研究者詳細 - 星野　一生

論文 - 星野　一生

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Neutral-density dependence of EIR- and MAR-dominated detachment regimes in hydrogen plasmas with multi-ion-temperature effects

Natsume H., Tanaka H., Togo S., Sawada K., Hoshino K., Ohno N.

Physics of Plasmas 33 （ 7 ） 072506 2026年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1070664X

A multi-ion-temperature model for hydrogen plasmas has been newly developed and implemented in the linear divertor plasma simulation code LINDA-NU to investigate detachment mechanisms involving atomic and molecular processes. In this model, H+, H<inf>2</inf>+, and H<inf>3</inf>+ ions are treated as separate fluids with independent temperatures, enabling a detailed assessment of ion-species-dependent thermal and momentum dynamics in detached plasmas. Plasma detachment transitions were studied by scanning atomic and molecular neutral densities. An increase in atomic hydrogen density leads to electron–ion recombination-dominated detachment, where charge exchange with atomic neutrals governs momentum loss. In contrast, increasing molecular hydrogen density results in molecular-activated recombination (MAR)-dominated detachment, in which molecular ions become quantitatively significant and strongly influence particle, momentum, and energy balances. A comparison with a simplified single-ion-temperature model demonstrates that the multi-ion-temperature treatment is essential for accurately describing the MAR-dominated regime, where multi-ion temperatures significantly modify molecular reaction rates, leading to qualitative differences in plasma profiles.

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Code-to-code comparison of SOLPS-ITER and SONIC edge transport codes in JT-60SA tokamak plasma conditions

Rubino G., Yamoto S., Coster D., Falchetto G., Wiesen S., Sozzi C., Hoshino K., Homma Y., Nakano T., Hayashi N., Aiba N.

Fusion Engineering and Design 228 115759 2026年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 09203796

Plasma boundary codes are widely used to interpret and predict SOL/divertor plasmas. The comparison of results obtained by these codes is one of the options to improve the SOL/divertor plasma modelling by defining possible limitations and enhancing the fidelity of future simulations. The aim of this study is to validate the transport models of SOL/divertor transport codes SOLPS-ITER and SONIC by terms-to-terms comparison, from the viewpoint of physical explanation rather than the numerical viewpoint. In this study, the comparison of the results between SOLPS-ITER and SONIC on pure-D plasma conditions with attached and partially detached plasmas in JT-60SA has been performed as a first step of the benchmarking activity. The calculation conditions such as computational grids, boundary conditions, and atomic/molecule database, are set to be as close as possible. In summary, the comparison can be concluded that no substantial difference can be seen although some differences in the plasma parameters have been observed. The main difference can be seen as the higher ratio of pressure drop in SOLPS-ITER than SONIC in front of the divertors in the partially detached case. It was found that the difference can be understood by the following process. The initial difference originated from the electron temperature and ion temperature in the vicinity of divertor plates. The electron temperature in SONIC near outer strike point is slightly higher than that of SOLPS-ITER. The difference in the electron temperature results in broader D+ particle source by ionization of neutrals and higher D+ ion density. Higher D+ ion and neutral densities in SOLPS-ITER leads to a larger momentum loss by charge exchange and thus the plasma loses its pressure. This result suggests that the small difference in the initial plasma parameters can be propagated by the interaction between neutrals, which affects the estimation of degree of detachment.

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Stability analysis of divertor plasma transport using the CSD model

Yamada K., Hoshino K., Hatayama A.

Plasma Physics and Controlled Fusion 68 （ 7 ） 075025 2026年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 07413335

