ABSTRACT
A high-speed vacuum ultraviolet monochromator is developed for the HL-2A tokamak through the introduction of a novel channel electron multiplier in a modified Seya-Namioka spectrometer. A good signal to noise ratio of above 2000 is attained in the development phase of the system with typical operating parameters for observing routine HL-2A plasmas. The wavelength calibration is performed using characteristic line emissions from a hollow cathode light source with helium and argon discharges. The first measurement result of the monochromator at a sample rate of 60 kHz is presented in comparison with the visible Dα signals.
ABSTRACT
A flat-field extreme ultraviolet (EUV) spectrometer working in the 20-500 Å wavelength range with fast time response has been newly developed to measure line emissions from highly ionized tungsten in the Experimental Advanced Superconducting Tokamak (EAST) with a tungsten divertor, while the monitoring of light and medium impurities is also an aim in the present development. A flat-field focal plane for spectral image detection is made by a laminar-type varied-line-spacing concave holographic grating with an angle of incidence of 87°. A back-illuminated charge-coupled device (CCD) with a total size of 26.6 × 6.6 mm(2) and pixel numbers of 1024 × 255 (26 × 26 µm(2)/pixel) is used for recording the focal image of spectral lines. An excellent spectral resolution of Δλ0 = 3-4 pixels, where Δλ0 is defined as full width at the foot position of a spectral line, is obtained at the 80-400 Å wavelength range after careful adjustment of the grating and CCD positions. The high signal readout rate of the CCD can improve the temporal resolution of time-resolved spectra when the CCD is operated in the full vertical binning mode. It is usually operated at 5 ms per frame. If the vertical size of the CCD is reduced with a narrow slit, the time response becomes faster. The high-time response in the spectral measurement therefore makes possible a variety of spectroscopic studies, e.g., impurity behavior in long pulse discharges with edge-localized mode bursts. An absolute intensity calibration of the EUV spectrometer is also carried out with a technique using the EUV bremsstrahlung continuum at 20-150 Å for quantitative data analysis. Thus, the high-time resolution tungsten spectra have been successfully observed with good spectral resolution using the present EUV spectrometer system. Typical tungsten spectra in the EUV wavelength range observed from EAST discharges are presented with absolute intensity and spectral identification.
ABSTRACT
A space-resolved vacuum ultraviolet (VUV) spectroscopy using a 3 m normal incidence spectrometer has been developed to measure the impurity profile in the edge ergodic layer composed of stochastic magnetic field by which the edge plasma in the large helical device (LHD) is uniquely characterized. It vertically measures the spatial profile of VUV lines emitted from impurities in the wavelength range of 300-3200 Å. The wavelength interval, Δλ, which can be measured in a single discharge, is about 37 Å. A spectral resolution of 0.153 Å, which results from an entrance slit width of the spectrometer of 20 µm, is adopted. The vertical observation range, ΔZ, can be switched by taking a convex mirror in and out, which enables both the edge profile measurement focused on the ergodic layer and the full profile measurement covering an entire vertical size of the LHD plasma, e.g., 165 ≤ ΔZ ≤ 200 mm and 1000 ≤ ΔZ ≤ 1250 mm for the R(ax)=3.6 m configuration, respectively, which shows a slight wavelength dependence. Precise calibrations on the line dispersion, spectral resolution, vertical range of the observable region, and the spatial resolution have been performed with a unique method. As a preliminary result, the ion temperature profile is obtained for CIV at 1548.20 Å in the second order (denoted as 1548.20 × 2 Å) in high-density helium discharges in addition to the emission profile with a time resolution of 100 ms in a multitrack CCD operation mode. The poloidal flow in the ergodic layer based on the Doppler-shift measurement of CIV at 1548.20 × 2 Å is also observed in high-density hydrogen discharges.
