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論文

国際核融合エネルギー研究センターの高性能計算機システムHeliosを利用した国内シミュレーション研究プロジェクトの進展

石澤 明宏*; 井戸村 泰宏; 今寺 賢志*; 糟谷 直宏*; 菅野 龍太郎*; 佐竹 真介*; 龍野 智哉*; 仲田 資季*; 沼波 政倫*; 前山 伸也*; et al.

プラズマ・核融合学会誌, 92(3), p.157 - 210, 2016/03

幅広いアプローチ協定に基づいて国際核融合エネルギー研究センター(IFERC)の計算機シミュレーションセンター(CSC)に設置された高性能計算機システムHeliosは、2012年1月に運用を開始し、日欧の磁気核融合シミュレーション研究に供用され、高い利用率の実績を示すとともに、炉心プラズマ物理から炉材料・炉工学にわたる広い分野で多くの研究成果に貢献している。本プロジェクトレビューの目的は、国内の大学や研究機関においてHeliosを利用して進められているシミュレーション研究プロジェクトとその成果を一望するとともに、今後予想される研究の進展を紹介することである。はじめにIFERC-CSCの概要を示した後、各研究プロジェクト毎にその目的、用いられる計算手法、これまでの研究成果、そして今後必要とされる計算を紹介する。

論文

Cross-scale interactions between electron and ion scale turbulence in a tokamak plasma

前山 伸也*; 井戸村 泰宏; 渡邉 智彦*; 仲田 資季*; 矢木 雅敏; 宮戸 直亮; 石澤 明宏*; 沼波 政倫*

Physical Review Letters, 114(25), p.255002_1 - 255002_5, 2015/06

 被引用回数:37 パーセンタイル:5.92(Physics, Multidisciplinary)

実イオン・電子質量比かつ実$$beta$$値のマルチスケールジャイロ運動論的乱流シミュレーションをはじめて実現した。ここで、$$beta$$値はプラズマ圧力と磁気圧の比によって与えられ、微視的不安定性に対する電磁効果を特徴付ける。電子・イオンスケールのスケール間相互作用を明らかにするために両スケールにおける数値解析を行った。実質量比のスケール分離があっても、イオンスケール乱流が電子スケールストリーマを消去し、イオンだけでなく電子の熱輸送も支配する。また、イオンスケールモードが有限$$beta$$効果によって安定化される場合には乱流輸送への電子スケールダイナミクスの寄与は無視できなくなり、イオンスケール乱流輸送を増大させることがわかった。

論文

On push-forward representations in the standard gyrokinetic model

宮戸 直亮; 矢木 雅敏; Scott, B. D.*

Physics of Plasmas, 22(1), p.012103_1 - 012103_9, 2015/01

 被引用回数:2 パーセンタイル:82.1(Physics, Fluids & Plasmas)

標準ジャイロ運動論モデルでは2つの形式的に異なる流体モーメントの押し出し表現を考えることができる。従来から用いられている表現ではジャイロ中心変換に関係する部分がジャイロ中心分布関数の引き戻し表現として現れる。これはジャイロ中心変換を生成するスカラー関数を含むが、通常、その最低次の解しか用いない。これは分極項を含むスカラー流体モーメントの押し出し表現を具体的に求めるときも同様である。しかしながら、分極ドリフトによる粒子フラックスなど有限ラーマ-半径効果による項を含む粒子フラックスの押し出し表現を導くためには、通常よりも高次の解が必要になる。一方、ジャイロ中心部分と案内中心部分が統合され、分布関数の押し出し表現が出てこないもう一つの表現を用いると、最低次の解で十分である。

論文

Fokker-Planck simulation of runaway electron generation in tokamak disruptions

奴賀 秀男; 松山 顕之; 矢木 雅敏; 福山 淳*

Plasma and Fusion Research (Internet), 10, p.1203006_1 - 1203006_2, 2015/01

The runaway electron generation during tokamak disruptions is investigated by kinetic simulations. Three dimensional Fokker-Planck simulations (2D in momentum space and 1D in radial direction) are carried out with various values of $$tau_q$$ and the results are compared with the previous simulation results that use runaway generation rate derived by Conner and Hastie. From the simulation results, It is pointed out that the importance of the hot-tail effect for the description of the runaway electron generation mechanism with fast thermal quench.

