Heat flux mitigation characteristics in the radiative divertors with multi-peaks of heat flux in the large helical device

ORCID　0000-0001-6090-5010In this study, the effects of impurity seeding on the one- and two-dimensional heat flux profiles of the large helical device divertor with a multi-peaks of heat flux were investigated, focusing on the heat-flux-reduction characteristics at the two peaks. The divertor heat flux profile was reconstructed using finite element analysis based on the temperature distribution measured via infrared thermography. The divertor heat flux profiles were calculated using the EMC3-EIRENE code considering the impurity seeding. In general, the calculated results were consistent with the experimental observations, including the heat flux value and reduction via impurity seeding. The reduction in the divertor heat flux due to impurity seeding was almost similar at different heat flux peaks and exhibited little dependence on the heating power variance. The difference between the low- and high-heating-power cases is discussed by tracing the magnetic field lines from the divertor plate and considering the penetration depth of the impurity particles into the scrape-off layer plasma.journal articl

Prediction of mode structure using a novel physics-embedded neural ODE method

We designed a new artificial neural network called Exposed latent state neural ordinary differential equation with physics (ExpNODE-p) by modifying the neural ordinary differential equation (NODE) framework to successfully predict the time evolution of the two-dimensional mode profile in nonlinear saturated stage. Starting from the magnetohydrodynamic equations, simplifying assumptions were applied based on physical properties and symmetry considerations of the energetic-particle-driven geodesic acoustic mode (EGAM) to reduce complexity. Our approach embeds known physical characteristics directly into the neural network architecture by exposing latent differential states, enabling the model to capture complex features in the nonlinear saturated stage that are difficult to describe analytically. ExpNODE-p was evaluated using a dataset generated from first-principles simulations of the EGAM instability, focusing on the nonlinear saturated stage where the mode properties (e.g. frequency) are quite difficult to capture. Compared to state-of-the-art models such as ConvLSTM, ExpNODE-p achieved superior performance in both accuracy and training efficiency for multi-step predictions. Additionally, the model exhibited strong generalization capabilities, accurately predicting mode profiles outside the training dataset and capturing detailed features and asymmetries inherent in the EGAM dynamics. Our results establish ExpNODE-p as a powerful tool for creating fast, accurate surrogate models of complex plasma phenomena, opening the door to applications that are computationally intractable with first-principles simulations.journal articl

EUV and Soft X-ray Spectroscopy of Highly Charged Heavy Ions Using LHD: Research Ranging from Industrial Light Sources to Basic Atomic Physics

ORCID　0000-0001-6536-9034This article reviews various achievements in spectroscopy of highly charged ions of a variety of heavy elements injected into the Large Helical Device (LHD) plasmas. We focus on discrete and quasi-continuum spectra observed in extreme ultraviolet (EUV) and soft X-ray wavelength ranges using multiple grazing incidence spectrometers. In particular, the atomic number dependence and temperature dependence of the spectral features have been investigated more comprehensively than ever before over extremely wide ranges based on comparisons with theoretical models and other experimental data. Consequently, the series of studies could provide an experimental database valuable for investigations of basic atomic physics issues specific to highly charged heavy ions, as well as the applications to industrial light source developments.journal articl

Determination of tritium inventory in carbon divertor tiles used in deuterium plasma experiment by induction heating method

ORCID　 0000-0001-9941-1958The tritium inventory of the plasma-facing materials in a Large Helical Device (LHD) was analyzed using a thermal desorption system. In this study, we developed an induction heating system for the thermal desorption of tritium from carbon divertor tiles, which has several advantages, including a rapid temperature rise, internal heating via eddy currents, and heating with the original size and shape of the carbon tile. The apparatus was capable of heating a carbon divertor tile to a temperature greater than 1373 K over 20 min. Following a 12-h heating period at 1423 K, the tritium release rate from the carbon tile was 99.7 %. The chemical form of the tritium released from the divertor tiles was approximately 80 % molecular hydrogen, with the remaining tritium in the form of water vapor. The results of the tritium analysis of the divertor tiles suggest that the tritium distribution in the divertor region is not uniform, and that the flux varies from location to location in the LHD. To improve the accuracy of the tritium inventory analysis in an LHD vacuum vessel, it is essential to conduct a tritium inventory analysis of other plasma-facing materials.journal articl

Influence of Stark Broadening on Ion Temperature Measurement for ITER Divertor Diagnosis

