm. After the installation of the undulator in a 2.7-m straight section of the Saga-LS storage ring, the performance of the beamline with varied line spacing plane grating monochromator was examined by measurements of peak energy, photon flux, and energy resolution. The beamline is opened for the experimental use in the energy region between 32 and 800 eV using fundamental and the higher harmonics.

High coherent flux at the NSLS-II coherent soft x-ray beamline

The optical design of the coherent soft x-ray (CSX) and fast polarization switching beamline at NSLS-II has been optimized to deliver photon flux in the range of 1012 − 1013 ph/s in the energy range from 200 to 2000 eV. Two identical APPLE-II undulators will be operated in two different modes: canted and phased. In the canted mode, one beam will be directed to the coherence branch and the other to the fast polarization switching branch. In the phased mode, the two undulators will be operated as a single device to deliver the highest coherence flux to the coherence branch. The monochromator for the coherence branch is a focusing variable line spacing plane grating monochromator designed to deliver a moderate resolving power (2000 to 3000). The horizontal focusing onto the exit slit will be performed with a cylindrical bendable mirror located downstream of the monochromator and having similar demagnification as the monochromator.

Design of a VLS Grating-Lens Hybrid Component for Miniature Spectrometer

we propose a micro varied-line-spacing (VLS) grating-lens hybrid component that integrates the collimation, dispersion and focus functions for use in a miniature spectrometer. We adopt the principle of least wave change to design and optimize the VLS grating pattern on the spherical lens surface, and then build a model in ZEMAX to simulate the spectral resolution performance. The best spectral resolution turns out to be 1nm at the center wavelength of the target wavelength range. The resolution across the wavelength range from 450nm to 650nm is better than 4nm.

High acceptance high resolution soft X-ray grating spectrometer: Choice of optical design

Elementary excitations in correlated electron systems produce low intensity spectral features that may be observed by resonant inelastic X-ray scattering (RIXS). In the soft X-ray region, ruled gratings are used to obtain optimum energy dispersion. Gratings have to operate at small grazing incidence so that in practice the angular acceptance is small. Schemes have been proposed using varied line spacing plane gratings combined with large focusing mirrors to increase acceptance. Here we analyze the relative performance of spherical and elliptical mirrors by means of numerical calculations of the light path function. We show that the use of an elliptical mirror provides higher resolving power over an extended energy range. Thus the 50–1000eV energy range can be covered by two gratings only and the resolving power can be adapted to the experimental conditions.

Space-resolved extreme ultraviolet spectrometer system for impurity behavior research on experimental advanced superconducting Tokamak

A flat-field extreme ultraviolet (EUV) spectrometer, which covers a wavelength range of 5–50nm, has been developed for the experimental advanced superconducting tokamak (EAST). The spectrometer mainly consists of an entrance slit, a varied line spacing groove grating (1200 grooves/mm at the grating center), and a charge coupled device (CCD). The spectral resolution of the spectrometer is Δλ=0.015nm at 20nm when the width of entrance slit is set at 0.03mm. The spectrometer is capable of surveying impurity line emissions, for both intrinsic low-Z impurities and highly ionized medium- and high-Z impurities. The obtained best time resolution is 25ms and with entrance slit height at 1mm the spatial resolution has reached 45mm. Preliminary experimental results from the recent EAST campaign were presented. The system was shown to be able to measure the spectral lines of C, O, He, Li and metal impurities under various auxiliary heating conditions. By translating the CCD with a step motor, spectral lines from a full wavelength of 5–50nm can be covered. In particular the wavelength interval of 15–20.5nm showed strong line emissions of metal impurities. Time evolutions of second order spectral line of C VI (6.674nm) and Ni XVI (19.219nm) lines are presented and detailed analysis is done while taking into account electron density, electron temperature, auxiliary heating power, C III (464.7nm) and XUV intensities. The profile of Ni XVI is presented to show its good spatial resolution. The EUV spectrometer will be an important addition to the current EAST diagnostic systems in the area of impurity survey and transport research.

Continuous Tuning of a Diode Laser Over 8.4 THz Near 1550 nm With a Chirped Grating

We achieve mode-hop-free tuning of an extended-cavity diode laser with an original technique. The external coupler is a chirped (varied line-space) diffraction grating. A simple linear translation of the grating is then sufficient to obtain continuous tuning. The measured continuous tuning range has reached 66 nm (8.4 THz) near a wavelength of 1550 nm with a relatively constant output power of 3 mW.

A ray-tracing program and two cases of its application at the Shanghai EBIT laboratory

Based on ray-tracing procedures/techniques, we have carried out simulations on the operation of (i) a toroidal mirror used along with a vacuum ultraviolet scanning monochromator and (ii) variable line spacing gratings such as those used in flat-field spectrometers. In case (i), a toroidal mirror was installed in the optical beam line for normal incidence spectroscopy at the Shanghai electron beam ion trap (EBIT) (Zou Y and Hutton R 2005 Phys. Scr.T120 47) based on simulation results. Case (2) yielded results that can be used in aligning flat-field spectrometers; the results of the simulations were tested by experiment and were found to be in good agreement.

