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LHD Thomson Scattering Diagnostics
No full textORCID 0000-0002-7023-1539Thomson scattering diagnostic systems are widely used to measure the local electron temperature and density of plasmas, which are one of the most important plasma parameters. The Large Helical Device (LHD) Thomson scattering system was designed and developed from 1991 to 1998, and has been in operation without any major problem since the LHD second experiment campaign in 1998. The LHD Thomson scattering system measures the pseudo-continuous time evolution of electron temperature and density profiles at 144 spatial points along the LHD major radius. The LHD Thomson scattering system can measure electron temperature and density spatially from the inner boundary to the outer boundary, and temporally from the birth to the destruction of LHD plasmas. The performance is still one of the best in the world. In this paper, we discuss improvements and newly obtained results of this system with emphasis on the results from 2010 to 2024.journal articl
表面クラック構造を有するアクリルコーティングによる液体窒素中の沸騰伝熱促進
No full textORCID 0000-0002-5682-3343A coating with surface-crack structures was observed to promote boiling heat transfer. An acrylic resin that develops cracks on its surface through natural drying was applied to a copper plate; subsequently, quenching experiments using liquid nitrogen were conducted. As a result, boiling heat transfer enhancement was observed across almost the entire temperature range during the cooling process, and the cooling time was considerably shortened. Near room temperature, complete film boiling disappeared, and an increase in heat flux was observed compared with the bare surface. In addition, the critical heat flux increased to approximately twice that of the bare surface. It is highly likely that the enhancement of surface wettability because of surface roughness contributed to boiling heat transfer enhancement.journal articl
Global Gyrokinetic Simulations of Isotope Effects under Ambipolar Electric Fields and Advances Toward Whole-Volume Modeling
No full textORCID 0000-0003-2513-0453We review global gyrokinetic simulation studies on plasma transport in the Large Helical Device using XGC-S. XGC-S is an extended version of X-point Gyrokinetic Code for stellarators and has been progressively verified throughout the code development process. Verification tests of neoclassical transport successfully demonstrate the generation of an ambipolar electric field due to ripple-trapped particles. We perform quasi-linear analyses of the ion temperature gradient mode under the influence of the ambipolar electric field. The results reveal that the ambipolar electric field and the heavy hydrogen component in mixed isotope plasmas can lead to the favorable isotope effect observed in recent deuterium experiments. We also present recent efforts in code development toward whole-volume simulations, including the helical divertor region. A mesh generation scheme based on field-line tracing and the construction of curved surfaces perpendicular to the magnetic field would be promising for global field calculations in the whole-volume simulations.journal articl
Determination of tritium inventory in carbon divertor tiles used in deuterium plasma experiment by induction heating method
Full text linkORCID 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
Full text linkThe 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
Visualization of characteristic plasma radiation structure in radiative collapse on Large Helical Device
Full text linkCharacteristics 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
Spatial Structure of Low-Frequency Hall-MHD Wave Propagation in a Field-Reversed Configuration Plasma
Full text linkA linear Hall-MHD simulation code is developed to investigate the spatial structure of two-dimensional wave propagation excited in a field-reversed configuration (FRC) equilibrium plasma. A low-frequency oscillating magnetic field at 160 kHz is externally applied by a ring coil installed concentrically with the device axis in the open-field region. The generated toroidal magnetic field propagates primarily along magnetic field lines at a phase velocity comparable to that of shear Alfvén waves. Due to the Hall effect, toroidal magnetic fluctuations penetrate into the closed-field region near the separatrix over a distance on the order of the ion skin depth. In the core region, where magnetic fluctuations vanish, ion density oscillations become dominant.journal articl
Prediction of Radiative Collapse in the Large Helical Device Plasma Discharges using Convolutional Neural Networks
Full text linkPredicting and preventing abrupt plasma termination incidents pose considerable challenges in nuclear fusion research. In the Large Helical Device (LHD), this occurrence is referred to as radiative collapse. During radiative collapse, impurity particles induce energy dissipation via radiation, hindering the maintenance of plasma discharges. Our approach aims to predict radiative collapse by analyzing the visible light emitted during such events. LHD uses approximately ten cameras to continuously observe plasma discharges, resulting in the accumulation of substantial video data from previous experiments. Using these images, convolutional neural network (CNN) models were trained to identify discharge states and subsequently applied to plasma discharge videos of the plasma discharges as a predictor. As a result, a determination model was developed, capable of discerning between stable and collapsed plasma discharge states with an accuracy of 91.5% ± 4% using plasma discharge images. Notably, this model demonstrated the potential to predict radiative collapse approximately three frames (66–132 ms) in advance. An examination of the model’s focal points revealed consistency with findings from prior research.journal articl
プロジェクトレビュー：国際エネルギー機関（IEA）PWI協定に拠る国際共同研究 国際共同研究の入口として、そしてその成果
Full text linkORCID 0000-0003-0161-0938国際エネルギー機関（IEA）の技術協力プログラム（TCP）の一つであるプラズマ・壁相互作用（PWI）TCP（以後PWI 協定）は，日本，欧州，米国，豪州，英国が参加する多国間協定であり，日本では核融合科学研究所が執行機関となっている．PWI 協定に拠る国際共同研究は，プラズマ・壁相互作用に関係する，材料分析，周辺・ダイバータプラズマ物理，プラズマ計測などの国際共同研究で多くの成果を挙げている．またこれらの国際共同研究は，学生・若手研究者が国際共同研究へ参加する入口ともなってきた．そこで本稿では，PWI 協定の概要，同協定に拠る国際共同研究の成果を紹介するとともに，特に，学生や若手研究者が，どのように国際共同研究を始めて，どのように外国研究機関で研究を行ったか，彼の地に滞在中にどのような経験をしたのか，も紹介する．journal articl
Machine learning-based hydrogen recycling model for predicting rovibrational distributions of released molecular hydrogen on tungsten materials via molecular dynamics simulations
Full text linkUnderstanding the hydrogen recycling process is crucial for comprehending the behavior of detached plasma in nuclear fusion devices. To achieve this, a molecular dynamics (MD) model is being developed to predict the distribution of translational energies and rovibrational states of hydrogen atoms and molecules released from the plasma-facing materials. Neutral transport simulations, utilizing distributions obtained from the MD model as boundary conditions, are also a powerful tool for analyzing the impact of recycled hydrogens on edge plasma. However, the MD model requires significant computational resources to obtain distributions under varying material and irradiation conditions such as material temperature and incident energy. Therefore, developing effective models that seamlessly integrate neutral transport simulation with hydrogen recycling models is crucial. Machine learning techniques are employed to develop predictive models capable of forecasting distributions of energies and rovibrational states of released hydrogen atoms and molecules. Furthermore, a model considering the incident energy distribution (shifted-Maxwellian) is developed by integrating the monochromatic distribution with the shifted-Maxwellian distribution.journal articl