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In this article, we demonstrate the inference of two-dimensional tungsten concentration profiles in tokamak plasmas, using Gaussian process tomography applied to bolometry and assuming a specific model for the tungsten cooling factor. In ITER, tungsten has been selected for divertor material due to its low tritium retention and ability to handle large heat and particle flux loads. On the other hand, this will cause tungsten impurities to enter the bulk plasma through various plasma-wall interaction processes. Therefore, a detailed understanding of tungsten impurity transport and active control of the impurity transport in tokamaks is crucial. The computational complexity of the method described in their article O n 2 m compares favorably to a simple least-squares approach O n 3 , n represents the number of pixels and m the number of measurement channels, hence bringing real-time tungsten profile monitoring within reach (

Turbulent transport of impurity ions with hollow density profiles (HDPs), which are widely observed in magnetically confined plasmas and desirable for fusion reactor, is self-consistently investigated. A full gyrokinetic description is employed for main and impurity ions. Instead of conventional ion temperature gradient (ITG, including impurity ITG) and trapped electron modes (TEMs), impurity modes (IMs), driven by impurity ion density gradient opposite to that of electrons, are considered. The impurity ion flux induced by IMs is shown to be approximately one order of magnitude higher than that induced by TEMs when both kinds of modes coexist. Main ITG and electron temperature gradient (ETG) are found to reduce influx of impurity ions significantly, resembling temperature screening effect of neoclassical transport of impurity ions. The simulation results such as peaking factor of the HDPs and the effects of main ITG are found in coincidence with the evidence observed in argon injection experiment on HL-2A tokamak. Thus, the IM turbulence is demonstrated to be a plausible mechanism for the transport of impurity ions with HDPs. A strong main ITG, ETG, and a low electron density gradient are expected to be beneficial for sustainment of HDPs of impurity ions and reduction of impurity accumulation in core plasma.

The radial profiles of the FeXVII 3s-2p and 3d-2p transitions emitted in the wavelength range of 15 − 17 A have been observed in the Large Helical Device (LHD). The chord-integrated radial profiles are converted into radial emissivity profiles by means of Abel inversion. The emissivity ratios among FeXVII n = 3−2 transitions calculated from the radial emissivity profiles are compared with calculations based on a collisional-radiative (CR) model developed for Fe ions. The result reasonably confirms the effect of electron temperature and density on the emissivity ratio. However, the emissivity of the 3C (2p 53d 1 P 1 → 2p 6) transition is obviously lower than the prediction from the CR model. This discrepancy is consistent with measurements in the solar corona and other laboratory plasmas.

Space-resolved vacuum ultraviolet (VUV) spectroscopy using a 3-m normal incidence spectrometer is utilized to measure the impurity emission profile in the edge and divertor plasmas of the Large Helical Device (LHD). It measures the vertical profile of VUV lines emitted in the wavelength range of 300–3200 °A. CII, CIII, CIV, and CV lines emitted from carbon ions are successfully measured, and their ion temperatures are derived from the Doppler broadening. Vertical profiles of the emission intensity and the ion temperature are measured simultaneously for the CIV line. The emission intensity profile, which has several peak structures, is reasonably explained by considering the relation between the C3+ ion distribution and the geometry used for the observations.

Impurity accumulation is studied for neutral beam-heated discharges after hydrogen multi-pellet injection in Large Helical Device (LHD). Iron density proflles are derived from radial proflles of EUV line emissions of FeXV-XXIV with the help of the collisional-radiative model. A peaked density proflle of Fe 23+ is simulated by using one-dimensional impurity transport code. The result indicates a large inward velocity of i6 m/s at the impurity accumulation phase. However, the discharge is not entirely afiected by the impurity accumulation, since the concentration of iron impurity, estimated to be 3.3£10 i5 to the electron density, is considerably small. On the other hand, a ∞at proflle is observed for the carbon density of C 6+ , which is derived from the Zefi proflle, indicating a small inward velocity of i1 m/s. These results suggest atomic number dependence in the impurity accumulation of LHD, which is similar to the tokamak result.