Variable line spacing plane grating monochromator for first LNLS undulator beamline

In early 2007 an elliptically polarized Apple II undulator was installed on an LNLS storage ring. It will provide photons to the first PGM beamline at LNLS designed to deliver photons ranging from 100 to 1000 eV with a resolving power of over 10 000. The beamline is about to start the commissioning process and this paper describes mainly the monochromator design and construction.

Numerical optimization of spherical variable-line-spacing grating X-ray spectrometers.

Operation of an X-ray spectrometer based on a spherical variable-line-spacing (VLS) grating is analyzed using dedicated ray-tracing software allowing fast optimization of the grating parameters and spectrometer geometry. The analysis is illustrated with optical design of a model spectrometer to deliver a resolving power above 20400 at a photon energy of 930 eV (Cu L-edge). With this energy taken as reference, the VLS coefficients are optimized to cancel the lineshape asymmetry (mostly from the coma aberrations) as well as minimize the symmetric aberration broadening at large grating illuminations, dramatically increasing the aberration-limited vertical acceptance of the spectrometer. For any energy away from the reference, corrections to the entrance arm and light incidence angle on the grating are evaluated to maintain the exactly symmetric lineshape. Furthermore, operational modes when these corrections are coordinated are evaluated to maintain either energy-independent focal curve inclination or maximal aberration-limited spectrometer acceptance. The results are supported by analytical evaluation of the coma term of the optical path function. This analysis thus gives a recipe for designing a high-resolution spherical VLS grating spectrometer operating with negligible aberrations at large acceptance and over an extended energy range.

The in-focus variable line spacing plane grating monochromator

The in-focus variable line spacing plane grating monochromator is based on only two plane optical elements, a variable line spacing plane grating and a plane pre-mirror that illuminates the grating at the angle of incidence that will focus the required photon energy. A high throughput beamline requires only a third optical element after the exit slit, an aberration corrected elliptical toroid. Since plane elements can be manufactured with the smallest figure errors, this monochromator design can achieve very high resolving power. Furthermore, this optical design can correct the deformations induced by the heat load on the optics along the dispersion plane. This should allow obtaining a resolution of 10 meV at 1 keV with currently achievable figure errors on plane optics. The position of the photon source when an insertion device center is not located at the center of the straight section, a common occurrence in new insertion device beamlines, is investigated.

First results from the online variable line spacing grating spectrometer at FLASH

A high resolution online grating spectrometer has been set up at the free electron laser in Hamburg FLASH at DESY in Hamburg. This new photon diagnostic device allows to parasitically measure the spectral distribution of the FEL laser pulses on a single shot basis and therefore gives complementary spectral and temporal pulse information for user experiments and FEL operation. The spectrometer combines a plane mirror and two plane variable line spacing (VLS) gratings and is installed at the non-monochromized beamline branch at FLASH. In contrast to a standard grating the blaze angle is optimized for the zero-order radiation transferred to the experimental stations, leaving approximately 1–10% intensity for recording the wavelength in the spectrometer. The detector, an intensified CCD camera imaging the dispersed beam on a Ce:YAG crystal, can follow the focal plane of the gratings covering a wavelength ranging from 6 to 60 nm. The resolving power of the spectrometer is expected to range from 500 to 8000 depending on the FEL wavelength.

Vacuum ultraviolet impurity spectroscopy on the Alcator C-Mod tokamak

Vacuum ultraviolet spectroscopy is used on the Alcator C-Mod tokamak to study the physics of impurity transport and provide feedback on impurity levels to assist experimental operations. Sputtering from C-Mod's all metal (Mo+W) plasma facing components and ion cyclotron range of frequency antenna and vessel structures (sources for Ti, Fe, Cu, and Ni), the use of boronization for plasma surface conditioning and Ar, Ne, or N(2) gas seeding combine to provide a wealth of spectroscopic data from low-Z to high-Z. Recently, a laser blow-off impurity injector has been added, employing CaF(2) to study core and edge impurity transport. One of the primary tools used to monitor the impurities is a 2.2 m Rowland circle spectrometer utilizing a Reticon array fiber coupled to a microchannel plate. With a 600 lines/mm grating the 80<λ<1050 Å range can be scanned, although only 40-100 Å can be observed for a single discharge. Recently, a flat-field grating spectrometer was installed which utilizes a varied line spacing grating to image the spectrum to a soft x-ray sensitive Princeton Instruments charge-coupled device camera. Using a 2400 lines/mm grating, the 10<λ<70 Å range can be scanned with 5-6 nm observed for a single discharge. A variety of results from recent experiments are shown that highlight the capability to track a wide range of impurities.

