Space-resolved Extreme Ultraviolet Spectrometer Research Articles

Performance improvement of space-resolved extreme ultraviolet spectrometer by use of complementary metal-oxide semiconductor detectors at the Experimental Advanced Superconducting Tokamak.

Two pairs of space-resolved extreme ultraviolet (EUV) spectrometers working at 5-138 Å with different vertical observation ranges of -7 ≤ Z ≤ 19 and -18 ≤ Z ≤ 8cm have been newly developed to observe the radial profile of impurity line emissions and to study the transport of high-Z impurity ions intrinsically existing in EAST tokamak plasmas. Both spectrometers are equipped with a complementary metal-oxide semiconductor (CMOS) detector (Andor Marana-X 4.2B-6, Oxford Instruments) with sensitive area of 13.3 × 13.3mm2 and number of pixels equal to 2048 × 2048 (6.5 × 6.5 µm2/pixels). Compared to the currently operating space-resolved EUV spectrometers with a charge-coupled detector (CCD: 1024 × 255 pixels, 26 × 26 µm2) working at 30-520 Å, this spectrometer's performance was substantially improved by using the CMOS detector. First, the spectral resolution measured at full width at half maximum was improved in the whole wavelength range, e.g., Δλ1/2_CMOS = 0.092 Å and Δλ1/2_CCD = 0.124 Å at C VI 33.73 Å and Δλ1/2_CMOS = 0.104 Å and Δλ1/2_CCD = 0.228 Å at Mo XXXI 115.999 Å, thus enabling a more accurate analysis of spectra with complicated structure such as tungsten unresolved transition array in the range 45-65 Å. Second, the temporal resolution was largely improved due to the high-speed data acquisition system of the CMOS detector, e.g., Δt_CMOS = 15 ms/frame and Δt_CCD = 200 ms/frame at routine operation in the radial profile measurement. Third, signal saturation issues that occurred when using the old CCD sensor during impurity accumulation now disappeared entirely using the CMOS detector due to lower exposure time at high readout rates, which largely improved the observation performance in similar impurity burst events. The above-mentioned performance improvements of the space-resolved EUV spectrometer led to a rapid change in the W XXXIII (52.22 Å) radial profile during a single cycle of low-frequency sawtooth oscillation with fst = 5-6Hz at a sufficient detector count rate.

Line identification of boron and nitrogen emissions in extreme- and vacuum-ultraviolet wavelength ranges in the impurity powder dropping experiments of the Large Helical Device and its application to spectroscopic diagnostics

An impurity powder dropper was installed in the 21st campaign of the Large Helical Device experiment (Oct. 2019–Feb. 2020) under a collaboration between the National Institute for Fusion Science and the Princeton Plasma Physics Laboratory for the purposes of real-time wall conditioning and edge plasma control. In order to assess the effective injection of the impurity powders, spectroscopic diagnostics were applied to observe line emission from the injected impurity. Thus, extreme-ultraviolet (EUV) and vacuum-ultraviolet (VUV) emission spectra were analyzed to summarize observable impurity lines with B and BN powder injection. Emission lines released from B and N ions were identified in the EUV wavelength range of 5–300 Å measured using two grazing incidence flat-field EUV spectrometers and in the VUV wavelength range of 300–2400 Å measured using three normal incidence 20 cm VUV spectrometers. BI–BV and NIII–NVII emission lines were identified in the discharges with the B and BN powder injection, respectively. Useful B and N emission lines which have large intensities and are isolated from other lines were successfully identified as follows: BI (1825.89, 1826.40) Å (blended), BII 1362.46 Å, BIII (677.00, 677.14, 677.16) Å (blended), BIV 60.31 Å, BV 48.59 Å, NIII (989.79, 991.51, 991.58) Å (blended), NIV 765.15 Å, NV (209.27, 209.31) Å (blended), NVI 1896.80 Å, and NVII 24.78 Å. Applications of the line identifications to the advanced spectroscopic diagnostics were demonstrated, such as the vertical profile measurements for the BV and NVII lines using a space-resolved EUV spectrometer and the ion temperature measurement for the BII line using a normal incidence 3 m VUV spectrometer.