ABSTRACT
A space-resolved vacuum ultraviolet (VUV) spectroscopy using a 3 m normal incidence spectrometer has been developed to measure the impurity profile in the edge ergodic layer composed of stochastic magnetic field by which the edge plasma in the large helical device (LHD) is uniquely characterized. It vertically measures the spatial profile of VUV lines emitted from impurities in the wavelength range of 300-3200 Å. The wavelength interval, Δλ, which can be measured in a single discharge, is about 37 Å. A spectral resolution of 0.153 Å, which results from an entrance slit width of the spectrometer of 20 µm, is adopted. The vertical observation range, ΔZ, can be switched by taking a convex mirror in and out, which enables both the edge profile measurement focused on the ergodic layer and the full profile measurement covering an entire vertical size of the LHD plasma, e.g., 165 ≤ ΔZ ≤ 200 mm and 1000 ≤ ΔZ ≤ 1250 mm for the R(ax)=3.6 m configuration, respectively, which shows a slight wavelength dependence. Precise calibrations on the line dispersion, spectral resolution, vertical range of the observable region, and the spatial resolution have been performed with a unique method. As a preliminary result, the ion temperature profile is obtained for CIV at 1548.20 Å in the second order (denoted as 1548.20 × 2 Å) in high-density helium discharges in addition to the emission profile with a time resolution of 100 ms in a multitrack CCD operation mode. The poloidal flow in the ergodic layer based on the Doppler-shift measurement of CIV at 1548.20 × 2 Å is also observed in high-density hydrogen discharges.
ABSTRACT
A space-resolved grazing-incidence flat-field extreme ultraviolet (EUV) spectrometer has been developed in the HL-2A tokamak to measure vertical impurity emission profiles with simultaneous spectral, temporal, and spatial resolution. The spectrometer working in the wavelength range of 30-500 Å has been equipped with a gold-coated varied-line-spacing holographic grating with curvature of 5606 mm and a back illuminated charge-coupled device with size of 6.6 × 26.6 mm(2) (255 × 1024 pixels). A lower half of the HL-2A plasma with averaged minor radius of 40 cm is observed when the spectrometer with horizontal dispersion is placed at a distance of 7.5 m away from the plasma center. An excellent spatial resolution of 12 mm is achieved when a space-resolved slit with vertical width of 0.5 mm is adopted. The radial profiles of intrinsic impurities in several ionization stages have been measured with high throughput and extremely low stray light.
ABSTRACT
A flat-field space-resolved extreme ultraviolet (EUV) spectrometer system working in wavelength range of 10-130 Å has been constructed in the Large Helical Device (LHD) for profile measurements of bremsstrahlung continuum and line emissions of heavy impurities in the central column of plasmas, which are aimed at studies on Zeff and impurity transport, respectively. Until now, a large amount of spike noise caused by neutral particles with high energies (≤180 keV) originating in neutral beam injection has been observed in EUV spectroscopy on LHD. The new system has been developed with an aim to delete such a spike noise from the signal by installing a thin filter which can block the high-energy neutral particles entering the EUV spectrometer. Three filters of 11 µm thick beryllium (Be), 3.3 µm thick polypropylene (PP), and 0.5 µm thick polyethylene terephthalate (PET: polyester) have been examined to eliminate the spike noise. Although the 11 µm Be and 3.3 µm PP filters can fully delete the spike noise in wavelength range of λ ≤ 20 Å, the signal intensity is also reduced. The 0.5 µm PET filter, on the other hand, can maintain sufficient signal intensity for the measurement and the spike noise remained in the signal is acceptable. As a result, the bremsstrahlung profile is successfully measured without noise at 20 Å even in low-density discharges, e.g., 2.9 × 10(13) cm(-3), when the 0.5 µm PET filter is used. The iron n = 3-2 Lα transition array consisting of FeXVII to FeXXIV is also excellently observed with their radial profiles in wavelength range of 10-18 Å. Each transition in the Lα array can be accurately identified with its radial profile. As a typical example of the method a spectral line at 17.62 Å is identified as FeXVIII transition. Results on absolute intensity calibration of the spectrometer system, pulse height and noise count analyses of the spike noise between holographic and ruled gratings and wavelength response of the used filters are also presented with performance of the present spectrometer system.
ABSTRACT
A grazing-incidence flat-field extreme ultraviolet (EUV) spectrometer has been newly developed in HL-2A tokamak. Typical spectral lines are observed from intrinsic impurities of carbon, oxygen, iron, and extrinsic impurity of helium in the wavelength range of 20 Å-500 Å. Bremsstrahlung continuum is measured at different electron densities of HL-2A discharges to calibrate absolute sensitivity of the EUV spectrometer system and to measure effective ionic charge, Z(eff). The sensitivity of a vacuum ultraviolet (VUV) spectrometer system is also absolutely calibrated in overlapped wavelength range of 300 Å-500 Å by comparing the intensity between VUV and EUV line emissions.