Thermal instability of divertor plasma transport in steady-state operation is a critical issue in reactor design. Identifying the thermally stable regime is crucial for the successful operation of a fusion reactor. This study aims to explore the regime of stable operation for divertor plasma and to investigate how this stability depends on key engineering parameters. For this purpose, a linear stability analysis is applied to equilibrium solutions of the particle, momentum, and energy balance equations obtained from the core-SOL-divertor model, providing a novel application of point-model-based analysis. In this regard, this study specifically addresses plasma heat control via radiation within the divertor volume. While alternative heat exhaust strategies utilizing strong radiation localized near the X-point—such as the X-point radiator regime—are also highly important, they are outside the scope of this work. Under this framework, this study presents two main findings through model calculations for JT-60U and JA-DEMO. First, the divertor plasma exhibits three thermally stable states: low-recycling, high-recycling, and weakly detached, while the intermediate states between them are unstable. Within the context of the localized divertor-volume radiation, the weakly detached state is identified as the expected operational regime in future fusion reactors. Second, as initial examples demonstrating the capability of the stability analysis framework for future extensive parametric surveys, the stability of the weakly detached state is found to be primarily sensitive to the exhaust power; increasing power raises the lower temperature limit and significantly narrows the stable range. Variations in the major radius have little influence on stability, whereas a reduction in the minor radius slightly raises the lower stability limit within the parameter range examined. The proposed stability evaluation framework provides qualitative guidance for selecting reactor design parameters that expand the stable operational window of divertor plasmas.

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SONIC simulation studies of power and particle exhaust, and impurity concentration for JA DEMO tokamak design

Asakura N., Hoshino K., Yamamoto T., Sakamoto Y.

Nuclear Fusion 66 （ 8 ） 086020 2026年06月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00295515

SONIC divertor code facilitates simultaneous calculations of seeding impurity (Ar) and fusion product (He) transport. D<inf>2</inf> and Ar gases were injected at the outer midplane and upper location of the outer divertor, respectively. He exhausts in the detached divertor have been investigated for a reference series of JA DEMO with an increasing deuterium gas puff rate (50–200 Pa m3s−1) where P<inf>sep</inf> ∼ 250 MW and the radiation fraction is 0.8. The outer divertor heat load (q<inf>tot</inf>div) peaked in the attached region on the partially detached target, and the peak-q<inf>tot</inf>div was reduced to lower than 10 MW m−2 by increasing the D<inf>2</inf> puff rate while maintaining a relatively low n<inf>e</inf>sep (2 × 1019 – 2.5 × 1019m−3). Neutral and gas pressures in the divertor (P<inf>D0</inf>div, P<inf>D2</inf>div) were evaluated at exhaust slots of the dome and in the sub-divertor. The total pressure (P<inf>Dtot</inf>div) in the D-puff scan was increased from 1.4 to 3 Pa at the outer exhaust slots, and the outer peak-q<inf>tot</inf>div was proportional to (P<inf>Dtot</inf>div)−0.7. Plasma flow reversals with M<inf>//</inf> = 0.2 – 0.3 were generated near the detach–attach boundary above the outer divertor target. The influence of the flow reversal on the plasma and impurity transport was determined. Convective heat flux was reversed, which significantly affected the ion heat flux profile in the upstream SOL. The flow reversal reduced the impurity shielding and radiation loss in the outer divertor. Inner-enhanced in–out asymmetry of Ar concentration (c<inf>Ar</inf>) was large near the separatrix in SOL, suggesting that Ar ions were transported from the inner SOL to the confined plasma. In the confined plasma (edge), both c<inf>Ar</inf>edge and c<inf>He</inf>edge were reduced and poloidally uniform, where c<inf>Ar</inf>edge was reduced to the marginal level and c<inf>He</inf>edge was below the design reference by system code assumption (c<inf>Ar</inf>main: 0.6%, c<inf>He</inf>main: 7%). Consequently, these SONIC results are generally consistent with the JA DEMO higher-κ design, and larger D-puff (and smaller Г<inf>Ar,puff</inf>) operation is preferable for the large power exhaust in the divertor and the impurity control in the edge plasma.

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First SOLPS-ITER Simulation Study of Japanese DEMO on Leakage of Seeded Ar Impurity

Si H., Kawamura G., Asakura N., Hoshino K., Seto H., Sakamoto Y., Yagi M.