論文

Alfv$'{e}$n acoustic channel for ion energy in high-beta tokamak plasmas revisited with a linear gyrokinetic model (LIGKA)

Bierwage, A.; Lauber, P.*; 相羽 信行; 篠原 孝司; 矢木 雅敏

Proceedings of 14th IAEA Technical Meeting on Energetic Particles in Magnetic Confinement Systems (Internet), 8 Pages, 2015/00

A recently proposed Alfv$'{e}$n acoustic self-heating channel for burning plasmas - where fast-ion-driven shear Alfv$'{e}$n waves transfer energy to sound waves which then heat the bulk ions - is reexamined using a linear gyrokinetic model. A local eigenvalue analysis shows that the ion sound branches required for such a self-heating channel are so strongly damped that they are effectively non-existent when $$T_{rm e} approx 1.7times T_{rm i}$$. However, when $$T_{rm e}$$ is increased by a factor 2.8, low-frequency sound continua in the range of beta-induced Alfv$'{e}$n acoustic eigenmodes (BAAE) are recovered and their excitation becomes feasible. This raises the question whether higher-frequency sound branches in the frequency range of beta-induced Alfv$'{e}$n continuum modes (BACM) and the associated self-heating channels may exist under reactor-relevant conditions. Moreover, it is shown that modifications of the continuous spectra by fast ions may need to be taken into account.

論文

Drift resonance effect on stochastic runaway electron orbit in the presence of low-order magnetic perturbations

松山 顕之; 矢木 雅敏; 影井 康弘; 中島 徳嘉*

Nuclear Fusion, 54(12), p.123007_1 - 123007_14, 2014/12

 被引用回数:7 パーセンタイル:48.08(Physics, Fluids & Plasmas)

The stochastization mechanisms of such high-energy RE drift orbits are investigated by 3D orbit following in tokamak plasmas. Drift resonance is shown to play an important role indetermining the onset of stochastic drift orbits for different electron energies, particularly in cases with low-order perturbations that have radially global eigenfunctions. The drift resonance due to the coupling between the cross-field drift motion with radially global modes yields a secondary island structure in the RE drift orbit. Only for highly relativistic REs, the widths of secondary drift islands are comparable with those of magnetic islands due to the primary resonance, thus the stochastic threshold becoming sensitive to the RE energy. Because of poloidal asymmetry due to toroidicity, the thresholdbecomes sensitive not only to the relative amplitude but also to the phase difference between the modes.

論文

Multi-time-scale energetic particle dynamics in JT-60U simulated with MHD activity, sources and collisions

Bierwage, A.; 篠原 孝司; 藤堂 泰*; 矢木 雅敏

Proceedings of 25th IAEA Fusion Energy Conference (FEC 2014) (CD-ROM), 8 Pages, 2014/10

The global nonlinear hybrid code MEGA was extended with a fast ion source and a collision model, so that the formation of the fast ion slowing-down distribution (long time scale, 1ms-1s) and the interaction between the fast ions and MHD waves (short time scale, 0.001-1ms) can be simulated simultaneously. This self-consistent approach, without artificial interfaces, allows to simulate meso-time-scale dynamics (0.1-10ms), such as recurring bursts of MHD activity and the resulting relaxations of the fast ion distribution. When applied to a JT-60U plasma driven by 400 keV negative-ion-based neutral beams (N-NB), the code successfully reproduced experimentally observed bursts of chirping modes. Thus validated, the new multi-time-scale simulations allows for the first time to study the physics of burstiness and frequency chirping in a realistic setting, and first results are presented and discussed. The advances made constitute an important step towards predictive simulations.

論文

Effects of centrifugal modification of magnetohydrodynamic equilibrium on resistive wall mode stability

白石 淳也; 相羽 信行; 宮戸 直亮; 矢木 雅敏

Nuclear Fusion, 54(8), p.083008_1 - 083008_8, 2014/08

 被引用回数:8 パーセンタイル:43.33(Physics, Fluids & Plasmas)

磁気流体力学(MHD: Magnetohydrodynamics)平衡におけるトロイダル回転の遠心力効果を考慮することにより、MHD安定性における回転の効果を自己無撞着に取り入れた。抵抗性壁モード(RWM : Resistive Wall Mode)の安定性に対する回転の効果を解析するため、新しいコードを開発した。真空領域と抵抗性壁における電磁気学的な問題を解くモジュールRWMaCを開発し、回転プラズマにおける線形理想MHDダイナミクスを記述するFrieman-Rotenberg方程式を解くMINERVAコードに組み込んだ。MINERVA/RWMaCにより、トロイダル回転によるMHD平衡の変化が、RWMの成長率を減少させることを初めて示した。加えて、トロイダル回転が線形ダイナミクスにのみ影響を与える場合には安定化窓が存在しない場合でも、平衡の変化により安定化窓が現れることを示した。回転は、平衡の圧力、電流密度、質量密度を変化させ、回転の効果を含むポテンシャルエネルギーを変化させる。