Observation of line emissions from Ni-like W46 + ions in wavelength range of 7–8 Å in the Large Helical Device

Tungsten W46+ lines were successfully observed in the extreme ultraviolet (EUV) wavelength range of 7 ∼ 8 Å in the Large Helical Device (LHD). Tungsten ions are distributed in the neutral beam injection (NBI) heated LHD plasma by injecting a pellet consisting of a small piece of tungsten metal wire enclosed by a carbon tube. While the electron temperature has a sudden drop due to the pellet injection, it can be recovered by applying electron cyclotron heating (ECH) together with continuous NBI heating. It is found that a W46+ line at 7.93 Å is emitted when the central electron temperature ranges around 3.4 keV with relatively high intensity and is isolated from other intrinsic impurity lines. The 7.93 Å line consists of two lines of forbidden transitions which are blended with each other; an electric quadrupole (E2) transition at 7.928 Å and a magnetic octupole (M3) transition at 7.938 Å. Observation of W46+ lines in the stellarator experiments is reported for the first time in the present study while the lines have been already observed in several tokamak experiments. The electron temperature dependence of the emission intensity of the 7.93 Å line agreed well with that of the fractional abundance of W46+ ions calculated using the ionization and recombination rate coefficients available in the ADAS database under the assumption of the collisional ionization equilibrium. The 7.93 Å line observed in this study will be used as tools for further spectroscopic researches, such as the measurements of spatial profile of W46+ ions using a space-resolved EUV spectrometer developed in LHD.

Space-resolved extreme ultraviolet spectrometer for impurity diagnostics in HL-2A

Abstract A space-resolved extreme ultraviolet (EUV) spectrometer system has been developed in the HL-2A tokamak for impurity monitoring and transport studies. A charge-coupled-device (CCD) detector is mounted on a two-stage wavelength scanning mechanism for measuring the spatial profile of impurity line emissions in the wavelength range of 30−600 A. A good spatial profile of impurity line emissions is observed throughout the lower half of the HL-2A plasma along the vertical direction by installing a space-resolved slit in front of an entrance slit. The vertical observation range is ∼43 cm covering a wide plasma area from the plasma central region to the vicinity of divertor separatrix X-point. In order to make a critical analysis of the impurity transport, in particular, in the plasma edge, an accurate position calibration in the observed vertical impurity line intensity profile is necessary. For the purpose a thin metal bar is installed at the diagnositc port. Then, the vertical position of the impurity profile can be accurately determined throughout the entire wavelength range. Based on the present method, the local emissivity profile of impurity line emissions is well reconstructed with good accuracy through Abel inversion technique. Resultant CIV and CVI local emissivity profiles show a clear difference in the carbon transport between ohmic and NBI heating phases.

Local profiles of line emission of impurity ions in rotating fusion plasmas

We analyze the poloidal distribution of impurity ions, such as C V, C VI and O VIII, in fast rotating KSTAR plasmas using the impurity emission lines in the 1–7 nm wavelength range obtained from the space-resolved extreme ultra violet spectrometer. This is aided particularly by a new tomographic reconstruction technique with limited viewing angles. The vacuum and extreme ultraviolet spectrometer system at ITER and its prototypes installed at KSTAR are one of the main diagnostic systems to monitor the impurity species in fusion plasmas; however, they have an insufficient field of view to utilize the conventional reconstruction method. The new algorithm developed in this work considers a curvilinear grid of pixels mapped onto the magnetic equilibrium and a weight factor considering toroidal rotational effect on impurity species. Validation tests with poloidally asymmetric images of synthetic plasma demonstrate that this method is highly useful for emission diagnostics that have a narrow field of view. The experimental results of the spatially resolved spectra from KSTAR plasma clearly show that even the light impurities such as carbon and oxygen are significantly affected by the toroidal rotation. Accordingly, the method is expected to be utilized for estimating the poloidal density profile of each charge state of an impurity species, which will play a key role in impurity transport studies.