Contributions to Plasma Physics 2026年06月

研究論文（国際会議プロシーディングス）, 共著, 査読有り, ISSN 08631042

For JA DEMO, one of the grand challenges is efficiently handling the extreme power loads onto plasma-facing components, particularly the divertor targets. Impurity seeding is an essential strategy for increasing the radiation losses and thereby reducing the heat load on the targets. A critical remaining issue is the trade-off between the retention of radiative impurities in the divertor region to maximize the radiation loss, and their leakage to the upstream region, which degrades the core plasma confinement. To address this issue, based on the current baseline divertor geometry of JA DEMO, the SOLPS-ITER code is applied for the first time to evaluate the divertor power exhaust and Ar impurity distribution across various Ar seeding levels in detail. The simulation results show that increasing the Ar seeding level is highly beneficial for reducing the heat load on the targets. Moreover, in the near SOL there is a strong flow reversal of the main D ions and Ar impurity ions from the lower outer divertor target toward the upstream. This flow reversal promotes the transport of Ar ions out of the lower outer divertor region, leading to significant core contamination. As a result, such leakage severely constrains the viability of using high seeding levels for heat-load control in the current JA DEMO divertor geometry and plasma operating scenario.

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Relationship between Surface-Produced H⁻ Density and the Potential Structure of an Ion-Ion Plasma near the Extraction Aperture in Negative Hydrogen Ion Sources

Hayashi K., Hoshino K., Miyamoto K., Hatayama A.

Plasma and Fusion Research （一般社団法人プラズマ・核融合学会） 21 （ 0 ） 1401019 2026年03月

研究論文（学術雑誌）, 共著, 査読有り

The so-called “ion-ion” plasma, which consists primarily of negative and positive ions, has been observed near the extraction aperture in negative hydrogen ion (H⁻) sources with a substantial amount of surface-produced negative ions, according to several experiments. In this study, to elucidate the characteristics of ion-ion plasmas with substantial surface H⁻ production, the relationship between surface-produced negative ion density and electric potential is investigated using three-dimensional Particle-In-Cell simulations with the KEIO-BFX code. The results reveal that the plasma in a negative ion source can be classified into four distinct regions based on the relationship between H⁻ density and electric potential: Region (I), where H⁻ follows the Boltzmann relation; Region (II), where H⁺ follows the Boltzmann relation; Region (III), a transition region between the plasma region and the beam acceleration region; and Region (IV), the beam acceleration region. In negative ion sources with surface-produced H⁻ ions, plotting the H⁻ density against the electric potential offers a useful method for identifying the plasma meniscus, which is crucial for optimizing extracted beam quality and ensuring proper operation of beam extraction systems.

An Estimation of the Heat Load Distribution on the Divertor Target Using the CSD Model

Yamada K., Hoshino K., Kawamura G., Si H., Hatayama A.

Contributions to Plasma Physics 2026年03月

研究論文（国際会議プロシーディングス）, 共著, 査読有り, ISSN 08631042

Handling the heat load on the divertor target is a critical issue in the design of fusion reactors. Estimating the heat load, supported by computationally efficient simulations with varying design parameters, provides valuable input to high-level design discussions and the development of design guidelines. This study aims to estimate the spatial distribution of the heat load on the target at a low computational cost. The Core-SOL-Divertor (CSD) model is employed to provide the integrated heat load on the target. The CSD output forms the basis of an analytical approach to solving the heat transport equation, which yields the spatial distribution of the heat load on the target. The resulting heat load distribution shows good agreement with SOLPS-ITER. The predicted distribution and its dependence on design parameters provide useful guidance for the preliminary design of fusion reactors.

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Deep Learning of Hydrogen Trapping Sites in Tungsten for Atomistic Plasma-Wall Simulations

Saito S., Takeuchi K., Nakamura H., Oda Y., Sawada K., Hoshino K., Homma Y., Yamoto S., Uchida Y.

Plasma and Fusion Research （一般社団法人プラズマ・核融合学会） 21 （ Special_1 ） n/a 2026年

研究論文（学術雑誌）, 共著, 査読有り

Understanding hydrogen trapping in tungsten is crucial for accurate modeling of plasma-wall interactions in fusion devices. In this study, we developed a deep learning model to predict hydrogen trapping sites, which serve as essential input parameters for kinetic Monte Carlo (kMC) simulations. The model employs a U-Net-based convolutional neural network that directly maps three-dimensional potential energy distributions to trapping site positions. Training data were generated from atomistic calculations using the embedded atom method (EAM) potential, and ground-truth trapping sites were systematically identified by force relaxation. The trained model achieved an F1 score of approximately 0.76, with most predicted sites coinciding with the true minima within ± 1 voxels (1 voxel = 0.1 Å per side). Visual comparisons confirmed the ability of the model to capture both global and local features of the potential energy landscape. In terms of efficiency, the proposed approach reduced prediction time by more than three orders of magnitude compared with conventional force-relaxation searches, enabling predictions in less than one second on a GPU. These results demonstrate that deep learning provides an accurate and computationally efficient method for identifying trapping sites. Future extensions include incorporating more complex defect structures and integrating the model into molecular dynamics-kMC hybrid frameworks for large-scale plasma-wall interaction simulations.

