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

Up-down asymmetry measurement of tungsten distribution in large helical device using two extreme ultraviolet (EUV) spectrometers

Two space-resolved extreme ultraviolet spectrometers working in wavelength ranges of 10-130 Å and 30-500 Å have been utilized to observe the full vertical profile of tungsten line emissions by simultaneously measuring upper- and lower-half plasmas of LHD, respectively. The radial profile of local emissivity is reconstructed from the measured vertical profile in the overlapped wavelength range of 30-130 Å and the up-down asymmetry is examined against the local emissivity profiles of WXXVIII in the unresolved transition array spectrum. The result shows a nearly symmetric profile, suggesting a good availability in the present diagnostic method for the impurity asymmetry study

Observation of W IV–W VII line emissions in wavelength range of 495–1475 Å in the large helical device

Vacuum ultraviolet spectra of line emissions from tungsten ions at lower ionization stages have been measured in the large helical device (LHD) using a high-resolution 3 m normal incidence spectrometer in the wavelength range of 495–1475 Å. Tungsten was introduced in the LHD plasma by injecting a coaxial tungsten impurity pellet. Many tungsten lines of W IV–W VII were successfully observed in low-temperature plasmas just after the tungsten pellet injection. It is found that some W VI lines are emitted with extremely high intensity and entirely isolated from other intrinsic impurity lines, in particular, W VI at 605.926 Å (5d–6p), 639.683 Å (5d–6p), 677.722 Å (5d–6p), 1168.151 Å (6s–6p) and 1467.959 Å (6s–6p). The result strongly suggests that those lines may be useful for the spectroscopic study in ITER and other magnetic fusion devices with tungsten materials as the plasma facing component. The ion temperature was also measured from Doppler broadening of W V and W VI lines. The result indicates that the measured ion temperature is clearly higher than the ionization energy of such ions. The reason is discussed with regarding to the pellet injection

EUV spectral shape variation of tungsten unresolved transition arrays in electron temperature range of 2–4 keV observed in the Large Helical Device

Spectroscopic studies of emissions released from tungsten ions combined with a pellet injection technique have been conducted in the Large Helical Device. The tungsten Unresolved Transition Array (UTA) spectrum was observed in the wavelength ranges of extreme ultraviolet (EUV) 6–60 Å and 130–340 Å, and the electron temperature dependence of the UTA spectral shape was investigated in the electron temperature region < 4.3 keV. The UTAs of W24+–W33+ at 20–33 Å, W37+–W42+ at 45–47 Å, W27+–W29+ at 48–55 Å, and W7+– W27+ at 170–210 Å were observed. Unidentified UTAs were also found at 230–270 Å and 280–320 Å. As the electron temperature increased further above 4 keV, the W37+–W42+ UTA at 45–47 Å was maintained, while the other UTAs became less intense

Three-dimensional structure of radiative cooling in impurity seeded plasmas in the Large Helical Device

Three-dimensionally localization of radiative cooling due to nitrogen (N2) seeding for divertor detachment was detected experimentally. Since the localization along some magnetic field lines induces toroidal asymmetry of heat load reduction on divertor plates, it should be avoided for fusion reactors. The three-dimensionally localized structure was extracted using Principal Component Analysis (PCA) from two-dimensional radiation images measured with an InfraRed imaging Video Bolometer (IRVB). By applying PCA to 34 images each in N2 seeded plasmas with toroidally-asymmetric heat load reduction and in neon (Ne) seeded plasmas with toroidally-symmetric heat load reduction, a radiation feature in N2 seeded plasmas was found as one of the principal components (PC). The three-dimensional transport code EMC3-EIRENE indicated that the ionization in one of the divertor legs is enhanced in nitrogen seeding compared with Ne seeding due to the difference in the first ionization energy. The magnetic field lines from the divertor leg were along the extracted radiation structure and were terminated by the divertor where the heat load decreased due to the N2 seeding. These results indicate that three-dimensionally localized structure of radiative cooling was detected experimentally

Observation of carbon impurity flow in the edge stochastic magnetic field layer of Large Helical Device and its impact on the edge impurity control

The parallel flow of carbon impurity in a thick stochastic magnetic field layer called the \u27ergodic layer\u27 located at the edge plasma of the Large Helical Device (LHD) is studied by space-resolved vacuum ultraviolet (VUV) spectroscopy, using a 3 m normal incidence spectrometer. A full vertical profile of C3+ impurity flow is evaluated from the Doppler shift of the second order of CIV line emission (2  ×  1548.20 Å) at a horizontally-elongated plasma position of LHD. The carbon flow at the top and bottom edges in the ergodic layer has the same direction toward the outboard side along the major radius direction. The observed flow quantitatively agrees with the simulation results calculated with a 3D simulation code, EMC3-EIRENE. It experimentally verifies the validity of edge parallel flow driving the impurity screening

Confinement improvement during detached phase with RMP application in deuterium plasmas of LHD