A normal incidence vacuum ultraviolet (VUV) and a grazing incidence extreme ultraviolet (EUV) spectrometers have been developed for the edge and core impurity measurements in HL-2A tokamak. The VUV and the EUV spectrometers cover wavelength ranges of 300~3200 A and 50~500 A, respectively. A spatial resolution of 2 mm has been achieved for the VUV spectrometer when a space-resolved slit 50 μm in width is used. The space-resolved slit is placed between the entrance slit and the grating of the spectrometer. The spectral resolutions of 0.15 A for the VUV spectrometer in the wavelength coverage of the concave 1200 grooves/mm grating and of 0.22 A for the EUV spectrometer at λ=200 A with a flat-field laminar-type holographic grating are obtained. The sensitivity of the VUV spectrometer was calibrated in situ with the plasma bremsstrahlung radiation. The experimental results from both spectrometers are presented, especially the line intensity radial profiles measured by the VUV spectrometer.

A cylindrical carbon pellet with a size of 1.2L × 1.2 mm to 1.8L × 1.8 mm and a velocity of 100 m/s to 300 m/s was injected into large helical device (LHD) for an efficient fueling based on its deeper deposition instead of hydrogen gas puffing and ice pellet injection. Electron density increment of Δne = 1014 cm−3 is successfully obtained by single carbon pellet injection without plasma collapse. Typical density and temperature of the ablation plasma of the carbon pellet, e.g., 6.5 × 1016 cm−3 and 2.5 eV for CII, are examined respectively by spectroscopic method. A confinement improvement up to 50% compared to ISS-95 stellarator scaling is clearly observed in a relatively low-density regime of ne = 2×1013 cm−3 to 4×1013 cm−3, and high ion temperature Ti(0) of about 6 keV is also observed with an internal transport barrier at ne = 1.2×1013 cm−3. In particular, the improvement in the ion temperature largely exceeds that observed in hydrogen gas-puffed discharges, which typically ranges below 3 keV.

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.

Magnetic dipole forbidden (M1) transition was studied in large helical device (LHD) and F-, Si- and Ti-like Ml transitions are successfully observed for highly ionized Ar, Kr, Mo and Xe ions. The wavelengths measured in visible range for the heavy elements, which are carefully determined with extremely small uncertainties of 0.02 ~ 0.05 A as a standard wavelength of usual electric dipole (E1) plasma emissions, are compared with theoretical predictions. The result shows a good agreement with recent Hatree-Fock calculation including semi-empirical adjustment. The M1 intensity for the F-like ions is examined by analyzing the intensity ratio of M1 to E1. Density dependence of the ratio is experimentally verified by comparing with collisional- radiative model calculation on level population. The M1/E1 line ratio for the F-like ions is applied to the α (He2+) particle diagnostics in ITER, in which a steady-state operation of burning plasmas based on D-T fusion reaction is expected with α particle heating. Unfortunately, the present estimation suggests a negative result for the α particle measurement because the ratio is largely enhanced by the collisional excitation with bulk ions due to high ion temperature of ITER of 10 keV as assumed and the resultant effect of the collisional excitation with α particles becomes less. Meanwhile, the Ml transition, in particular, Ti-like WLIII (W52+) transition (3627 A) emitted in visible range, is very useful for diagnostics of the impurity behavior and the core plasma parameters in ITER.

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.

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.

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.

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.

A space-resolved extreme ultraviolet (EUV) spectrometer working in wavelength range of 50-500 A 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 A) and HeII (303.78 A) 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.

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.