Model Improvement for Isotope Effects in a Negative Ion Source Using KEIO-MARC and a Rate Equation Model

H. Soejima, T. Shibata, H. Nakano, K. Hayashi, K. Miyamoto, K. Hoshino

Journal of Physics: Conference Series 3237 012015 2026年

研究論文（国際会議プロシーディングス）, 共著, 査読有り

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Source-term models based on the Boltzmann collision integral for hydrogen-isotope ion-atom collisions in fluid simulations

Umezaki D., Yamoto S., Hoshino K., Asakura N., Kawamura G.

Physics of Plasmas 32 （ 12 ） 122508 2025年12月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1070664X

In divertor plasmas, ion–atom collisions critically influence plasma transport and heat flux to divertor targets. Fluid neutral models are widely used in divertor simulations due to their low computational cost and favorable convergence properties. However, source terms representing ion–atom collisions often differ across codes because of variations in the cross section data and in the physical approaches used to model collisions between hydrogen isotope ions and atoms of identical nuclei. This study systematically evaluates these differences and establishes more consistent modeling approaches based on the Boltzmann collision integral (BCI). Using the BCI, we investigate particle transport in velocity space and assess the effect of scattering angles and differential cross sections (DCSs). Several analytical expressions—rarely used in divertor studies—are reviewed. For energy transfer, we derive a new BCI-based expression and validate the analytical expressions by comparing with the direct BCI evaluation. It is also pointed out that the commonly used charge–exchange (CX) cross section data no longer reflect recent experimental and theoretical developments. For deuteron-deuterium atom collisions at T = 10 eV, the CX-based model overestimates the friction force by more than 30% compared with the BCI-based model. For energy transfer between deuterons at T = 20 eV and deuterium atoms at T = 10 eV, the CX-based model overestimates the transferred energy by approximately 25%. The use of DCSs and the analytical expressions introduced in this study is therefore recommended. Implementing them in place of the conventional CX-based model may modify the predicted plasma profiles and the assessment of the detached divertor.

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Machine learning-based hydrogen recycling model for predicting rovibrational distributions of released molecular hydrogen on tungsten materials via molecular dynamics simulations

Saito S., Iida M., Nakamura H., Sawada K., Hoshino K., Kobayashi M., Hasuo M., Homma Y., Yamoto S.

Nuclear Materials and Energy 43 101942 2025年06月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

Understanding 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.

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Improvement of a CSD model toward detached plasma analyses by introducing effective loss factors based on plasma spatial distributions

Yamada K., Hoshino K., Hatayama A.

Plasma Physics and Controlled Fusion 67 （ 4 ） 045023 2025年03月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 07413335

CSD (Core, SOL and Divertor) models sampling a fusion device with three points (Core, SOL and Divertor) aim at understanding fusion plasmas in a wide range of parameter space. Conventional CSD models estimate interactions among plasmas, neutrals and impurities, which are incorporated as loss factors in basic equations in models, with the use of physical quantities of plasmas at three sampled points. Physical quantities are assumed to be uniform in each of the three regions. Neglecting plasma spatial distributions in a divertor prevents the models from accurately evaluating loss factors at low temperatures. This is because loss factors strongly depend on the location-dependent temperature when the temperature is low. Consequently, conventional CSD models have difficulties in reproduction of the detachment. This research increases sampling points for the calculation of loss factors in a divertor. Loss factors at each point are integrated in the volume, then effective loss factors (ELF) for the divertor are obtained. Conventional loss factors in transport equations are replaced with ELFs for an improved CSD model proposed in this study. As a result, the plasma detachment is successfully reproduced in the CSD model without changing the equations. A CSD model for qualitative analyses with low computational costs has been developed to study plasma detachment.

