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Journal Articles

Multi-scale turbulence simulation suggesting improvement of electron heated plasma confinement

Maeyama, Shinya*; Watanabe, Tomohiko*; Nakata, Motoki*; Nunami, Masanori*; Asahi, Yuichi; Ishizawa, Akihiro*

Nature Communications (Internet), 13, p.3166_1 - 3166_8, 2022/06

 Times Cited Count:4 Percentile:90.56(Multidisciplinary Sciences)

Turbulent transport is a key physics process for confining magnetic fusion plasma. Recent theoretical and experimental studies of existing fusion experimental devices revealed the existence of cross-scale interactions between small (electron)-scale and large (ion)-scale turbulence. Since conventional turbulent transport modelling lacks cross-scale interactions, it should be clarified whether cross-scale interactions are needed to be considered in future experiments on burning plasma, whose high electron temperature is sustained with fusion-born alpha particle heating. Here, we present supercomputer simulations showing that electron scale turbulence in high electron temperature plasma can affect the turbulent transport of not only electrons but also fuels and ash. Electron-scale turbulence disturbs the trajectories of resonant electrons responsible for ion-scale micro-instability and suppresses large-scale turbulent fluctuations. Simultaneously, ion-scale turbulent eddies also suppress electron-scale turbulence. These results indicate a mutually exclusive nature of turbulence with disparate scales. We demonstrate the possibility of reduced heat flux via cross-scale interactions.

Journal Articles

Implementation of a gyrokinetic collision operator with an implicit time integration scheme and its computational performance

Maeyama, Shinya*; Watanabe, Tomohiko*; Idomura, Yasuhiro; Nakata, Motoki*; Nunami, Masanori*

Computer Physics Communications, 235, p.9 - 15, 2019/02

 Times Cited Count:5 Percentile:52.31(Computer Science, Interdisciplinary Applications)

We have implemented the Sugama collision operator in the gyrokinetic Vlasov simulation code, GKV, with an implicit time-integration scheme. The new method is versatile and independent of the details of the linearized collision operator, by means of an operator splitting, an implicit time integrator, and an iterative Krylov subspace solver. Numerical tests demonstrate stable computation over the time step size restricted by the collision term. An efficient implementation for parallel computation on distributed memory systems is realized by using the data transpose communication, which makes the iterative solver free from inter-node communications during iteration. Consequently, the present approach achieves enhancement of computational efficiency and reduction of computational time to solution simultaneously, and significantly accelerates the total performance of the application.

Journal Articles

Cross-scale interactions between turbulence driven by electron and ion temperature gradients via sub-ion-scale structures

Maeyama, Shinya*; Watanabe, Tomohiko*; Idomura, Yasuhiro; Nakata, Motoki*; Ishizawa, Akihiro*; Nunami, Masanori*

Nuclear Fusion, 57(6), p.066036_1 - 066036_10, 2017/05

 Times Cited Count:13 Percentile:64.76(Physics, Fluids & Plasmas)

Multi-scale plasma turbulence including electron and ion temperature gradient (ETG/ITG) modes has been investigated by means of electromagnetic gyrokinetic simulations. Triad transfer analyses on nonlinear mode coupling reveal cross-scale interactions between electron and ion scales. One of the interactions is suppression of electron-scale turbulence by ion- scale turbulence, where ITG-driven short-wavelength eddies act like shear flows and suppress ETG turbulence. Another cross-scale interaction is enhancement of ion-scale turbulence in the presence of electron-scale turbulence. This is caused via short-wavelength zonal flows, which are created by the response of passing kinetic electrons in ITG turbulence, suppress ITG turbulence by their shearing, and are damped by ETG turbulence. In both cases, sub-ion-scale structures between electron and ion scales play important roles in the cross-scale interactions.

Journal Articles

Progress report of Japanese simulation research projects using the high-performance computer system Helios in the International Fusion Energy Research Centre

Ishizawa, Akihiro*; Idomura, Yasuhiro; Imadera, Kenji*; Kasuya, Naohiro*; Kanno, Ryutaro*; Satake, Shinsuke*; Tatsuno, Tomoya*; Nakata, Motoki*; Nunami, Masanori*; Maeyama, Shinya*; et al.

