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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.
Miki, Kazuhiro; Idomura, Yasuhiro
Plasma and Fusion Research (Internet), 10(Sp.2), p.3403068_1 - 3403068_4, 2015/07
We identify linear properties of the energetic-particle-induced geodesic acoustic mode (EGAM) using eigenmode analysis based on the gyrokinetic theory. From the perturbed gyrokinetic equation with energetic particles, we derive a dispersion relation of the EGAM. The behaviors of the roots vary depending on the safety factor. Taking into account of the finite-orbit-width (FOW) effects, we examine variations of the growth rates of the EGAM for various beam intensities. The analyses indicate that the FOW effects are small, within several percent of the growth rates, for experimentally relevant machine sizes.
H transitionsMiki, Kazuhiro; Diamond, P. H.*; Hahn, S.-H.*; Xiao, W. W.*; G
rcan, 
. D.*; Tynan, G. R.*
Physics of Plasmas, 20(8), p.082304_1 - 082304_11, 2013/08
Times Cited Count：14 Percentile：52.85(Physics, Fluids & Plasmas)We report on model studies of stimulated LH transitions. These studies use a reduced mesoscale model. Model studies reveal that LH transition can be triggered by particle injection into a subcritical state. Particle injection changes edge mean flow shear via changes of density and temperature gradients. The change of edge mean flow shear is critical to turbulence collapse and the subsequent stimulated transition. For low ambient heating, strong injection is predicted to trigger a transient turbulence collapse. Repetitive injection at a period less than the lifetime of the collapsed state can thus maintain the turbulence collapse. The total number of injected particles required is much smaller than that required for a transition by gas puffing. We thus show that internal injection is more efficient than gas puffing of comparable strength. We also observe that zonal flows do not play a critical role in stimulated transitions.
H and HL transitionsMiki, Kazuhiro; Diamond, P. H.*; Fedorczak, N.*; Grcan, . D.*; Malkov, M.*; Lee, C.*; Kosuga, Yusuke*; Tynan, G. R.*; Xu, G. S.*; Estrada, T.*; et al.
Nuclear Fusion, 53(7), p.073044_1 - 073044_10, 2013/07
Times Cited Count：24 Percentile：72.36(Physics, Fluids & Plasmas)Understanding the LH and HL transitions is crucial to successful ITER operation. In this paper we present novel theoretical and modelling study results on the spatio-temporal dynamics of the transition. We place a special emphasis on the role of zonal flows and the micromacro connection between dynamics and the power threshold dependences. The model studied evolves five coupled fields in time and one space dimension, in simplified geometry. The content of this paper is (a) the model fundamentals and the space-time evolution during the LIH transition, (b) the physics origin of the well-known 
B-drift asymmetry in power threshold, (c) the role of heat avalanches in the intrinsic variability of the LH transition, (d) the dynamics of the HL back transition and the physics of hysteresis.
L back transitionMiki, Kazuhiro; Diamond, P. H.*; Schmitz, L.*; McDonald, D. C.*; Estrada, T.*; Grcan, . D.*; Tynan, G. R.*
Physics of Plasmas, 20(6), p.062304_1 - 062304_9, 2013/06
Times Cited Count：19 Percentile：64.31(Physics, Fluids & Plasmas)Since ITER will operate close to threshold and with limited control, the HL back transition is a topic important for machine operations as well as physics. Using a reduced mesoscale model, we investigate ELM-free HL back transition dynamics in order to isolate transport physics effects. Model studies indicate that turbulence spreading is the key process which triggers the back transition. The transition involves a feedback loop linking turbulence and profiles. The I-phase appears during the back transition following a slow power ramp down, while fast ramp-downs reveal a single burst of zonal flow during the back transition. The I-phase nucleates at the pedestal shoulder, as this is the site of the residual turbulence in H-mode. Hysteresis in the profile gradient scale length is characterized by the Nusselt number. Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number.
Kishimoto, Yasuaki; Miki, Kazuhiro*; Li, J. Q.*; Miyato, Naoaki; Wang, Z. X.*; Anderson, J.*
Proceedings of 22nd IAEA Fusion Energy Conference (FEC 2008) (CD-ROM), 8 Pages, 2008/10
A new class of transient transport near the critical gradient (CG) referred to as GAM growing intermittency due to the collision-less GAM damping has been found based on Landau-fluid simulation. Here, we present a new predator-prey model to understand the essential features of the growing intermittency. We have also extended the simulation model by taking into account the collisional zonal flow (ZF) damping. Due to the mixture of two kinds of damping mechanisms, i.e. the GAM damping and collisional damping, the growing intermittency is found to recursively appear accompanied with complex envelope modulation to ZFs over collisional (or transport) time scale. Furthermore, we have investigated the effect of zonal pressure (ZP) near the CG, which also works as a dissipation mechanism. The multiple dissipation mechanisms are found to synergetically couple each other and lead plasmas to complex dynamical transport over long time scale.