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DISCOVER: New integrated code to simulate helium detached plasma in a linear device

Natsume H., Masuda S., Tanaka H., Migita R., Mano R., Sugiura K., Sawada K., Hoshino K., Hayashi Y., Ohno N.

Physics of Plasmas 32 （ 2 ） 022508 2025年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1070664X

A detached plasma integrated simulation code with various elastic/inelastic reactions (DISCOVER) was developed for better understanding of detached helium plasma experiments. DISCOVER, which includes a neutral transport code, fluid code, and collisional radiative code, considers elastic and inelastic collisions between recycling neutrals and bulk particles, as well as the transport of metastable atoms [He( 21S) and He( 23S)]. The code was used for simulating helium plasma in the linear plasma device NAGDIS-II. The transition from attached to detached plasma was demonstrated in the simulation by increasing the upstream electron density through an increase in the gas supply from the plasma source while maintaining a fixed downstream gas injection rate. In addition, similar transition was also observed by introducing neutral particles from downstream, while maintaining a fixed upstream electron density. In the detached plasma, the density of He( 23S) near the end target was approximately seven times higher than that in the attached plasma. Furthermore, it was evident that the transport of He( 23S) enhanced the electron energy rate coefficient and influenced the spatial distribution of the detached plasma.

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Evaluation of local erosion and deposition on the W-monoblock of JA-DEMO divertor

Oya M., Hoshino K., Asakura N., Sakamoto Y., Ohno N., Hanada K.

Nuclear Materials and Energy 41 101793 2024年12月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

We developed and performed a 3D Monte-Carlo simulation on local erosion and deposition in outer divertor target of JA-DEMO. The local erosion and deposition were evaluated considering the divertor plasma parameters and the roof shape of W-monoblocks. First, for local erosion simulation, the trajectories of D ions and impurity (He and Ar) ions were traced before irradiating the monoblock surface. The physical sputtering was calculated by these ions irradiation, considering their charge states, incident-energies and -angles. The highest sputtering flux was produced by multiply-charged Ar ions (Ar3+–Ar5+). The sputtering flux strongly depended on the poloidal positions along the divertor target. Although almost no erosion (sputtering) was obtained in low-temperature zone (∼1 eV), the erosion occurred in mid- (∼10 eV) and high-temperature (>10 eV) zone. Second, for local deposition simulation, the trajectories of W atoms emitted from the monoblock surface were traced. After transport (including ionization and recombination) in the plasma, W atoms and ions were deposited on the monoblock surface. The deposition occurred even on the area where plasmas did not irradiate (shadow). This was due to perpendicular diffusion to magnetic field lines, gyro-orbit effects for W ions and straight-line motion for neutral W atoms. The poloidal distribution of the deposition fluxes was similar to that of the sputtering fluxes. In low-temperature zone, no deposition flux occurred due to very low sputtering flux. The deposition flux in mid-temperature zone increased significantly and that in high-temperature zone remained high.

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Investigation of spatial distribution characteristics of emission intensity in detachment on the divertor simulator TPDsheet-ICR

Tonegawa A., Okada N., Miura K., Natume H., Sato K.N., Hoshino K., Hayashi Y., Masuzaki S., Yamoto S.

Nuclear Materials and Energy 41 101802 2024年12月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

For the first time, we have revealed the spatial structure of a detached plasma with electron–ion recombination (EIR) and molecular-activated recombination (MAR) in response to changes in ion temperature (T<inf>i</inf>) and gas pressure. This was achieved using high-density sheet plasma generated by a linear divertor simulator (TPDsheet-ICR). The parallel-plate electrodes were positioned above and below the sheet plasma (∼1019 m−3). We varied the T<inf>i</inf> from 3 to 7 eV using ion cyclotron resonance heating and investigated the parameters of the detached plasma near the target in the divergent field region. Plasma emission intensities at Balmer series (H<inf>α</inf> and H<inf>γ</inf>) and Fulcher band wavelengths (610 ± 10 nm) were measured with a high-speed camera equipped with an Arbaa prism. Additionally, a Langmuir probe measured the electron density and temperature of the plasma. Our results show that as the T<inf>i</inf> increases, EIR generated near the plasma periphery disappears, while MAR gradually forms near the plasma center during the transition from detached to attached plasma. This transition occurs because the increase in T<inf>i</inf> in the detached plasma transfers energy from ions to electrons, raising the high-energy component of the electrons, resulting in EIR disappearance and the onset of MAR.