Purazuma, Kaku Yugo Gakkai-Shi, 92(3), p.157 - 210, 2016/03

The high-performance computer system Helios which is located at The Computational Simulation Centre (CSC) in The International Fusion Energy Research Centre (IFERC) started its operation in January 2012 under the Broader Approach (BA) agreement between Japan and the EU. The Helios system has been used for magnetised fusion related simulation studies in the EU and Japan and has kept high average usage rate. As a result, the Helios system has contributed to many research products in a wide range of research areas from core plasma physics to reactor material and reactor engineering. This project review gives a short catalogue of domestic simulation research projects. First, we outline the IFERC-CSC project. After that, shown are objectives of the research projects, numerical schemes used in simulation codes, obtained results and necessary computations in future.

Journal Articles

Improved strong scaling of a spectral/finite difference gyrokinetic code for multi-scale plasma turbulence

Maeyama, Shinya; Watanabe, Tomohiko*; Idomura, Yasuhiro; Nakata, Motoki; Nunami, Masanori*; Ishizawa, Akihiro*

Parallel Computing, 49, p.1 - 12, 2015/11

 Times Cited Count:7 Percentile:54.32(Computer Science, Theory & Methods)

Journal Articles

Effects of toroidal rotation shear and magnetic shear on thermal and particle transport in plasmas with electron cyclotron heating on JT-60U

Yoshida, Maiko; Honda, Mitsuru; Narita, Emi*; Hayashi, Nobuhiko; Urano, Hajime; Nakata, Motoki; Miyato, Naoaki; Takenaga, Hidenobu; Ide, Shunsuke; Kamada, Yutaka

Nuclear Fusion, 55(7), p.073014_1 - 073014_9, 2015/07

 Times Cited Count:14 Percentile:60.53(Physics, Fluids & Plasmas)

Conditions without the increases in the thermal and particle transport with ECH have been experimentally investigated in positive magnetic shear (PS), weak magnetic shear (WS) and reversed magnetic shear (RS) plasmas with internal transport barriers (ITBs) on JT-60U. The ion heat diffusivity around an internal transport barrier in the ion temperature ($$T_{rm i}$$-ITB) remains constant with ECH when a large negative toroidal rotation shear is formed before the ECH. The condition does not depend on the electron to ion temperature ratio ($$T_{rm e}$$/$$T_{rm i}$$) and ECH power. The electron heat diffusivity around a $$T_{rm e}$$-ITB stays constant with ECH when the magnetic shear is negative around the Te-ITB region. Effective particle transport remains constant or reduces during ECH under the condition of negative magnetic shear.

Journal Articles

Integrated modelling of toroidal rotation with the 3D non-local drift-kinetic code and boundary models for JT-60U analyses and predictive simulations

Honda, Mitsuru; Satake, Shinsuke*; Suzuki, Yasuhiro*; Yoshida, Maiko; Hayashi, Nobuhiko; Kamiya, Kensaku; Matsuyama, Akinobu; Shinohara, Koji; Matsunaga, Go; Nakata, Motoki; et al.

Nuclear Fusion, 55(7), p.073033_1 - 073033_11, 2015/07

 Times Cited Count:5 Percentile:25.37(Physics, Fluids & Plasmas)

Journal Articles

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

Maeyama, Shinya*; Idomura, Yasuhiro; Watanabe, Tomohiko*; Nakata, Motoki*; Yagi, Masatoshi; Miyato, Naoaki; Ishizawa, Akihiro*; Nunami, Masanori*

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

 Times Cited Count:81 Percentile:95.09(Physics, Multidisciplinary)

Multiscale gyrokinetic turbulence simulations with the real ion-to-electron mass ratio and $$beta$$ value are realized for the first time, where the $$beta$$ value is given by the ratio of plasma pressure to magnetic pressure and characterizes electromagnetic effects on microinstabilities. Numerical analysis at both the electron scale and the ion scale is used to reveal the mechanism of their cross-scale interactions. Even with the real- mass scale separation, ion-scale turbulence eliminates electron-scale streamers and dominates heat transport, not only of ions but also of electrons. When the ion-scale modes are stabilized by finite-$$beta$$ effects, the contribution of the electron-scale dynamics to the turbulent transport becomes non-negligible and turns out to enhance ion-scale turbulent transport.

