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2. Applications to Vacancy-Hydrogen Atom Cluster Formation in Tungsten
No full textORCID 0000-0002-5302-073X密度汎関数理論（DFT）を用いることにより，任意の元素・原子配置の多電子波動関数に基づいて，様々な格子欠陥を含んだ結晶構造が乱れた系でのエネルギーや力場の非経験的な計算を行うことができる．本章では，このDFT の特長を活かしながら著者らによって行われた，核融合炉ダイバータ材料のタングステン中で形成される空孔‐ 水素クラスタのエネルギーと安定構造に関する研究について紹介する．また，最近取り組み始めた，より大きなサイズの系や長時間スケールの分子動力学シミュレーションのためのDFT に基づいた機械学習力場の開発についても簡単に紹介する．journal articl
Tritium Management for LHD Deuterium Plasma Experiments
No full textORCID 0000-0001-9941-1958In high-temperature plasma experiments using deuterium gas in large fusion devices, tritium is produced by the deuterium fusion reaction. Although the amount of tritium produced is not large, it is a radioactive material, so it is essential to develop safe handling systems and obtain public acceptance. Here, we summarize tritium safety management and tritium behavior in the facility, and have monitored the results of six years of deuterium plasma experiments in the Large Helical Device (LHD). More than 95% of the tritium exhausted from the LHD vacuum vessel was recovered by a tritium removal system. The extremely low concentrations of tritium discharged from the stack into the environment were monitored for each chemical form of tritiated water vapor, tritiated molecular hydrogen, and tritiated hydrocarbons, and were verified to be well below the levels specified by management at the National Institute for Fusion Science (NIFS). Although the tritium handled in the LHD deuterium experiment was a small amount, the operational experience and the instruments thereby developed would likely be useful for tritium safety management in future fusion reactors.journal articl
Spectroscopic Diagnostics of Non-Maxwellian Electron Velocity Distribution Function in the Large Helical Device
No full textThis paper reviews studies of non-Maxwellian electron velocity distribution function (EVDF) measured via line emission spectroscopy and spectropolarimetry in the Large Helical Device (LHD). Information on where and under what conditions the non-Maxwellian EVDFs are generated can substantially affect plasma confinement. Since different atomic transitions exhibit different sensitivities to momenta of incident electrons, spectroscopic analysis of line intensities and their polarization enables the investigation of both the shape and anisotropy of the EVDF. Measurement techniques and their results are summarized across a broad temperature range, covering both edge and core plasmas in LHD. The results are compared with plasma parameters obtained from other diagnostic systems, and the dynamics of passing and trapped electrons are discussed.journal articl
Nonlinear excitation of energetic particle-induced geodesic acoustic mode via resonance overlap in CFQS
No full textORCID 0000-0001-8002-8641Energetic particle (EP) driven instabilities are investigated in the Chinese First Quasi-axisymmetric Stellarator using the hybrid kinetic-magnetohydrodynamic code MEGA. EPs are described as the anisotropic slowing-down distribution with pitch angle variable peak value Λ0. Simulations reveal that for Λ0 ≈ 0.25 or 0.65, two energetic-particle-modes (EPMs) with close growth rates coexist during the linear phase, and poloidal/toroidal mode numbers are m/n = 3/1 and m/n = 6/2, respectively. For 0.3 ⩽ Λ0 ⩽ 0.6, the 6/2 EPM is the dominant linear instability, and in the nonlinear phase, an EP-induced geodesic acoustic mode (EGAM) is excited and ultimately becomes the dominant instability. The nonlinear excitation mechanism of the EGAM is identified as resonance overlap, based on the analysis of the EP perturbed distribution function δf. During the linear growth of the 6/2 EPM, hole-clump structure of δf forms within its resonance region in energy E and pitch angle variable Λ space. As the 6/2 EPM saturates and its frequency chirps, the dominant clump structure in (Λ, E) space migrates towards and gradually overlaps with the EGAM resonance region. This resonance overlap triggers the nonlinear excitation of the EGAM. This is the first demonstration of resonance overlap inducing mode nonlinear excitation in (Λ, E) space and in a stellarator.journal articl
Consideration on relation between penetrated power and topological charge of millimeter-wave vortex in magnetized plasma
Full text linkORCID 0009-0009-9605-513XIt was demonstrated that the vortex field of a hybrid mode can propagate in the magnetized plasma region where plane waves are unable to propagate due to the cut-off condition. In this study, the dependence of the penetrated power of injected as millimeter-wave vortices of the hybrid mode in magnetized plasma is analyzed using the Finite-Difference Time-Domain (FDTD) method. The effect of the radius size of the corrugated waveguide on the penetrated power is described, revealing its significant contribution to reducing the deviation of topological charge in the hybrid mode. Furthermore, it was found that the penetrated power of the vortex field in magnetized plasma strongly depends on the topological charge l and the deviation of topological charge.journal articl
Design and test of a waveguide filter for reflected electromagnetic waves
