Daiji Kato

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.

Super-saturated hydrogen effects on radiation damages in tungsten under the high-flux divertor plasma irradiation

Tungsten is a prime candidate as the divertor material of the ITER and DEMO reactors, which would be exposed to unprecedentedly high-flux plasmas as well as neutrons. For a better characterization of radiation damages in the tungsten under the divertor condition, we examine influences of super-saturated hydrogen on vacancies in the tungsten. The present calculations based on density functional theory (DFT) reveal unusual phenomena predicted at a super-saturated hydrogen concentration: (1) strongly enhanced vacancy concentration with the super-saturated hydrogen concentration is predicted by a thermodynamics model assuming multiple-hydrogen trapping, i.e. hydrogen clusters formation, in the vacancies; and (2) DFT molecular dynamics revealed that hydrogen clusters can prevent a vacancy from recombining with the neighboring crowdion-type self-interstitial-atom. This suggests that neutron damage effects will be increased in the presence of the hydrogen clusters.

Extreme ultraviolet spectroscopy and atomic models of highly charged heavy ions in the Large Helical Device

We report recent results of extreme ultraviolet (EUV) spectroscopy of highly charged heavy ions in plasmas produced in the Large Helical Device (LHD). The LHD is an ideal source of experimental databases of EUV spectra because of high brightness and low opacity, combined with the availability of pellet injection systems and reliable diagnostic tools. The measured heavy elements include tungsten, tin, lanthanides and bismuth, which are motivated by ITER as well as a variety of plasma applications such as EUV lithography and biological microscopy. The observed spectral features drastically change between quasicontinuum and discrete depending on the plasma temperature, which leads to some new experimental identifications of spectral lines. We have developed collisional-radiative models for some of these ions based on the measurements. The atomic number dependence of the spectral feature is also discussed.

Spectral evolution of soft x-ray emission from optically thin, high electron temperature platinum plasmas

The soft x-ray spectra of heavy element plasmas are frequently dominated by unresolved transition array (UTA) emission. We describe the spectral evolution of an intense UTA under optically thin conditions in platinum plasmas. The UTA was observed to have a peak wavelength around 4.6 nm at line-of-sight averaged electron temperatures less than 1.4 keV at electron densities of (2.5–7.5) × 1013 cm−3. The UTA spectral structure was due to emission from 4d–4f transitions in highly charged ions with average charge states of q = 20–40. A numerical simulation successfully reproduced the observed spectral behavior.

Visible emission spectroscopy of highly charged tungsten ions in LHD: I. Survey of new visible emission lines

We found 12 unknown visible emission lines from the core plasma of large helical device with highly charged tungsten ions accumulated. The observation was made with our home-built echelle spectrometer, which covers the wavelength range of 450-715 nm with a wavelength resolution of<0.05 nm for two lines of sight; one line passes both the core and edge plasmas and the other passes only the edge plasma. These emission lines are attributed to highly charged tungsten ions because (1) they were observed only after a tungsten pellet was injected into the plasma, (2) they were observed only from the core plasma where the electron temperature is 1 keV, (3) they show line broadenings that are close to the Doppler widths of tungsten ions with 1 keV temperature and (4) the wavelengths of some of these emission lines are close to the calculation results for tungsten ions in the charge state of 25-28.

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

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.

Mechanism of dominance of the Breit interaction in dielectronic recombination

The recent theoretical and experimental studies show that the Breit interaction plays a dominant role in the dielectronic recombination for some particular transitions. The detailed mechanism of why the Breit interaction is dominant for such a process is still unknown. In this work, we performed a simulation and decomposed each individual term in the transition matrix level and found that the Breit interaction is dominant when the leading term ( with the larger of r1 and r2) contribution of the two-electron Coulomb interaction is vanished. Based on this mechanism, we explained why the dielectronic capture strength to state is much stronger than the one to as well as why the Breit interaction plays a dominant role in the anisotropic parameters. Furthermore, the present finding may guide us to search some physical processes in which the Breit interaction is dominant by simply analyzing the coupling coefficients for a given isoelectronic sequence.

Database and Related Activities in Japan

We have constructed and made available atomic and molecular (AM) numerical databases on collision processes such as electron‐impact excitation and ionization, recombination and charge transfer of atoms and molecules relevant for plasma physics, fusion research, astrophysics, applied‐science plasma, and other related areas. The retrievable data is freely accessible via the internet. We also work on atomic data evaluation and constructing collisional‐radiative models for spectroscopic plasma diagnostics. Recently we have worked on Fe ions and W ions theoretically and experimentally. The atomic data and collisional‐radiative models for these ions are examined and applied to laboratory plasmas. A visible M1 transition of W26+ ion is identified at 389.41 nm by EBIT experiments and theoretical calculations. We have small non‐retrievable databases in addition to our main database. Recently we evaluated photo‐absorption cross sections for 9 atoms and 23 molecules and we present them as a new database. We establis...

Extreme ultraviolet spectra of multiply charged tungsten ions

We present extreme ultraviolet spectra of multiply charged tungsten ions observed with an electron beam ion trap. The observed spectra are compared with previous experimental results and theoretical spectra obtained with a collisional radiative model.

Theoretical study on the soft X-ray spectra of E1 transition of W LV ion

Jiaoxia Yang∗, Xiaobin Ding∗1, Rui Sun∗, Fumihiro Koike†, Izumi Murakami‡, Daiji Kato‡, Hiroyuki A Sakaue‡, Nobuyuki Nakamura §, Chenzhong Dong∗ ∗ Key Laboratory of Atomic and Molecular Physics and Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China † Department of physics, Sophia University, Tokyo 102-8554, Japan ‡ National Institute for Fusion Science, Toki, Gifu 509-5292, Japan § Institute for Laser Science, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan Synopsis A collisional-radiative model was constructed to investigate the spectra of W LV ion from EBIT. A good agreement with previous experiment was obtained and some new transition lines were predicted. The research on the energy level structure and radiation properties of various Tungsten(W) ions was largely spurred by the decision of chosen tungsten as the armor material of ITER divertor due to its favourable properties. In order to monitor and control the tungsten ion impurities produced when the plasma interact with the divertor, the throughout knowledge of the tungsten ions especially the radiation properties are necessary.