論文

Simulation study of nonlocal transport from edge to core in tokamak plasmas

矢木 雅敏; 松山 顕之; 宮戸 直亮; 滝塚 知典*

Contributions to Plasma Physics, 54(4-6), p.363 - 367, 2014/06

 パーセンタイル:100(Physics, Fluids & Plasmas)

In this work, the simulation study of nonlocal transport from edge to core in tokamak plasmas has been performed using 4-field reduced MHD model. The cylindrical particle source is applied in the plasma edge, after the steady state of resistive ballooning turbulence is attained. After a short time, the source is switched off and the plasma response is investigated. It is found that applying the particle source induces (0,0) and (1,0) modes of density fluctuations, where ($$m,n$$) indicates the set of poloidal mode number $$m$$ and the toroidal mode number n. These modes interact with each other by the nonlinear and/or toroidal couplings. The convective cell mode such as (1,0) mode is attributed to the nonlocal transport instead of the long-range fluctuation such as (1,1) mode excited by the turbulence. The simulation result indicates that two-dimensional transport in the poloidal plane plays a key role to describe the nonlocal transport.

論文

Comparison between kinetic-ballooning-mode-driven turbulence and ion-temperature-gradient-driven turbulence

前山 伸也; 石澤 明宏*; 渡邉 智彦*; 仲田 資季; 宮戸 直亮; 矢木 雅敏; 井戸村 泰宏

Physics of Plasmas, 21(5), p.052301_1 - 052301_12, 2014/05

 被引用回数:7 パーセンタイル:48.08(Physics, Fluids & Plasmas)

Electromagnetic turbulence driven by kinetic ballooning modes (KBMs) in high-beta plasma is investigated based on the local gyrokinetic model. Analysis of turbulent fluxes, norms and phases of fluctuations shows that KBM turbulence gives narrower spectra and smaller phase factors than those in ion-temperature-gradient (ITG)-driven turbulence. This leads to the smaller transport fluxes in KBM turbulence than those in ITG turbulence even when they have similar linear growth rates. From the analysis of the entropy balance relation, it is found that the entropy transfer from ions to electrons through the field-particle interactions mainly drives electron perturbations, which creates radially twisted modes by rapid parallel motions of electrons in a sheared magnetic geometry. The nonlinear coupling between the dominant unstable mode and its twisted modes is important for the saturation of KBM turbulence, in contrast to the importance of zonal flow shearing in ITG turbulence.

論文

Simulation study on nonlocal transport for peripheral density source

矢木 雅敏; 宮戸 直亮; 松山 顕之; 滝塚 知典*

Plasma and Fusion Research (Internet), 9, p.3403030_1 - 3403030_4, 2014/04

The nonlocal transport for peripheral density source is investigated by using 4-field reduced MHD model. The spherical particle source with a small diameter is applied in the plasma edge region, just after the quasi-steady state of resistive ballooning turbulence is attained. Then, the plasma transport is observed. The nonlocal transport appears at the location far from the edge source region. It is found that applying the particle source induces (0,0) and (1,0) modes of density fluctuations as well as (2,1), (3,1) modes, where ($$m$$,$$n$$) indicates the set of poloidal mode number m and the toroidal mode number $$n$$. (0,0) and (1,0) modes dominantly interact with each other by the convective nonlinearity and/or toroidal couplings. The formation of the spiral structure with poloidal rotation is observed, which makes a connection with core and edge regions.

論文

Stochastic transport of runaway electrons due to low-order perturbations in tokamak disruption

松山 顕之; 矢木 雅敏; 影井 康弘*

JPS Conference Proceedings (Internet), 1, p.015037_1 - 015037_4, 2014/03

既存のトカマク装置とITERにおけるディスラプション時の逃走電子の閉じ込め特性を理解するため、低トロイダルモード数の磁場摂動が存在する場合の相対論的ドリフト軌道を数値的に研究した。ディスラプション時のトカマク磁場配位は、良好な磁気面、低トロイダルモードの磁気島、ストキャスティック領域が混在した構造としてモデル化する。逃走電子のエネルギーや装置サイズ依存性に関して、短い時間スケールでの逃走電子損失につながるストキャスティック軌道の出現条件を調べた。その結果、低トロイダルモードと磁力線を横切るドリフト間の結合によって生じる共鳴軌道が磁気島の重畳条件に作用し、逃走電子の高エネルギーテールが低エネルギー成分に比べて相対的に低い磁場揺動レベルで損失することがわかった。短波長磁場乱流の研究で認識されていた高エネルギーの逃走電子ほど輸送係数が小さいという性質と比較して、低トロイダルモードの磁場摂動による逃走電子の損失過程では逆の傾向が現れることを明らかにした。