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Analyses of Particle and Energy Balances using a Core-SOL-Divertor (CSD) Model for Tokamak DEMO-Reactor Studies

Yamada K., Hoshino K., Hatayama A.

Plasma and Fusion Research （一般社団法人プラズマ・核融合学会） 19 （ 0 ） 1403034 - 1403034 2024年11月

研究論文（学術雑誌）, 共著, 査読有り

Understanding of key physics and engineering parameters is one of the most important issues to conduct fundamental design studies of future fusion reactors. Also, sensitivity analyses of performance by parameter surveys need to be done for optimizing a reactor. This research aims at a qualitative analysis to identify directions of design parameter optimization for fusion reactors. For this purpose, a Core-SOL-Divertor (CSD) model has been employed, because it is suitable for a wide range of parameter surveys by changing various parameters with low computational costs. This paper analyzes the particle and energy balances of the JT-60U divertor plasma by using a CSD model and reveals dependencies of the balances on design parameters of the device. In addition, the same CSD model is applied to conduct divertor plasma analyses for a future demonstration reactor (JA-DEMO). The result shows that it is possible to obtain a low temperature state of its divertor plasma as has been already reported by a two-dimensional SOL-Divertor integrated code. From these results, this paper shows that the CSD model is applicable to basic studies of directions and design concepts for future fusion reactors.

Recent progress of JT-60SA project toward plasma operation

Shirai H., Takahashi K., Di Pietro E., Abate D., Abdel Maksoud W., Abe H., Aiba N., Abe T., Akimitsu M., Ayllon-Guerola J., Arai T., Artaud J.F., Asakura N., Ashikawa N., Balbinot L., Barabaschi P., Baulaigue O., Belonohy E., Belpane A., Bin W., Bombarda F., Bolzonella T., Bonne F., Bonotto M., Botija J., Buermans J., Cabrera-Pérez S., Cardella A., Carralero D., Carraro L., Cavalier J., Cavinato M., Chernyshova M., Chiba S., Clement-Lorenzo S., Cocilovo V., Coda S., Coelho R., Coffey I., Collin B., Corato V., Cucchiaro A., Czarski T., Dairaku M., Davis S., Day C., Dela Luna E., De Tommasi G., Decool P., Di Pace L., Dibon M., Disset G., D’Lsa F., Ejiri A., Endo Y., Ezumi N., Falchetto G., Fassina A., Fejoz P., Ferro A., Fietz W., Figini L., Fornal T., Frello G., Fujita T., Fukuda T., Fukui K., Fukumoto M., Funaba H., Furukawa M., Futatani S., Gabellieri L., Gaio E., Galazka K., Garcia J., Garcia-Dominguez J., Garcia-Lopez J., Garcia-Munoz M., Garzotti L., Gasparini F., Gharafi S., Giacomelli L., Ginoulhiac G., Giruzzi G., Giudicotti L., Gonzalez-Martin J., Guillén-González R., Hajnal N., Hall S., Hamada K., Hanada K., Hanada M., Hasegawa K., Hatakeyama S., Hauer V., Hayashi N., Hayashi T., Heller R., Hidalgo-Salaverri J., Higashijima S.