A space-resolved extreme ultraviolet spectrometer for radial profile measurement of tungsten ions in the Experimental Advanced Superconducting Tokamak

In order to study the radial transport of tungsten ions in long-pulse H-mode discharges, a space-resolved spectrometer working at 30–520 Å has been newly developed to measure a radial profile of the tungsten line emission. The spectrometer is installed behind a long extension vacuum tube connected to a horizontal midplane diagnostic port of EAST tokamak. The long distance between the plasma and spectrometer, 8835.5 mm, enables observation of the radial profile of impurity line emissions in a wide vertical range of −8.5≤Z≤ 40 cm (−0.1≤ρ≤0.6). A good spectral resolution of Δλ0=4−5 pixels at the foot position of spectral line profiles and a high spatial resolution of ΔZ=2.5 cm are obtained in addition to a sufficient temporal resolution, e.g. 50 ms/frame. As a result, accurate radial profiles have been successfully obtained in EAST Ohmic and H-mode discharges for several impurity species such as carbon, oxygen, argon, iron and tungsten. The radial profiles of tungsten line emissions from W42+ – W45+ ions with 4p–4s transitions measured at two wavelength ranges of 45–70 Å and 120–140 Å are analyzed for the ion density evaluation based on the photon emissivity coefficient from ADAS database. The result shows that the density of W43+ – W45+ ions ranges at 2–6 × 108 cm−3 in steady H-mode discharges with Te(0)=3 keV and ne(0)=4 × 1013 cm−3.

Density evaluation of tungsten W24+, W25+, and W26+ ions using unresolved transition array at 27–34 Å in Large Helical Device

The extreme ultraviolet (EUV) spectra of a tungsten unresolved transition array (UTA) at 15–70 Å have been studied in Large Helical Device (LHD) by injecting a tungsten pellet. Vertical profiles of the UTA line are measured with a space-resolved EUV spectrometer. In our previous study, it has been found that the UTA line at wavelength intervals of 32.16–33.32, 30.69–31.71, and 29.47–30.47 Å is composed of a single ionization stage of W24+, W25+, and W26+, respectively. In this report, therefore, the densities of W24+, W25+, and W26+ ions are evaluated from the radial profile measured at the above-mentioned wavelength intervals. To evaluate ion density, the photon emission coefficients of W24+, W25+, and W26+ ions are calculated using a collisional-radiative (CR) model. The chord-integrated radial profile of UTA lines is converted to a local emissivity profile using the Abel inversion technique. The density profiles of W24+, W25+, and W26+ ions are thus obtained from the local emissivity profile and the photon emission coefficient in addition to the temperature and density profiles. The obtained density profile of the W24+ ion is analyzed in detail by investigating the dependences of the electron density and the number of tungsten particles injected by the tungsten pellet. The total tungsten ion density nW near ρ = 0.7 where the W24+ ion locates is also estimated from the W24+ ion density with fractional abundance in ionization equilibrium calculated with the Atomic Data and Analysis Structure (ADAS) code. The nW evaluated from the present CR model seems to be larger than that estimated from the number of tungsten particles injected by the pellet. Discussions are made with the nW evaluated from the photon emission coefficient in the CL version of the ADAS code.

Performance improvement of two-dimensional EUV spectroscopy based on high-frame-rate CCD and signal normalization method in Large Helical Device