Numerical transport study predicts that the edge surface layer in ergodic layer of Large Helical Device (LHD) has a favorable capability of impurity screening for materials of not only divertor plates but also vacuum vessel. In order to demonstrate the theoretical prediction, the density of iron originating in the LHD vacuum vessel made of stainless steel, which is not covered by carbon plates like tokamaks, is accurately determined with its radial profile using a space-resolved extreme ultraviolet (EUV) spectrometer, of which absolute intensity calibration is done with bremsstrahlung continuum. For the purpose effective intensity coefficients are precisely calculated for iron ions based on a collisional-radiative model. The iron ion density profiles of Fe14+, Fe15+, Fe22+ and Fe23+ are then evaluated with the radial emissivity profile reconstructed from chord-integrated profile and the effective intensity coefficient. The ratio of iron density to electron density integrated over the whole plasma volume can be finally calculated by fitting the iron density profile using one-dimensional impurity transport code. Thus, the analysis on the ratio gives a typical value of 8×10−7 in experimental campaign at last year. The entirely small value of the iron density demonstrates the theoretical prediction. The radial structure of transport coefficients are also obtained from the impurity transport code, showing a large inward convection velocity.

In fusion research, the stochastic magnetic field with 3-D magnetic field structure formed at the plasma edge has been energetically investigated to mitigate the divertor heat load. The edge stochastic magnetic field layer in the large helical device, which is intrinsically provided by a unique coil system for the plasma confinement, has also been studied for steady-state radiative divertor operation using impurity radiation. Therefore, impurity transport study in the 3-D magnetic field is crucially important. Images of impurity line emission in an extreme ultraviolet range are observed for the edge impurity transport study.

Radial profile measurement of Z(eff) using visible bremsstrahlung (5300 A) 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 A. 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%.

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.

We measured extreme ultraviolet spectra of Fe ions for plasmas produced in the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS). Iron was injected into the plasmas by using a tracer-encapsulated pellet. By controlling the neutral beam injection pattern, we could produce plasma with a central electron temperature of approximately 500 eV, which was suitable for producing Fe XVII ions. We measured seven Fe XVII lines. The intensity ratio for λ of 20.468 to 25.493 nm was consistent with the theoretically calculated value of 1.1. This calculated value was determined purely from the branching ratio due to the common upper level of these transitions, although Warren et al. [Astrphys. J. 685, 1277 (2008)] reported a larger ratio of approxinately 2 from Hinode EIS measurements. The other five ratios for Fe XVII lines in our LHD measurements were also consistent with the theoretical ratios calculated with a collisional-radiative model. A preferred atomic dataset for Fe XVII is suggested to obtain better agreement between the measured and calculated ratios.

Two-dimensional distribution of impurity lines emitted from ergodic layer with stochastic magnetic field lines in Large Helical Device (LHD) has been observed using a space-resolved extreme ultraviolet (EUV) spectrometer. The two-dimensional electron temperature distribution in the ergodic layer is successfully measured using the line intensity ratio of Li-like NeVIII 2s-3p ( 2 S1/2- 2 P3/2: 88.09 A, 2 S1/2- 2 P1/2: 88.13 A) to 2p-3s ( 2 P1/22 S1/2: 102.91 A, 2 P3/2- 2 S1/2: 103.09 A) transitions emitted from radial location near Last Closed Flux Surface (LCFS). The intensity ratio analyzed with ADAS code shows no dependence on the electron density below 10 14 cm −3 . The result indicates a little higher temperature, i.e., 220 eV, in the poloidal location at high-field side near helical coils called O-point compared to the temperature near X-point, i.e., 170 eV. The electron temperature profile is also measured at the edge boundary of ergodic layer using the line intensity ratio of Li-like CIV 2p-3d ( 2 P1/2- 2 D3/2: 384.03 A, 2 P3/2- 2 D5/2: 384.18 A) to 2p-3s ( 2 P1/2- 2 S1/2: 419.53 A, 2 P3/2- 2 S1/2: 419.71 A) transitions. The intensity ratios analyzed with CHIANTI, ADAS and T.Kawachi codes show a slightly higher temperature near O-point, i.e., 25 eV for CHIANTI, 21 eV for ADAS and 11 eV for T.Kawachi’s codes, compared to the temperature at X-point, i.e., 15 - 21 eV for CHIANTI, 9 - 15 eV for ADAS and 6 - 9 eV for T.Kawachi codes. It suggests that the transport coefficient in the ergodic layer is varied with three-dimensional structure.