論文

Relativistic guiding-center equations including slow equilibrium changes in magnetic coordinates

松山 顕之; 矢木 雅敏

Plasma and Fusion Research (Internet), 8, p.1403170_1 - 1403170_6, 2013/12

Guiding-center equations for relativistic particles are presented in axisymmetric toroidal geometry using Boozer coordinates. Effects of slow equilibrium change are included for describing the electron acceleration due to the induction field, which is a fundamental process of the runaway electron generation during disruptions. For a consistent treatment of the runaway orbit in finite pressure plasma, the equations are given in both canonical and noncanonical forms with retaining the radial covariant component of the equilibrium magnetic field. For this, the Lagrangian formulation by White and Zakharov is applied to axisymmetric equilibria with slowly-varying magnetic flux functions.

論文

Development of the transport-code framework for self-consistent predictions of rotation and the radial electric field

本多 充; 井手 俊介; 滝塚 知典*; 林 伸彦; 吉田 麻衣子; 矢木 雅敏; 藤田 隆明

Nuclear Fusion, 53(7), p.073050_1 - 073050_11, 2013/07

 被引用回数:14 パーセンタイル:28.64(Physics, Fluids & Plasmas)

A toroidal momentum solver has been modeled and integrated into the 1.5D integrated transport code TOPICS. A scheme that can uniquely determine the radial electric field $$E_r$$ without an iterative calculation has been developed. It is also used to compute the parallel and toroidal flows for each species based on the neoclassical transport theory. The combination of TOPICS and the orbit-following Monte Carlo code OFMC enables us to self-consistently predict the evolution of not only the density, temperature and safety factor but also the toroidal momentum, following $$E_r$$. The framework developed has been tested against JT-60U experiments and showed the predictive capability of toroidal rotation. Rapid toroidal rotation can be hardly expected in ITER if it is solely driven by neutral beam injection, whereas the residual stress may potentially generate the torque comparable to or greater than that by neutral beams.

論文

On the Gyrokinetic model in long wavelength regime

宮戸 直亮; Scott, B. D.*; 矢木 雅敏

Plasma Physics and Controlled Fusion, 55(7), p.074011_1 - 074011_6, 2013/07

 被引用回数:5 パーセンタイル:66.51(Physics, Fluids & Plasmas)

The standard gyrokinetic model is formulated for perturbations with short wavelength and small amplitude. In the long wavelength regime, the polarisation density term with electrostatic potential $$varphi$$ in the gyrokinetic quasi-neutrality equation goes to higher order than usual. Therefore, we should consider the other higher order terms to obtain $$varphi$$ from the equation. This issue is investigated from the point of view of the push-forward representation of particle density. The polarisation density term stems from the ${it gyro-centre}$ displacement vector. There is another piece associated with the ${it guiding-centre}$ transformation which is not considered in the standard model. The gyrokinetic Hamiltonian is modified by taking into account this piece. The quasi-neutrality equation is derived rigorously from the modified Hamiltonian through a variational method and we find several additional terms resulting from non-uniformity of magnetic field.

論文

Nonlinear variational method for predicting fast collisionless magnetic reconnection

廣田 真; Morrison, P. J.*; 石井 康友; 矢木 雅敏; 相羽 信行

Nuclear Fusion, 53(6), p.063024_1 - 063024_11, 2013/06

 被引用回数:7 パーセンタイル:54.94(Physics, Fluids & Plasmas)

トカマク放電中の鋸歯状崩壊現象を理解するため、無衝突プラズマ中で起きる速い磁気リコネクションのメカニズムを解析した。非線形の変位写像に対するエネルギー原理を用いることで、電子慣性によって駆動されるテアリングモードの爆発的成長を解析的に予測した。(磁気島の幅が電子慣性長より大きくなった)非線形段階では、ポテンシャルエネルギーの減少が線形段階よりも急峻になることがわかり、それがリコネクションの加速の要因となる。こうした理想流体の運動による自由エネルギーの解放は、非定常かつ強い対流を生成し、それは高温トカマクにおける小さな散逸効果では阻止することはできない。スラブ配位における直接数値シミュレーションにより、この非線形成長の理論予測を立証した。

論文

Stabilization of resistive wall modes by magnetohydrodynamic equilibrium change induced by plasma toroidal rotation