Full text linkA high power gyrotron system is used for plasma production and sustainment by electron cyclotron resonance heating (ECRH) in fusion reactors. An ECRH system in the Large Helical Device uses gyrotrons with megawatt output power. This megawatt electromagnetic wave transmitted through a circular corrugated waveguide is partially reflected back to the gyrotron and can be a cause of unstable gyrotron oscillations. The reflected beam may also propagate at an angle relative to the waveguide axis. To reduce the reflected beam while retaining as much of the forward-propagating gyrotron beam as possible, we aim to optimize the length of a filter consisting of a gap with absorbing walls between waveguide sections in a transmission line. A semi-analytical model is developed using diffraction theory to calculate the transmitted power of the HE11 mode as a function of filter gap distance for a wide range of beam angles, including a perfectly aligned beam to approximate the forward gyrotron beam. Before implementing this filter, we use a vector network analyzer to measure the scattering parameters for an experimental mock-up of this filter with 88.9 mm inner diameter waveguides, gap distances between 0 and 600 mm, and beam angles between 0° and 4°. These sophisticated models are used to inform the gap filter design and length optimization.journal articl
Progress in the development of radiation detectors for fusion neutron source A-FNS
No full textORCID 0000-0003-4555-1837The time evaluation of the intensity and spectrum of a neutron source provides the fundamental data for neutron irradiation experiments. Therefore, the study and development for measurement of radiation flux and spectrum with high accuracy is necessary. The development of fast neutron detectors has been advanced through a collaborative research undertaking between the National Institute for Fusion Science and the National Institutes for Quantum Science and Technology for Fusion Neutron Source, A-FNS, which is a high-energy and high-intensity neutron source based on the deuterium-lithium stripping reaction with its energy spectrum extending up to 55 MeV. d-D and d-T neutron spectrometers based on scintillation and diamond detectors were developed in the accelerator-based d-D and d-T neutron sources. The initial radiation measurements using the developed detectors were conducted in the Linear International Fusion Materials Irradiation Facility Prototype Accelerator (LIPAc). The d-D neutron spectrum and source emission rate were measured by a high-pressure 3He proportional counter and stilbene scintillation detector, respectively. Accordingly, the fast-neutron spectrum from the LIPAc beam dump was measured using an EJ-301 scintillation detector. Furthermore, an initial test of a single-crystal chemical-vapor-deposition diamond detector and the gamma ray spectroscopy using large-volume LaBr3:Ce scintillation detector were also conducted. The time evolution of the gamma ray pulse count rate was consistent with the beam injection pattern, i.e., 3 ms injection every 100 ms. The demonstration of the performance of the developed detectors at the current stage of LIPAc provided the basic experimental evidence necessary for designing a radiation monitor system for A-FNS.journal articl
TALIF measurements of atomic deuterium in toroidal divertor simulator NAGDIS-T
Full text linkThe behavior of neutral deuterium (D) atoms is important for understanding the physics in the divertor region
of nuclear fusion reactors. It is necessary to introduce an active measurement when considering the application
to detached plasmas, where recombination processes dominate the processes determining the population
distribution. In this work, we have developed a two-photon absorption laser-induced fluorescence (TALIF)
system in the toroidal divertor simulator NAGDIS-T to measure D atomic density. The absolute D atomic density
was obtained by calibrating the signal with the krypton TALIF signal. The gas pressure and power dependence
of the D atomic density is shown. The D atomic density was in the range of 1.6 × 1018–1.4 × 1019 m−3, and the
temperature was estimated to be <0.4 eV. The behavior of the D atoms is discussed in terms of the production
process.journal articl
Time-dependent density functional theory simulation for analyzing the neutralization process of hydrogen ions injected onto tungsten surfaces
Full text linkORCID 0000-0001-9050-2802We conducted time-dependent density functional theory simulations to investigate the neutralization process of a hydrogen ion injected at 100 eV onto the (110) surface of tungsten. Additionally, we introduced a method for evaluating the detection probability of electrons within a localized region. This probability serves as an indicator for identifying hydrogen in its various states: positive ion, neutral atom, and negative ion. Our findings indicate that following the collision, the probabilities of detecting hydrogen as a positive ion, neutral atom, and negative ion are approximately 25%, 50%, and 25%, respectively.journal articl
Effect of finite beta on neoclassical transport characteristics in CFQS quasi-axisymmetric stellarator
Full text linkORCID 0000-0003-0824-3448CFQS is the world’s first modular-coil-type quasi-axisymmetric stellarator, which is currently under construction. This study systematically investigates the effect of finite beta (volume-averaged beta, 〈β〉) on the neoclassical transport characteristics of CFQS plasmas. An intensification of the finite-β effect increases the neoclassical bootstrap current and amplifies variations in the major helical ripple component B1,1 and other non-axisymmetric magnetic components. During this process, the magnetic flux surfaces essentially remain at moderate β values (〈β〉 = 0.77%); however, at high β values (〈β〉 = 2.03%), the flux surface becomes significantly deformed due to intensified axisymmetry breaking. The neoclassical diffusion properties in the CFQS are investigated for different finite β values using the independently developed Monte Carlo Neoclassical Transport Simulation code. The charged-particle fluxes are estimated as functions of the radial electric field. Subsequently, the neoclassical diffusion coefficients and plasma parameters at moderate β (〈β〉 = 0.77%) and high β (〈β〉 = 2.03%) values are examined under ambipolar conditions. Electron roots are observed to provide strong confinement in the CFQS at various finite β values.journal articl