The Stark broadening of a Be II line (1s23d 2D – 1s24f 2F, 467.339 nm) under a magnetic field is evaluated with the divertor plasma of ITER in mind. The electron and ion perturbers are treated in the impact and static approximations, respectively. The perturbation term due to the magnetic field is included in the static approximation. The results show that the Stark broadening comes to be significantly large when the density is higher than 1021 m−3, and the ion temperature would be overestimated if the Stark broadening is not taken into account.journal articl

Numerical analysis of miter bend with spiral phase mirror

To excite optical vortices using high-power millimeter waves, we use a miter bend with a spiral phase mirror. Through numerical simulations, we demonstrate that vortex beams can be successfully excited by employing a spiral phase mirror that appropriately accounts for the phase difference between the input and output modes. The simulations also reveal the generation of higher-order modes caused by diffraction inherent to the miter bend structure and unintended reflections arising from the singularity at the optical axis of the spiral phase mirror. Additionally, we propose a method to estimate the topological charge, which corresponds to the vorticity, from real-valued data. The simulation results confirm that vortex beams are successfully excited as the dominant mode.journal articl

Visualization of characteristic plasma radiation structure in radiative collapse on Large Helical Device

Characteristics of plasma radiation structure in radiative collapse were visualized using a two-dimensional radiation measurement and AutoEncoder (AE) on the Large Helical Device of the National Institute for Fusion Science, Japan. The state without collapse was treated as normal, and the state in which collapse is evolving was treated as abnormal. Using the anomaly detection by the AE, the collapse could be detected ~ 0.36 s before the collapse as increase in abnormality. Moreover, the abnormal radiation structure could be visualized as the profile of the reconstruction error which appeared ~ 0.46 s before the collapse from inboard side of the torus plasma.journal articl

Water window soft X-ray spectra of Ce- to Gd-like Bi ions in an electron beam ion trap

The water window emission lines (20–40 Å) of highly charged bismuth (Bi) ions in laser produced plasmas are useful as light sources for biological microscopy of living cells. However, broad distributions of charge states and overlapping of transition arrays in the laser produced high density plasmas make precise line identification difficult. In this paper, we identified strong emission lines in the water window range from Bi ions decomposing to each charge state with a compact electron beam ion trap (CoBIT). To this end, we constructed a collisional radiative model accounting for detail atomic processes of the Bi ions interacting with mono-energetic electron beams. Each transition array observed in the experimental spectra was carefully identified with the present calculations. Distributions of the observed peaks show distinct features depending on charge state abundance in the CoBIT. We found that line emissions mediated via meta-stable excited states play an important role.journal articl

"Plasma and Fusion Cloud" Data analysis environment

ORCID　0000-0002-0441-6340The Fusion Cloud concept is currently underway at the National Institute for Fusion Science (NIFS). This concept is to deploy data collection and data analysis systems, which are assets developed through LHD experiments, to other laboratories in Japan through a high-speed network. The Open Data Server discussed in this paper is part of the Fusion Cloud concept, and data collected at LHD is currently being made publicly available. In the future, the data collected by various devices will be made widely available for use in fusion research, and will also be used for research beyond fusion and across different fields. In addition, in the future, not only the data but also a portable environment for analyzing these data will be provided on the cloud.journal articl

Corrosive behavior of structural F82H RAFM steel by LTZO ceramic breeder pebbles

ORCID　 0000-0001-8067-8732In this study, corrosion behavior of F82H reduced activation ferritic/martensitic steel (RAFM) by LTZO (Li2+xTiO3+y solid solution with 20 wt% Li2ZrO3) ceramic breeder pebbles was investigated at 773–998 K in an inert sweep gas condition (Ar + 0.1 % H2). Due to vapor gas release from the breeder pebbles and those penetrations, corrosion layer formed on the surface of the F82H steel. Glow discharge optical emission spectroscopy (GD-OES) and X-ray diffraction (XRD) identified the corrosion products as cubic, spinel, and rhombohedral Li–TM–O (TM: transition element in F82H such as Fe, Cr, and Mn). The growth of the corrosion layer thickness followed a parabolic curve at 833 K, yielding apparent diffusion coefficient of D = 6.95 × 10–13 cm2/s. Rapid growth was observed at 993 K after a parabolic growth which could be triggered by failure of the protective layer. A comparative analysis indicates a predominant effect of humidity and oxygen in the sweep gas on the growth rate, while the composition and shape of breeding materials have minor impacts.journal articl