Journal Articles

Roles of argon seeding in energy confinement and pedestal structure in JT-60U

Urano, Hajime; Nakata, Motoki; Aiba, Nobuyuki; Kubo, Hirotaka; Honda, Mitsuru; Hayashi, Nobuhiko; Yoshida, Maiko; Kamada, Yutaka; JT-60 Team

Nuclear Fusion, 55(3), p.033010_1 - 033010_9, 2015/03

 Times Cited Count:35 Percentile:88.32(Physics, Fluids & Plasmas)

Physics picture of improving energy confinement with argon seeding at high density has been investigated in JT-60U. Better confinement is sustained at high density by argon seeding accompanied by higher core and pedestal temperatures. Peaked density profiles are kept with argon seeding. Density peaking and dilution effects lower the pedestal density at a given averaged density. The pedestal density in the argon seeded plasmas which is relatively lower than that in a single deuterium puff enables the pedestal temperature to be higher. The density peaking is a key factor of sustaining better confinement in argon seeded H-mode plasmas.

Journal Articles

Integrated modeling of toroidal rotation with the 3D non-local drift-kinetic code and boundary models for JT-60U analyses and predictive simulations

Honda, Mitsuru; Satake, Shinsuke*; Suzuki, Yasuhiro*; Yoshida, Maiko; Hayashi, Nobuhiko; Kamiya, Kensaku; Matsuyama, Akinobu; Shinohara, Koji; Matsunaga, Go; Nakata, Motoki; et al.

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

Journal Articles

Role of seed impurity for H-mode plasmas in JT-60U

Urano, Hajime; Nakata, Motoki; Aiba, Nobuyuki; Kubo, Hirotaka; Honda, Mitsuru; Yoshida, Maiko; Hayashi, Nobuhiko; Kamada, Yutaka; JT-60 Team

Europhysics Conference Abstracts (Internet), 38F, p.P4.018_1 - P4.018_4, 2014/06

This paper reports the analysis result of heat transport and pedestal structure in H-mode plasmas with and without argon seeding on JT-60U.

Journal Articles

Exploring phase space turbulence in magnetic fusion plasmas

Watanabe, Tomohiko*; Idomura, Yasuhiro; Maeyama, Shinya; Nakata, Motoki; Sugama, Hideo*; Nunami, Masanori*; Ishizawa, Akihiro*

Journal of Physics; Conference Series, 510, p.012045_1 - 012045_11, 2014/05

 Times Cited Count:1 Percentile:51.27

Plasma turbulence accompanied with fluctuations of the distribution function and the electromagnetic fields develops on the phase space composed of the configuration space and the velocity space. Detailed structures of the distribution function in magnetic fusion plasmas are investigated by means of gyrokinetic simulations performed on massively parallel supercomputers. The gyrokinetic simulations of drift wave turbulence have demonstrated entropy transfer in the phase space, zonal flow enhancement by helical fields and the resultant transport reduction. The state-of-the-art high performance computing is utilized for a multi- scale turbulence simulation covering ion- and electron-scales and for a global-scale simulation of turbulent transport in a sub-ITER sized plasma.

Journal Articles

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

Maeyama, Shinya; Ishizawa, Akihiro*; Watanabe, Tomohiko*; Nakata, Motoki; Miyato, Naoaki; Yagi, Masatoshi; Idomura, Yasuhiro

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

 Times Cited Count:15 Percentile:61.45(Physics, Fluids & Plasmas)

Journal Articles

Progress of full-f gyrokinetic simulation toward reactor relevant numerical experiments

Idomura, Yasuhiro; Nakata, Motoki; Jolliet, S.*

Plasma and Fusion Research (Internet), 9(Sp.2), p.3503028_1 - 3503028_7, 2014/04

Full-f gyrokinetic simulations compute both turbulent transport and profile formations under fixed power, momentum, and particle input as in experiments. This approach has the capability of dictating plasma profiles, provided that time scale of the simulation is long enough to establish power, momentum, and particle balance conditions. Recent Peta-scale supercomputers made such long time scale simulations feasible, and full-f gyrokinetic simulations are applied to reactor relevant numerical experiments. In this paper, physical models, numerical approaches, and accuracy issues of the gyrokinetic full-f Eulerian code GT5D are summarized, and then, its recent applications to the scaling studies of turbulent transport with respect to plasma size and heating power are reviewed.