Space-resolved vacuum ultraviolet (VUV) spectroscopy using a 3 m normal incidence spectrometer has been developed in the large helical device (LHD) to study plasma transport in the ergodic layer by measuring the spatial profile of VUV lines from impurities emitted in the wavelength range of 300-3200 ?. Characteristics of the diagnostics system such as line dispersion, observable region and spatial resolution were evaluated. CIV spectra of 1548.20 × 2 ? were measured clearly. Intensity and ion temperature profiles were obtained simultaneously using CIV emissions in high-density discharges. Dependencies of the CIV intensity profiles on the electron density and magnetic configurations were observed.

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.

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.

An iron density profile is accurately determined in Large Helical Device (LHD) using a space-resolved extreme ultraviolet (EUV) spectrometer, the absolute intensity calibration of which is carried out by bremsstrahlung continuum measurement. The effective intensity coefficients R (eV cm3 s-1) of FeXV to FeXXIV are precisely calculated on the basis of a collisional-radiative model for iron density determination. The total iron density at the plasma center is found to be almost 4 or 5 orders of magnitude smaller than the electron density. The application of the present result to the study of impurity transport demonstrates a new way of examining the radial structure of transport coefficients and of determining the total impurity density.

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.

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...

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.

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.

Visible bremsstrahlung emission profiles have been studied over a wide range of electron densities in H2 gas puff and solid-H2-pellet-fueled discharges of large helical device (LHD). Peaked profiles are observed in high-density discharges (n e≥1014 cm-3) with pellet injection, whereas hollow profiles appeared in the normal discharges (n e≤1014 cm-3) with gas puffing. The total bremsstrahlung radiation is analyzed from the visible bremsstrahlung profile by integrating the energy and plasma volume. It is found that the total bremsstrahlung radiation rapidly increases with the density in the pellet discharges, of which the increment is scaled by the square of density, while it is roughly constant against the density in the gas puff discharges. The total bremsstrahlung radiation becomes equal to the total radiation loss in the pellet discharges. The ratio of the total bremsstrahlung radiation to the total input power has only a range of 3–10% in the gas puff discharges. In contrast, the ratio increases with the density and reaches 30–40% in the pellet discharges. Flat Z eff profiles are observed not only in the gas puff discharges but also in the pellet discharges. This indicates that no impurity accumulation occurs in the high-density operation with H2 pellet injection.

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 variety of density profiles observed in the Large Helical Device (LHD) have suggested an interesting core impurity transport. The edge impurity transport in the ergodic layer formed by stochastic magnetic field lines with long connection length (10-2000 m) can also exhibit interesting phenomena in the competition of perpendicular and parallel transport. The LHD discharge is highly robust against impurity buildup, and operation is possible essentially up to the global power balance limit because current-driven instability does not principally exist. The LHD plasma has therefore provided information on many interesting physics issues closely related to impurities. Recent results of impurity-related physics experiments in the LHD are briefly reviewed. The specific contents presented here are (1) core impurity behavior with perpendicular transport, (2) edge impurity behavior with parallel transport, (3) high-Z discharges with high ion temperature, (4) impurity pellet injection with improved plasma performance, (5) impurity pellet ablation in the presence of energetic ions with high heat flux and (6) observation of magnetic dipole forbidden transitions for high-Z elements. A result from the Compact Helical System (CHS) is used only in the impurity pellet ablation study, because detailed data have not yet been obtained from the LHD. Finally, the results are summarized and future directions in these topics are noted.