Traveling-wave Thomson scattering and optical undulators for high-yield EUV and X-ray sources

We present a novel high-yield Thomson scattering geometry that takes advantage of compact electron bunches, as available in advanced, low-emittance linear accelerators or laser wakefield accelerators. In order to avoid the restrictions on the X-ray photon yield imposed by the Rayleigh limit, we use ultrashort, pulse-front tilted laser pulses in a side-scattering geometry. Such a traveling-wave setup allows an overlap of electron and laser beams, even after propagating over distances much longer than the Rayleigh length. Experimental designs are discussed and optimized for different scattering angles. Specifically, to minimize group delay dispersion at large scattering angles >10°, we propose the use of varied-line spacing (VLS) gratings for spatio-temporal laser pulse shaping. Compared to head-on (180°) Thomson scattering, interaction lengths are in the centimeter to meter range and photon numbers for ultrashort X-ray pulses can increase by several orders of magnitudes.

Space-resolved extreme ultraviolet spectrometer for impurity emission profile measurement in Large Helical Device

A space-resolved extreme ultraviolet (EUV) spectrometer working in 60-400 A range has been developed to observe impurity emission profiles in core and edge plasmas of Large Helical Device (LHD). A flat focus is made for plane surface detector by using a varied line spacing holographic grating with an angle of incidence of 87 degrees. An excellent spectral resolution of 0.22 A at 200 A is then obtained after careful adjustment of the optical components. In the profile measurement of the LHD plasmas, the toroidal resolution is important as well as the radial resolution because the magnetic surfaces of LHD quickly change when the observation chord is tilted at a slightly different toroidal location. Horizontal dispersion is therefore selected for the present spectrometer. As a result, the toroidal resolution of 75 mm is achieved at the plasma position. An enough radial resolution of 10 mm is also obtained at spatial-resolution slit width of 0.2 mm. In order to measure the full radial profile of LHD plasmas the spectrometer is placed at a distance of approximately 9200 mm away from the plasma center and a backilluminated charge-coupled device with a size of 6.6x26.6 mm(2) is set vertical to the horizontal dispersion. Half of the LHD plasma, i.e., approximately 50 cm, can be measured along the vertical direction at horizontally elongated plasma cross section. A full vertical profile can be obtained by changing the vertical angle of the EUV spectrometer. As an example the full vertical profile of edge C IV (312.4 A) emission is presented. A wavelength interval of 35-65 A can be simultaneously observed, which varies according to the wavelength to be measured. A local emission profile of Fe XX (132.67 A) is also presented after Abel inversion as a typical example of the core EUV emission. Finally, the EUV spectrometer is absolutely calibrated using EUV bremsstrahlung continuum profile in comparison with absolute values of visible bremsstrahlung continuum profile because the bremsstrahlung continuum can be easily detected in high-density discharges of LHD.

A compact soft X-ray spectrograph combining high efficiency and resolution

A compact and light weight soft X-ray spectrograph covering 5−35 nm and employing a toroidal mirror and a variable line space reflection grating has been newly developed. Particular emphasis has been placed on achieving a large collection solid angle (1.9 × 10−3 sr) and a high efficiency of the components in order to enable Thomson Scattering plasma diagnostics which has a small total cross section (6.65 × 10−25 cm2). The instrument achieves a signal-to-noise ratio of 5 with a 13.5 nm source which isotropically emits 2.5 × 105 photons. A resolution λ/Δλ = 330 was measured at 21 nm and the dispersion was calibrated. The instrument is housed inside a DN 100 CF ultra high vacuum manipulator (43 × 46 × 47 cm3) which allows positioning relative to the source within ±5 mm and ±50 mm in X,Y and Z direction, respectively. It can be used with or without entrance pinhole and is equipped with a motorized grating, a filter wheel with five filters, and a shutter. Altogether, these features make the spectrograph a versatile instrument which can be employed in a variety of physics applications such as line and bremsstrahlung spectroscopy or Thomson scattering.

Development of soft X-ray multilayer laminar-type plane gratings and varied-line-spacing spherical grating for flat-field spectrograph in the 1–8 keV region

W/C and Co/SiO 2 multilayer laminar-type holographic plane gratings (groove density 1/σ = 1200 lines/mm) in the 1–8 keV region are developed. For the Co/SiO 2 grating the diffraction efficiencies of 0.41 and 0.47 at 4 and 6 keV, respectively, and for the W/C grating 0.38 at 8 keV are observed. Taking advantage of the outstanding high diffraction efficiencies into practical soft X-ray spectrographs a Mo/SiO 2 multilayer varied-line-spacing (VLS) laminar-type spherical grating (1/σ = 2400 lines/mm) is also developed for use with a flat field spectrograph in the region of 1.7 keV. For the Mo/SiO 2 multilayer grating the diffraction efficiencies of 0.05–0.20 at 0.9–1.8 keV are observed. The FWHMs of the measured line profiles of Hf-Mα 1(1644.6 eV), Si-Kα 1(1740.0 eV), and W-Mα 1 (1775.4 eV) are 13.7 eV, 8.0 eV, and 8.7 eV, respectively.