In the Large Helical Device (LHD), two-dimensional (2D) distribution of edge impurity line emissions from the ergodic layer with a three-dimensional (3D) magnetic field structure has been observed in the wavelength range of 30–650 Å by horizontally scanning a space-resolved extreme ultraviolet (EUV) spectrometer during a steady phase in electron cyclotron heating (ECH) and ion cyclotron range of heating (ICRF) discharges with long pulse duration (τd ≥ 10 s). The 2D distribution of impurity line emissions can be measured at the upper or lower half in the full vertical range of LHD plasmas by a single horizontal scan. The performance of 2D EUV spectroscopy, however, was not sufficient for discussing the impurity transport in the ergodic layer from the viewpoints of the quality of 2D images and the requirement of long pulse discharges. The 2D EUV spectrometer is therefore modified by installing a high-frame-rate charge-coupled detector (CCD) and increasing its horizontal scanning speed. As a result, the quality of 2D images is significantly improved with increased horizontal spatial resolution, and then the 2D measurement is also possible for neutral beam injection (NBI) discharges with short pulse duration (τd ∼ 3 s). In addition, temporal and shot-by-shot intensity variations in the edge EUV emission are significantly reduced by applying a signal normalization method using another EUV spectrometer with high time resolution. A 0.5 µm poly(ethylene terephthalate) (PET) filter is also installed in front of an entrance slit of the space-resolved EUV spectrometer. The spike noise originating from high-energy neutral particles, which are enhanced in low-density NBI discharges, has been successfully reduced. Typical examples of the 2D distribution of impurity line emissions with improved quality are presented for several impurity species.

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.

Two-dimensional measurement of edge impurity emissions using space-resolved extreme ultraviolet spectrometer in Large Helical Device

A space-resolved extreme ultraviolet (EUV) spectrometer working in wavelength range of 50-500 Å has been developed to measure two-dimensional distribution of impurity spectral lines emitted from edge plasma of Large Helical Device (LHD), in which the magnetic field is formed by stochastic magnetic field with three-dimensional structure called ergodic layer. The two-dimensional measurement of edge impurity line emissions is carried out by scanning horizontally the observation chord of the space-resolved EUV spectrometer during single LHD discharge. Images of CIV (312.4 Å) and HeII (303.78 Å) are presented as the first result. The results are compared with ones calculated from the edge chord length in the ergodic layer of LHD plasma.

Extension of Wavelength Range in Absolute Intensity Calibration of Space-Resolved EUV Spectrometer for LHD Diagnostics

A space-resolved extreme ultraviolet (EUV) spectrometer has been upgraded by extending the wavelength range to 30-650 A to explore impurity line emissions existing at shorter and longer wavelength sides. The absolute intensity calibration is implemented for measurement in the extended wavelength based on bremsstrahlung profiles simultaneously measured in EUV and visible ranges. For the purpose a wider entrance slit of 200µm and a wider space-resolved slit of 1.0 mm are adopted to increase the number of photons to the spectrometer. As a result, the bremsstrahlung intensity can be enhanced by order of magnitude. A centrally peaked high-density discharge at ne(0) ≥ 10 14 cm −3 is also used for the accurate calibration. Thus, the calibration becomes possible, even in longer wavelength side at λ ≥ 400 A. The result at shorter wavelength range of 30-90 A shows a flat calibration factor, suggesting sudden changes of holographic grating efficiency and CCD detection efficiency, while the result at longer wavelength side of λ ≥ 400 A shows a simple extension of the previous calibration factor.

Absolute intensity calibration of flat-field space-resolved extreme ultraviolet spectrometer using radial profiles of visible and extreme ultraviolet bremsstrahlung continuum emitted from high-density plasmas in Large Helical Device

A precise absolute intensity calibration of a flat-field space-resolved extreme ultraviolet (EUV) spectrometer working in wavelength range of 60-400 Å is carried out using a new calibration technique based on radial profile measurement of the bremsstrahlung continuum in Large Helical Device. A peaked vertical profile of the EUV bremsstrahlung continuum has been successfully observed in high-density plasmas (n(e) ≥ 10(14) cm(-3)) with hydrogen ice pellet injection. The absolute calibration can be done by comparing the EUV bremsstrahlung profile with the visible bremsstrahlung profile of which the absolute value has been already calibrated using a standard lamp. The line-integrated profile of measured visible bremsstrahlung continuum is firstly converted into the local emissivity profile by considering a magnetic surface distortion due to the plasma pressure, and the local emissivity profile of EUV bremsstrahlung is secondly calculated by taking into account the electron temperature profile and free-free gaunt factor. The line-integrated profile of the EUV bremsstrahlung continuum is finally calculated from the local emissivity profile in order to compare with measured EUV bremsstrahlung profile. The absolute intensity calibration can be done by comparing measured and calculated EUV bremsstrahlung profiles. The calibration factor is thus obtained as a function of wavelength with excellent accuracy. It is also found in the profile analysis that the grating reflectivity of EUV emissions is constant along the direction perpendicular to the wavelength dispersion. Uncertainties on the calibration factor determined with the present method are discussed including charge-coupled device operation modes.

