Motoshi Goto

Relations between the ionization or recombination flux and the emission radiation for hydrogen and helium in plasma

On the basis of the collisional-radiative models for neutral hydrogen, and neutral and ionized helium, the relationship between the ionization flux or the recombination flux and the photon emission rate of a representative visible line of each species is investigated. It is found that both fluxes are proportional to the photon emission rate and that the proportionality factor depends rather weakly on the plasma parameters in the ranges of practical interest. This implies that the observed emission line intensity can be a good measure of the ionization flux or the recombination flux. The relation between the total radiation power rate and the ionization or recombination flux is also considered. For a hydrogen plasma in ionization balance the Balmer-α line intensity takes the maximum value near the optimum temperature of 1.3 eV, while for plasmas out of ionization balance it takes the minimum near that temperature. This latter characteristic corresponds to the recently observed “inverse edge-localized mode”...

Experimental study of impurity screening in the edge ergodic layer of the Large Helical Device using carbon emissions of CIII to CVI

Four resonance transitions of CIII (977.03 A:2s2 S10-2s2p P11), CIV (1550 A:2s S2-2p P2), CV (40.27 A:1s2 S10-1s2p P11), and CVI (33.73 A:1s S2-2p P2) have been observed in vacuum ultraviolet and extreme ultraviolet regions to study the edge carbon impurity transport in the Large Helical Device ergodic layer. Here, CIII and CIV indicate the carbon influx at the outside boundary of the ergodic layer and CV and CVI indicate the ions in higher ionization stages, which have already experienced the transport in the ergodic layer. The intensity ratio of CV+CVI to CIII+CIV, therefore, represents the degree of impurity screening, which has been analyzed with different edge plasma parameters and ergodic magnetic field structures. The ratio decreases by two orders of magnitude with an increase in electron density ne in the range of 1–8×1019 m−3. The CV and CVI emissions tend to decrease with ne, whereas the CIII and CIV emissions monotonically increase with ne. The result suggests an enhancement of the impurity scr...

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.

Radial profile measurement of impurity line emissions using space-resolved 3m vacuum ultraviolet spectrometer in LHD

A space-resolved 3m vacuum ultraviolet (vuv) spectrometer has been developed to measure a radial (vertical) profile of impurity line emissions in the Large Helical Device (LHD). The system consists of a 3m normal incidence spectrometer equipped with a space-resolved slit and a back-illuminated charge-coupled device (CCD) detector, view-angle adjustable mirrors, and a toroidal slit. A full vertical profile of the vuv line radiated from the LHD plasma is observed with a convex cylindrical mirror by extending the view angle. The vertical profiles of intrinsic impurities of carbon (C III–C V) and externally injected impurities of neon (Ne VIII), aluminum (Al XI), and argon (Ar VIII) are successfully obtained with a time interval of 125ms in the VUV range of 500–3100A. The ion temperature profiles are also measured from the C IV line emitted in the ergodic layer of LHD.

A study on plasma edge boundary in ergodic layer of LHD based on radial profile measurement of impurity line emissions

Vertical profiles of edge impurity emissions have been measured in upper half region of elliptical plasmas at horizontally elongated plasma cross section in large helical device (LHD). The vertical profiles near upper O-point located just below helical coil are analyzed to study the plasma edge boundary of the ergodic layer consisting of stochastic magnetic field lines with connection lengths of 30 ≤ Lc ≤ 2000 m. As a result, C3+ ion emitting CIV spectrum is identified as the ion existing in the farthest edge of the ergodic layer. The peak position of CIV (312.4 A: 1s23p 2P1/2,3/2-1s22s 2S1/2) vertical profile does not change at all in a wide temperature range of 150 ≤ Te(ρ = 1) ≤ 400 eV, whereas it moves inside the ergodic layer when Te(ρ = 1) is reduced below a threshold temperature, e.g., 130 eV at Rax = 3.75 m configuration. It is found that the C3+ ion exists at the boundary between ergodic layer and open magnetic filed layer at which the Lc distributes in lengths of 5 to 30 m. The result indicates t...

Zeff profile diagnostics using visible bremsstrahlung continuum for nonaxisymmetric plasmas with finite β in large helical device

An astigmatism-corrected Czerny–Turner-type visible spectrometer coupled with a charge-coupled device has been installed in large helical device (LHD) to measure visible bremsstrahlung continuum. A full vertical profile has been observed from the elliptical plasmas at horizontally elongated plasma cross section through a 44 fiber parallel array with vertical observation length of ∼1 m. Line emissions can be easily eliminated by use of the visible spectrometer instead of an interference filter. A nonmonotonic bremsstrahlung emission profile, which is originated in the thick ergodic layer surrounding the main plasma, has been observed for normal discharges in all the magnetic configurations of LHD. After analyzing the detailed structure, the lower half of the vertical bremsstrahlung emission profile is found to be free of the strong edge bremsstrahlung emission in inwardly shifted magnetic configurations (Rax≤3.60 m). The nonmonotonic bremsstrahlung emission disappeared in extremely high-density discharges ...

A fast-time-response extreme ultraviolet spectrometer for measurement of impurity line emissions in the Experimental Advanced Superconducting Tokamak

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.

Development of quantitative atomic modeling for tungsten transport study using LHD plasma with tungsten pellet injection

Quantitative tungsten study with reliable atomic modeling is important for successful achievement of ITER and fusion reactors. We have developed tungsten atomic modeling for understanding the tungsten behavior in fusion plasmas. The modeling is applied to the analysis of tungsten spectra observed from plasmas of the large helical device (LHD) with tungsten pellet injection. We found that extreme ultraviolet (EUV) emission of W24+ to W33+ ions at 1.5–3.5 nm are sensitive to electron temperature and useful to examine the tungsten behavior in edge plasmas. We can reproduce measured EUV spectra at 1.5–3.5 nm by calculated spectra with the tungsten atomic model and obtain charge state distributions of tungsten ions in LHD plasmas at different temperatures around 1 keV. Our model is applied to calculate the unresolved transition array (UTA) seen at 4.5–7 nm tungsten spectra. We analyze the effect of configuration interaction on population kinetics related to the UTA structure in detail and find the importance of two-electron-one-photon transitions between 4p54dn+1– 4p64dn−14f. Radiation power rate of tungsten due to line emissions is also estimated with the model and is consistent with other models within factor 2.

Hydrogen transport diagnostics by atomic and molecular emission line profiles simultaneously measured for large helical device

We observe the Balmer-α, -β, and -γ lines of hydrogen atoms and Q branches of the Fulcher-α band of hydrogen molecules simultaneously with their polarization resolved for large helical device. From the fit including the line splits and the polarization dependences by the Zeeman effect, the emission locations, intensities, and the temperatures of the atoms and molecules are determined. The emission locations of the hydrogen atoms are determined outside but close to the last closed flux surface (LCFS). The results are consistent with a previous work (Phys. Plasmas 12, 042501 (2005)). On the other hand, the emission locations of the molecules are determined to be in the divertor legs, which is farer from those of the atoms. The kinetic energy of the atoms is 1 ∼ 20 eV, while the rotational temperature of molecules is ∼0.04 eV. Additionally, substantial wings, which originate from high velocity atoms and are not reproduced by the conventional spectral analysis, are observed in the Balmer line profiles. We dev...

Zeff measurement using extreme ultraviolet bremsstrahlung emission in LHDa)

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%.