ABSTRACT
Spectra measured by an extreme ultraviolet (EUV) spectrometer frequently suffer large spike noise when Large Helical Device is operated in low-density range (≤ 3 × 10(13) cm(-3)) with neutral beam injection (NBI). The spike noise completely disappears in electron cyclotron heating discharges. In order to examine the effect of NBI, a carbon filter with thickness of 150 nm was installed in the EUV spectrometer. As a result, the spike noise was reduced by an order of magnitude. It is experimentally verified that the spike noise is caused by escaping high-energy neutral particles resulting from the circulating high-energy hydrogen ions borne from NBI.
ABSTRACT
Vertical profile of neon line emissions in 30-650 Å wavelength range has been observed in horizontally elongated plasma cross section of Large Helical Device (LHD). Intensity ratio between the neon line emissions is studied to measure the radial profile of electron temperature in the edge stochastic magnetic field layer of LHD. The edge temperature profile successfully obtained from the line ratio of NeVIII 2s-3p to 2p-3s transitions is compared with the simulation based on three-dimensional edge transport code. The result shows a reasonably good agreement with the edge temperature profile analyzed from atomic data and analysis structure code. The electron temperature at last closed flux surface measured from the intensity ratio is also in good agreement with that measured from Thomson scattering.
ABSTRACT
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.
ABSTRACT
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.
ABSTRACT
Radial profile measurement of Z(eff) using visible bremsstrahlung (5300 Å) in the Large Helical Device (LHD) has often encountered difficulties because the intensity profile was largely deformed by the nonuniform visible bremsstrahlung emissions from the edge ergodic layer surrounding the core plasma. A space-resolved flat-field extreme ultraviolet (EUV) spectrometer has been newly adopted to measure the Z(eff) profile using the EUV bremsstrahlung continuum in the wavelength range of 70-75 Å. The EUV bremsstrahlung intensity profiles have been measured and checked for all the magnetic configurations with totally different magnetic field structures in the ergodic layer of LHD. It is found that the nonuniform bremsstrahlung emission from the thick ergodic layer can be entirely eliminated by use of the EUV emission with relatively high photon energy of 170 eV. As a result, the Z(eff) profile can be successfully measured for most of discharges regardless of magnetic field structures of the ergodic layer. The Z(eff) profiles measured in the EUV range are compared with those measured in the visible range at a magnetic configuration with the thinnest ergodic layer thickness. The result verifies that the use of the EUV bremsstrahlung continuum is an alternative way for the Z(eff) measurement in toroidal plasmas with nonuniform bremsstrahlung emissions at the edge. Typical results from the EUV bremsstrahlung measurement are presented showing a fairly flat Z(eff) profile with error bars of ±14%.
ABSTRACT
A 1 m normal incidence vacuum ultraviolet (VUV) spectrometer has been developed for spatial distribution measurement of edge impurity line emission in the wavelength range of 300-3200 A on HL-2A tokamak. A vertical profile of the impurity line emission is measured with a space-resolved slit placed between an entrance slit and a grating of the spectrometer. Two concave 1200 grooves/mm gratings blazed at 800 and 1500 A are set on a rotatable holder in the spectrometer, which gives wavelength dispersion of 0.12 mm/A. A back-illuminated charge-coupled device is used as a detector with an image size of 6.7 x 26.6 mm(2) (26 x 26 microm(2)/pixel). An excellent spatial resolution of 2 mm is obtained with good spectral resolution of 0.15 A when the space-resolved slit of 50 microm in width is used. The space-resolved spectra thus provide three radial profiles of emission line intensity, ion temperature, and poloidal rotation. The electron temperature can be measured by the intensity ratio, e.g., CIII 2s(2)-2s3p (386 A)/2s(2)-2s2p (977 A). The sensitivity of the spectrometer is calibrated in situ by using the VUV bremsstrahlung continuum radiation emitted from the tokamak plasma. A good performance of the spectrometer system for the edge impurity and temperature profile measurements is presented with results on Ohmic and H-mode discharges.
ABSTRACT
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.