Study on Radial Position of Impurity Ions in Core and Edge Plasma of LHD Using Space-Resolved EUV Spectrometer

Radial profiles of impurity ions of carbon, neon and iron were measured for high-temperature plasmas in large helical device (LHD) using a space-resolved extreme ultraviolet (EUV) spectrometer in the wavelength range of 60 Å to 400 Å. The radial positions of the impurity ions obtained are compared with the local ionization energies, Ei of these impurity ions and the electron temperatures TeZ there. The impurity ions with 0.3 keV ≤ Ei ≤ 1.0 keV are always located in outer region of plasma, i.e., 0.7 ≤ ρ ≤ 1.0, and those with Ei ≤ 0.3 keV are located in the ergodic layer, i.e., 1.0 ≤ ρ ≤ 1.1, with a sharp peak edge, where ρ is the normalized radial position. It is newly found that TeZ is approximately equal to Ei for the impurity ions with Ei ≤ 0.3 keV, whereas roughly half the value of Ei for the impurity ions with 0.3 keV ≤ Ei ≤ 1.0 keV. It is known that TeZ is considerably lower than Ei in the plasma edge and approaches to Ei in the plasma core. Therefore, this result seems to originate from the difference in the transverse transport between the plasma edge at ρ ≤ 1.0 and the ergodic layer at ρ ≥ 1.0. The transverse transport is studied with an impurity transport simulation code. The result revealed that the difference appearing in the impurity radial positions can be qualitatively explained by the different values of diffusion coefficient, e.g., D= 0.2 m2/s and 1.0 m2/s, which can be taken as a typical index of the transverse transport.

Fe14+ and Fe15+ Spatial Distributions in a LHD Plasma Reconstructed from Multiple-Viewing-Chord EUV-Emission Observation

We have observed Fe14+ (3s2 1S0 - 3s3p 1P1) and Fe15+ (3s 2S1/2 - 3p 2P3/2) emissions from a LHD plasma with a space-resolved extreme-ultraviolet spectrometer. The observed intensity distributions against the viewing chord for the respective emissions are reconstructed to the emission flux distributions in the plasma against the normalized radius of the poloidal cross section with a maximum entropy method. Both of the emissions localize in the periphery region, and the Fe14+ emission is located outer side than that of Fe15+. We calculate the charge state distribution of Fe ions against the normalized radius assuming the ionization equilibrium at the electron temperature and density, which are measured by a Thomson scattering method. The calculated result is consistent with the experimental one.

Survey of EUV Impurity Line Spectra and EUV Bremsstrahlung Continuum in LHD

Radial profiles of impurity spectral line emissions in extreme ultraviolet (EUV) range (60-400 A) are surveyed for impurity seeded discharges in Large Helical Device (LHD). The line emissions from light impurities such as helium, carbon and neon are located in the edge ergodic layer (1.0 ≤ ρ ≤ 1.2) except for CV and CVI, which are located in region of 0.8 ≤ ρ ≤ 1. In contract, the line emissions from heavy impurities such as iron are widely located over the whole plasma region as a function of ionization energy. On the other hand, the radial profile of bremsstrahlung continuum radiation is also clearly recorded in the EUV spectrum observed from high-density discharges with hydrogen pellet injection. Wavelength intervals available for the absolute sensitivity calibration of the present space-resolved EUV spectrometer system are listed with dominant line emissions which are useful as a wavelength marker.

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.