In order to explore the compatibility of good core plasma performance with divertor heat load mitigation, the interaction between cold edge plasma and core plasma transport, including the edge transport barrier (ETB), has been analysed in the divertor detachment discharges of deuterium plasmas in LHD with resonant magnetic perturbation (RMP) field application. The RMP application introduces a widened edge stochastic layer and sharp boundary in the magnetic field structure between the confinement region and the edge stochastic layer. The widened edge stochastic layer enhances impurity radiation and provides stable detachment operation as compared with the case without RMP. It is found that ETB is formed at the confinement boundary at the onset of detachment transition. However, as the detachment deepens, the resistive pressure gradient-driven MHD mode is excited, which degrades the ETB. At the same time, however, the core transport decreases to keep global plasma stored energy (Wp) unchanged, showing clear core-edge coupling. After a gradual increase of density fluctuation during the MHD activity, a spontaneous increase of Wp and the recovery of ETB are observed while the detachment is maintained. Then, the coherent MHD mode ceases and ELM-like bursts appear. In the improved mode, impurity decontamination occurs, and the divertor heat load increases slightly. Key controlling physical processes in the interplay between core and cold edge plasma are discussed. A comparison between deuterium and hydrogen plasmas shows that hydrogen plasmas exhibit similar features to the deuterium ones in terms of density and magnetic fluctuations, impurity decontamination towards higher confinement, etc. But most of the features are modest in the hydrogen plasmas and thus no clear confinement mode transition with clear ETB formation is defined. Better global confinement is obtained in the deuterium plasmas than the hydrogen ones at a higher radiation level

Vertical profiles and two-dimensional distributions of carbon line emissions from C2+−C5+ ions in attached and RMP-assisted detached plasmas of large helical device

In Large Helical Device (LHD), the detached plasma is obtained without external impurity gas feed by supplying an m/n = 1/1 resonant magnetic perturbation (RMP) field to a plasma with an outwardly shifted plasma axis position of Rax = 3.90 m where the magnetic resonance exists in the stochastic magnetic field layer outside the last closed flux surface. The plasma detachment is triggered by the appearance of an m/n = 1/1 island when the density, increased using hydrogen gas feed, exceeds a threshold density. The behavior of intrinsically existing impurities, in particular, carbon originating in the graphite divertor plates, is one of the important key issues to clarify the characteristic features of the RMP-assisted plasma detachment although the particle flux still remains on some divertor plates even in the detachment phase of the discharge. For this purpose, vertical profiles and two-dimensional (2-D) distributions of edge carbon emissions of CIII to CVI have been measured at extreme ultraviolet wavelength range, and the results are compared between attached and RMP-assisted detached plasmas. It is found that the CIII and CIV emissions located in the stochastic magnetic field layer are drastically increased near the m/n = 1/1 island O-point and in the vicinity of both inboard and outboard edge separatrix X-points during the RMP-assisted detachment, while those emissions are only enhanced in the vicinity of the outboard edge X-point in attached plasmas without RMP. The result clearly indicates a change in the magnetic field lines connecting to the divertor plates, which is caused by the growth of the m/n = 1/1 edge magnetic island. In contrast, the intensity of CVI emitted radially inside the magnetic island significantly decreases during the detachment, suggesting an enhancement of the edge impurity screening. The measured carbon distribution is analyzed with a three-dimensional edge plasma transport simulation code, EMC3-EIRENE, for the attached plasmas without RMP. It is found that the narrow strip-shaped impurity trace emitted along the edge X-point and its width are sensitive to the cross-field impurity diffusion coefficient, DZ⊥. As a result, the value of DZ⊥ of C3+ ions is evaluated to be 20 times larger than that of the bulk ions in the Rax = 3.90 m configuration, while the reason is unclear at present. The measured 2-D carbon distribution is also discussed and compared to the structure of the m/n = 1/1 magnetic island, which quickly expanded during the appearance of the plasma detachment

Indirect energy transfer channel between fast ions via nuclear elastic scattering observed on the large helical device

An energy transfer phenomenon between energetic ions, which cannot be explained only considering the Coulomb scattering process, was observed on a large helical device (LHD). This phenomenon often occurs in fusion reactivity enhancement and fast-ion slowing-down process that can be observed as a delay in the decay time of the D(d,n)3He neutron generation rate. The transferred energy required to induce such a reactivity enhancement or delay in the fast-ion slowing-down time (neutron decay time) was examined based on the Boltzmann−Fokker−Planck analysis in which a discrete energy transfer process, called nuclear elastic scattering (NES), is included. It was shown that even though the cross section of the NES is smaller than that of the Coulomb scattering, enough knock-on population appears in the energetic region in ion distribution function to induce the observable NES effects; thus, enough energy is transferred from beam ions to fast component of bulk ion distribution function indirectly and the transferred energy per unit time via NES is comparable to the Coulomb scattering rate. This study analytically demonstrates that the observed phenomena on LHD can be explained smoothly by considering the alternative indirect energy transfer channel between energetic ions, which can be comparable with the one via Coulomb scattering