Showing papers by "Hiroyuki A. Sakaue published in 2015"

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

Abstract: 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.

Extreme ultraviolet and visible spectroscopy of promethiumlike heavy ions

Abstract: We present extreme ultraviolet and visible spectra of promethiumlike tungsten and gold obtained with an electron beam ion trap. Although the contributions from a few charge states are involved in the spectra, the charge state of the ion assigned to the observed lines is definitely identified by the time-of-flight (TOF) analysis of the ions performed at the same time with the spectroscopic measurements. Experimental results are compared with collisional-radiative model calculations as well as previous experimental and theoretical studies. Some ambiguous identifications in the previous observations have been clarified owing to the definite identification by the TOF analysis.

Overview of transport and MHD stability study: Focusing on the impact of magnetic field topology in the Large Helical Device

Abstract: The progress in the understanding of the physics and the concurrent parameter extension in the large helical device since the last IAEA-FEC, in 2012 (Kaneko O et al 2013 Nucl. Fusion 53 095024), is reviewed. Plasma with high ion and electron temperatures (Ti(0) ~ Te(0) ~ 6 keV) with simultaneous ion and electron internal transport barriers is obtained by controlling recycling and heating deposition. A sign flip of the nondiffusive term of impurity/momentum transport (residual stress and convection flow) is observed, which is associated with the formation of a transport barrier. The impact of the topology of three-dimensional magnetic fields (stochastic magnetic fields and magnetic islands) on heat momentum, particle/impurity transport and magnetohydrodynamic stability is also discussed. In the steady state operation, a 48 min discharge with a line-averaged electron density of 1 × 1019 m−3 and with high electron and ion temperatures (Ti(0) ~ Te(0) ~ 2 keV), resulting in 3.36 GJ of input energy, is achieved.

Spectra of W 19 + -W 32 + observed in the EUV region between 15 and 55 Å with an electron-beam ion trap

Abstract: We present extreme ultraviolet spectra of highly charged tungsten ions (${\mathrm{W}}^{19+}\ensuremath{-}{\mathrm{W}}^{32+}$) in the wavelength range of $15\ensuremath{-}55\phantom{\rule{0.16em}{0ex}}\AA{}$ obtained with a compact electron-beam ion trap (CoBIT) and a grazing-incidence spectrometer at the National Institute for Fusion Science. The electron energy dependence of the spectra was investigated for electron energies from 490 to $1320\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. Identification of the observed lines was aided by collisional-radiative (CR) modeling of CoBIT plasma. Good quantitative agreement was obtained between the CR-modeling results and the experimental observations. The ion charge dependence of the $6g\text{\ensuremath{-}}4f,\phantom{\rule{0.16em}{0ex}}5g\text{\ensuremath{-}}4f,\phantom{\rule{0.16em}{0ex}}5f\text{\ensuremath{-}}4d,\phantom{\rule{0.16em}{0ex}}5p\text{\ensuremath{-}}4d$, and $4f\text{\ensuremath{-}}4d$ transition wavelengths were measured.

Intensity ratio of EUV emission lines in Fe XV studied with electron beam ion traps

Abstract: We present emission spectra of highly charged Fe ions in the extreme ultraviolet range obtained with two electron beam ion traps. The intensity ratio between the 3s3p3P2 - 3s3d 3D3 (233.9 A) and 3s3p 1P1 - 3s3d 1D2 (243.8 A) transitions in Fe XV is given for various electron beam parameters. The experimental ratios are compared with theoretical model calculation as well as our previous experiment, where significant discrepancy with theoretical model was found. The present result, which has been obtained with a higher resolution spectrometer to investigate possible line blending, confirms that the discrepancy is not due to line blending.

Characteristics of x-ray emission from optically thin high-Z plasmas in the soft x-ray region

Abstract: The characteristics of soft x-ray emission from optically thin high-Z plasmas of gold, lead and bismuth were investigated with the large helical device. Compared to optically thicker laser-produced plasmas, significantly different spectral structures were observed due to the difference in opacities and electron temperatures. Peak structures appearing in unresolved transition arrays were identified by calculations using atomic structure codes. The main contributors of discrete line emission in each case were Pd-, Ag-, and Rh-like ion stages. The present calculations point to the overestimation of contributions for 4p–4d transitions based on intensity estimates arising purely from gA distributions that predict strong emission from 4p–4d transitions. Understanding of such spectral emission is not only important for the completion of databases of high-Z highly ion charge states but also the development of promising high brightness sources for biological imaging applications.

Properties of the extreme ultraviolet emission from germanium and gallium plasmas

Abstract: We report on the usefulness of germanium (Ge) and gallium (Ga) plasmas as potential extreme ultraviolet (EUV) sources at both 6.x and 13.5 nm, wavelengths of interest for lithography and metrology applications. Spectra from plasmas produced using neodymium-doped yttrium-aluminum-garnet (Nd:YAG) lasers with pulse durations of 170 ps and 7 ns [full width at half-maximum] and a compact electron beam ion trap have been used to determine the charge states responsible while theoretical calculations using atomic structure Cowan and HULLAC codes enabled the identification of structure in the Ge spectrum arising from strong transitions of the type 3d – 4p and 3d – 4f. Transitions of the type 3d – 4f in Ge10+ and Ga11+ were found to lie in the 6.x nm region, while 3p−3d transitions from a range of stages in both elements contribute in the 13.5 nm region. We also studied the emission spectra of galinstan plasmas, and the results imply that galinstan might provide a promising candidate for EUV applications that explo...

Intensity ratio measurements for density sensitive lines of highly charged Fe ions

Abstract: Intensity ratio of density sensitive emission lines emitted from Fe ions in the extreme ultraviolet region is important for astrophysics applications. We report highresolution intensity ratio measurements for Fe ions performed at Tokyo EBIT laboratory by employing a flat-field grazing incidence spectrometer. The experimental intensity ratios of Fe X and Fe XII are plotted as a function of electron density for different electron beam currents. The experimental results are compared with the predicted intensity ratios from the model calculations.

Level-energy-dependent mean velocities of excited tungsten atoms sputtered by krypton-ion bombardment

Abstract: Visible emission spectra were acquired from neutral atoms sputtered by 35–60 keV Kr+ ions from a polycrystalline tungsten surface. Mean velocities of excited tungsten atoms in seven different 6p states were also obtained via the dependence of photon intensities on the distance from the surface. The average velocities parallel to the surface normal varied by factors of 2–4 for atoms in the different 6p energy levels. However, they were almost independent of the incident ion kinetic energy. The 6p-level energy dependence indicated that the velocities of the excited atoms were determined by inelastic processes that involve resonant charge exchange.

Intensity ratio in EUV spectra of highly charged Fe ions: Experimental evaluation of astrophysical plasma model

Abstract: We present spectra of highly charged Fe ions in the extreme ultraviolet region observed with a compact electron beam ion trap. The experimental intensity ratios obtained as a function of the effective electron density is used for evaluating plasma models used in the electron density diagnostics of the solar atmosphere.