白石 淳也; 相羽 信行; 宮戸 直亮; 矢木 雅敏

Proceedings of 24th IAEA Fusion Energy Conference (FEC 2012) (CD-ROM), 8 Pages, 2013/03

磁気流体力学(MHD: Magnetohydrodynamics)平衡におけるトロイダル回転の遠心力効果を考慮することにより、MHD安定性における回転の効果を自己無撞着に取り入れた。抵抗性壁モード(RWM: Resistive Wall Mode)の安定性に対する回転の効果を解析するため、新しいコードを開発した。真空領域と抵抗性壁における電磁気学的な問題を解くモジュールRWMaCを開発し、回転プラズマにおける線形理想MHDダイナミクスを記述するFrieman-Rotenberg方程式を解くMINERVAコードに組み込んだ。MINERVA/RWMaCにより、トロイダル回転によるMHD平衡の変化が、RWMの成長率を減少させることを初めて示した。加えて、トロイダル回転が線形ダイナミクスにのみ影響を与える場合には安定化窓が存在しない場合でも、平衡の変化により安定化窓が現れることを示した。回転は、平衡の圧力,電流密度,質量密度を変化させ、回転の効果を含むポテンシャルエネルギーを変化させる。

論文

Nonlinear simulation of energetic particle modes in JT-60U

Bierwage, A.; 相羽 信行; 篠原 孝司; 藤堂 泰*; Deng, W.*; 石川 正男; 松永 剛; 矢木 雅敏

Proceedings of 24th IAEA Fusion Energy Conference (FEC 2012) (CD-ROM), 8 Pages, 2012/10

Energetic particle mode (EPM) dynamics are simulated using a global nonlinear hybrid code (MEGA). The scenario considered is based on JT-60U shot E039672, just before the onset of a so-called Abrupt Large Event (ALE). The simulation model is gradually extended towards a more realistic representation of the JT-60U plasma, including realistic geometry, realistic pressure, and realistic fast ion distributions. It is found that the mode structure and frequency of the EPM is very robust and can be reproduced by simple models. In contrast, accurate prediction of the growth rate, fluctuation amplitude and fast ion transport may require realistic models. The progress made - in particular, the use of realistic geometry and a realistic fast ion distribution computed by an orbit-following Monte-Carlo code - constitutes an important step forward towards predictive simulations and integrated modelling of energetic particle dynamics in burning plasmas, such as in ITER and DEMO.

論文

Nonlinear simulation of energetic particle modes in high-beta tokamak plasma

Bierwage, A.; 相羽 信行; 藤堂 泰*; Deng, W.*; 石川 正男; 松永 剛; 篠原 孝司; 矢木 雅敏

Plasma and Fusion Research (Internet), 7, p.2403081_1 - 2403081_4, 2012/07

The global hybrid code MEGA is used to study the dynamics of energetic particle modes (EPM) in parameter regimes close to the onset of ideal magnetohydrodynamic (MHD) ballooning instability. This work is motivated by observations of energetic-particle-driven modes in the wall-stabilized regime (so called EWM) in JT-60U, the physics of which have not been fully understood yet. In this study, nonlinear simulations of energetic particle modes (EPM, high frequency, 40-60 kHz) are carried out under conditions, where EWM (low frequency, 1-3 kHz) are observed. Although, EPM are unlikely to interact with EWM directly, the energetic particle transport caused by EPM may affect the total pressure profile and, hence, the stability of kink-ballooning modes, which are thought to be the fundamental modes underlying EWM. The dynamics of EPM near ideal MHD ballooning stability boundary is examined for the first time.

論文

Simulation study of L/H transition with self-consistent integrated modelling of core and SOL/divertor transport

矢木 雅敏; 清水 勝宏; 滝塚 知典; 本多 充; 林 伸彦; 星野 一生; 福山 淳*

Contributions to Plasma Physics, 52(5-6), p.372 - 378, 2012/06

 被引用回数:4 パーセンタイル:74.44(Physics, Fluids & Plasmas)

We have developed a self-consistent integrated modelling of core and SOL/divertor transport. Thereby it enables us to investigate operation scenarios to be compatible with high confinement core plasma and detached divertor plasmas. A 1.5D core code TOPICS-IB and a 2D divertor code SONIC were coupled successively with a Multiple Program Multiple Data parallel computing system. For an integrated code including Monte-Carlo calculations, this system makes it possible to perform efficient simulations. The dynamic simulation for the L/H transition in JT-60SA is carried out by this integrated code with a CDBM transport model including the E$$times$$B shearing effect. Impacts of SOL/divertor transport on the L/H transition is studied.

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