Journal Articles

Local gyrokinetic Vlasov simulations with realistic tokamak MHD equilibria

Nakata, Motoki; Matsuyama, Akinobu; Aiba, Nobuyuki; Maeyama, Shinya; Nunami, Masanori*; Watanabe, Tomohiko*

Plasma and Fusion Research (Internet), 9, p.1403029_1 - 1403029_12, 2014/04

A local gyrokinetic Vlasov simulation code GKV is extended to incorporate realistic tokamak equilibria including up-down asymmetry, which are produced by a free-boundary 2D Grad-Shafranov equation solver MEUDAS. The developed codes have been verified by a cross-code benchmark test using Cyclone-base-case like MHD equilibrium, where good agreement in the dispersion relation of ion temperature gradient (ITG) driven mode has been confirmed. The extended GKV is applied to two types of shaped plasmas expected in JT-60SA tokamak devices, i.e., ITER-like and highly-shaped plasmas, and ITG-mode stability and residual zonal-flow level are investigated. Through the detailed comparisons, more favorable stability properties against the ITG mode are revealed for the highly-shaped case, where the lower ITG-mode growth rate and higher residual zonal-flow levels compared to the ITER-like case are identified.

Journal Articles

Kinetic ballooning mode turbulence simulation based on electromagnetic gyrokinetics

Maeyama, Shinya; Ishizawa, Akihiro*; Watanabe, Tomohiko*; Nakata, Motoki; Miyato, Naoaki; Idomura, Yasuhiro

Plasma and Fusion Research (Internet), 9, p.1203020_1 - 1203020_3, 2014/03

Journal Articles

Communication-overlap techniques for improved strong scaling of gyrokinetic Eulerian code beyond 100k cores on the K-computer

Idomura, Yasuhiro; Nakata, Motoki; Yamada, Susumu; Machida, Masahiko; Imamura, Toshiyuki*; Watanabe, Tomohiko*; Nunami, Masanori*; Inoue, Hikaru*; Tsutsumi, Shigenobu*; Miyoshi, Ikuo*; et al.

International Journal of High Performance Computing Applications, 28(1), p.73 - 86, 2014/02

 Times Cited Count:17 Percentile:77.32(Computer Science, Hardware & Architecture)

Journal Articles

Plasma size and power scaling of ion temperature gradient driven turbulence

Idomura, Yasuhiro; Nakata, Motoki

Physics of Plasmas, 21(2), p.020706_1 - 020706_4, 2014/02

 Times Cited Count:17 Percentile:63.73(Physics, Fluids & Plasmas)

The transport scaling with respect to plasma size and heating power is studied for ion temperature gradient driven turbulence using a fixed-flux full-$$f$$ gyrokinetic Eulerian code. It is found that when heating power is scaled with plasma size, the ion heat diffusivity increases with plasma size in a local limit regime, where $$delta$$$$f$$ simulations predict a gyro-Bohm scaling. In the local limit regime, the transport scaling is strongly affected by the stiffness of ion temperature profiles, which is related to the power degradation of confinement.

Journal Articles

Plasma size and collisionality scaling of ion-temperature-gradient-driven turbulence

Nakata, Motoki; Idomura, Yasuhiro

Nuclear Fusion, 53(11), p.113039_1 - 113039_16, 2013/11

 Times Cited Count:20 Percentile:66.63(Physics, Fluids & Plasmas)

Fixed-flux (FF), fixed-gradient (FG), and local fluxtube (FT) gyrokinetic simulations are systematically compared for ion temperature gradient (ITG) driven turbulence. The collisionality ($$nu^{ast}$$) dependence of ion heat diffusivity is verified through the inter-model comparisons. When the temperature gradient is far from the nonlinear critical value, the FF and FT models give a weak $$nu^{ast}$$-dependence, while the FG model shows a strong $$nu^{ast}$$-dependence. It has been revealed that the FG model provides such strong $$nu^{ast}$$-dependence through the change of ITG-mode stability due to $$nu^{ast}$$-dependent deformation of the velocity distribution function. The plasma size ($$rho^{ast}$$) scan in the FF simulations show a Bohm like transport scaling even in a local limit regime, $$rho^{ast -1} ge 300$$, where profile shear effects are weak. It has been clarified that the transient variations of local power balance are essential mechanisms leading to the Bohm like heat transport even at similar mean temperature gradients, where the burst amplitude and its frequency increase with the plasma size and the heating power. The mechanism is unique to the FF model. Comparisons of statistical characteristics in the local limit regime show differences in frequency spectra and probability density functions of the heat flux, while zonal flow structures and avalanche propagations properties are similar among these models.

Journal Articles

Computation-communication overlap techniques for parallel spectral calculations in gyrokinetic Vlasov simulations

Maeyama, Shinya; Watanabe, Tomohiko*; Idomura, Yasuhiro; Nakata, Motoki; Nunami, Masanori*; Ishizawa, Akihiro*

Plasma and Fusion Research (Internet), 8, p.1403150_1 - 1403150_8, 2013/11

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