Nuclear Fusion 64 （ 11 ） 112008 2024年11月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00295515

Superconducting (SC) tokamak JT-60SA plays an essential role in fusion research and development by supporting and complementing the ITER project, providing directions to the DEMO design activity and fostering next generation scientists and engineers. Since the short circuit incident at the terminal joints of equilibrium field coil #1 during the integrated commissioning (IC) in March 2021, both EU and JA implementing agencies (IAs) have examined how to ensure safe operation of JT-60SA by mitigating the risk of possible discharge occurrence inside the cryostat. Based on the experience of the global Paschen tests, the IAs have established a strategy of risk mitigation measures, which is a combination of (i) reinforcement of insulation, (ii) avoiding unnecessary voltage application to the coil systems and (iii) immediate de-energization of the coils when deteriorated vacuum conditions are detected. Thanks to the considerable efforts of the Integrated Project Team members, the IC restarted in May 2023. After confirmation of the SC state of the coil systems (TF, EF and CS), the coil energization test and the plasma operation phase 1 (OP-1) started. The first plasma was successfully achieved on 23 October 2023 with a limited value of voltage and current applied to the coils. The plasma configuration control was also confirmed with low plasma current and low auxiliary heating power conditions. Based on the IO-F4E-QST collaboration, activities of JT-60SA have been shared with the IO and provided an important lesson for ITER assembly and commissioning, and will provide an outstanding contribution to fusion research at large. After OP-1, maintenance & enhancement phase 1 (M/E-1) starts from January 2024, in which in-vessel components are installed, and heating and diagnostic systems are extensively upgraded to allow a high power heating experiment planned in OP-2. In order to make the best use of JT-60SA, a newly organized JT-60SA experiment team will refine the research plan for the future high heating power operation phase.

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Emission of high rovibrational hydrogen molecules under detached plasma conditions by recycling on the tungsten wall

Saito S., Nakamura H., Sawada K., Hoshino K., Kojima Y., Doi T., Kobayashi M., Hasuo M., Homma Y., Yamoto S.

Nuclear Fusion 64 （ 12 ） 126067 2024年10月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 00295515

It is well known that the rate coefficient of molecular-assisted recombination (MAR) varies by several orders of magnitude depending on the rovibrational states of the hydrogen molecules. A molecular dynamics simulation is performed to estimate the rovibrational states of recycled hydrogen molecules at the divertor in the JA-DEMO reactor under detached plasma conditions. The simulation results reveal that molecules in high rovibrational states are released even with low incident energy, which will be the dominant condition under detached plasma conditions. Molecules generated in this way can strongly affect the formation of the detached plasma via a molecular assisted-process such as MAR.

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Deep learning model for predicting the spatial distribution of binding energy from atomic configurations

Saito S., Sato S., Nakamura H., Takahashi C., Sawada K., Hoshino K., Kobayashi M., Hasuo M.

Japanese Journal of Applied Physics 63 （ 9 ） 09SP03 2024年08月

研究論文（国際会議プロシーディングス）, 共著, 査読有り, ISSN 00214922

Understanding plasma-material interaction is crucial for achieving steady-state operation of magnetic confinement fusion devices. Kinetic Monte Carlo (kMC) simulation is a powerful tool for investigating the motion of atoms in the plasma facing materials under the influence of this interaction. To predict trapping sites and migration energies necessary for kMC simulations, we developed a deep learning model based on pix2pix for predicting the spatial distribution of binding energy. Results show that the model can reproduce spatial distributions similar to the true values. However, larger errors occur in regions with steep value gradients.

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First results of multi-fluid modeling of detached hydrogen plasmas in a linear plasma device using fluid code LINDA-NU

Sugiura K., Ido T., Tanaka H., Natsume H., Masuda S., Hoshino K., Sawada K., Ohno N.

Contributions to Plasma Physics （Contributions to Plasma Physics） 64 （ 7-8 ） e202300150 2024年06月

研究論文（国際会議プロシーディングス）, 共著, 査読有り, ISSN 08631042

In order to simulate hydrogen (H) plasma in the linear plasma device NAGDIS-II, we have modified the fluid code LINDA-NU to allow the simultaneous calculation of multiple ion species consisting of hydrogen atomic ions ((Formula presented.)) and molecular ions ((Formula presented.)). In this simulation, H and (Formula presented.) neutrals are assumed to be uniformly distributed in space in order to obtain initial qualitative results. The fraction of (Formula presented.) ions increases as the (Formula presented.) molecular density increases, and the recombination process between (Formula presented.) and electrons is observed to reduce the particle flux to the target plate. With an increase in H density, the electron density increases due to the decrease in ion flow velocity due to the change exchange process, and the electron temperature decreases to less than 1 eV, leading to the detached plasma formation attributed to the electron-ion recombination process.

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