Li, J. Q.*; Kishimoto, Yasuaki; Miyato, Naoaki; Miki, Kazuhiro*; Anderson, J.*; Shi, B. R.*
Journal of Physics; Conference Series, 123, p.012027_1 - 012027_11, 2008/08
Times Cited Count：2 Percentile：61.73(Physics, Fluids & Plasmas)The nonlinear excitation and saturation mechanism of geodesic acoustic mode (GAM), as well as its radial structure, in tokamak plasmas are investigated by applying a newly well-benchmarked gyrofluid model. At first, an empirical closure relation for the conventional three-field gyrofluid modeling is presented for ion temperature gradient (ITG) fluctuations and the GAMs. The zonal flow (ZF) damping is precisely examined by comparing with theoretical predictions and other kinetic calculations. Then, a local code and the global version are advanced to simulate the nonlinear excitation of the GAMs by ITG fluctuations. It is found that the GAM instability can be nonlinearly excited under the competition between the nonlinear driving and the collisionless damping. The pump amplitude threshold of the GAM instability is higher than that of the ZF instability. Meanwhile, the unstable GAMs are mainly saturated by the intrinsic Landau damping. Furthermore, the radial structure of the GAMs is shown as 
.
Miki, Kazuhiro*; Kishimoto, Yasuaki; Li, J.*; Miyato, Naoaki
Physics of Plasmas, 15(5), p.052309_1 - 052309_14, 2008/05
Times Cited Count：13 Percentile：46.66(Physics, Fluids & Plasmas)Effects of geodesic acoustic modes (GAMs) on toroidal ion temperature gradient turbulence and associated transport near the critical gradient regime in tokamak plasma are investigated based on global Landau-fluid simulations and extended predator-prey modeling analyses. A new type of intermittent dynamics of transport accompanied with emission and propagation of the GAMs has been found. The GAMs transfer turbulence energy to wide radial region nonlocally. Stationary zonal flows gradually grow over many periods of quasi-periodic intermittent bursts and quench the turbulence, leading to a nonlinear up-shift of the linear critical gradient, the Dimits shift. This process is categorized as a new class of transient dynamics, growing intermittency. An extended minimal predator-prey model is proposed, which qualitatively reproduces the main features of the growing intermittency and approximately predicts its various time scales observed in the simulations.
Miki, Kazuhiro*; Kishimoto, Yasuaki; Miyato, Naoaki; Li, J. Q.*
Journal of Physics; Conference Series, 123, p.012028_1 - 012028_14, 2008/00
Times Cited Count：4 Percentile：78.33(Physics, Fluids & Plasmas)We have constructed a four-field minimum model that describes the growing intermittency of turbulence associated with the geodesic acoustic mode (GAM) observed in our toroidal Landau-fluid simulations. The intermittent dynamics are well reproduced by the model for the reference parameters used in the simulation. The model can also reproduce more nature of turbulent transport associated with the GAM, such as a single burst leading to a full quench of turbulence and also a steady state turbulence mixed with steady state zonal flows and GAMs. Investigating the behaviour of the solution trajectories around the fixed points in four-dimensional phase space, we examine the comprehensive properties of the model and identify the bifurcation property between Dimits shift and steady state turbulence regimes, which correspond to different eigen-states.
Miki, Kazuhiro*; Kishimoto, Yasuaki; Miyato, Naoaki; Li, J. Q.*
Physical Review Letters, 99(14), p.145003_1 - 145003_4, 2007/10
Times Cited Count：44 Percentile：82.91(Physics, Multidisciplinary)Turbulent transport near critical gradient in toroidal plasmas is studied based on global Landau-fluid simulations and an extended predator-prey theoretical model of ion temperature gradient (ITG) turbulence. A new type of intermittent transport associated with the emission and propagation of geodesic acoustic mode (GAM) is found near critical gradient regime, which is referred to GAM intermittency. The intermittency is characterized by new time scales of trigger, damping, and recursion due to GAM damping. During the recursion of intermittent bursts, stationary zonal flow increases with a slow time scale due to the accumulation of undamped residues and eventually quenches the turbulence, suggesting that a nonlinear up-shift of critical gradient, i.e. Dimits shift, is established through such a dynamical process.
Kishimoto, Yasuaki; Miki, Kazuhiro*; Miyato, Naoaki; Li, J. Q.*; Anderson, J.*
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
We found a new type of intermittent dynamics of transport near critical gradient associated with the emission and propagation of geodesic acoustic mode (GAM) coupled with zonal flow based on our global gyro-fluid ITG turbulence simulations. We may refer the prominent dynamics to GAM intermittency. The intermittent bursts are triggered by the onset of spatially propagating GAM when the turbulent energy exceeds a critical value. The GAMs suffer a collisionless damping during the propagation and nonlocally transfer local turbulence energy to wide radial region as that of zonal flow. The stationary component of zonal flows gradually increases over many periods of quasi-periodic intermittent bursts and eventually quenches the turbulence, leading to a nonlinear up-shift of the linear critical gradient, i.e., the Dimits shift. The gradual increase of zonal flow results from the accumulation of non-damped residual part during the propagation of GAMs in each cycle of bursts. The period of burst becomes shorter as the temperature gradient increases. An extended minimum model including dynamical GAM damping which qualitatively describes the intermittent dynamics obtained in our simulation is presented.
Miki, Kazuhiro
no journal, ,
A central issue in H-mode physics is to achieve control, not only understanding. Control of the L-H transition is desirable, on account of the successful fusion power plants. We here report on model studies of stimulated L-H transitions. We have developed a one-dimensional mesoscale model. Numerical studies reveal that L-H transitions can be triggered by particle injection into a subcritical state. Particle injection alters the edge mean flow shear. For low ambient heating, strong injection is predicted to trigger a transient turbulence collapse. We predict that repetitive injection at a period less than the lifetime of the collapsed state can thus maintain the turbulence collapse and so sustain a driven H-mode-like state. We also observe that zonal flows do not play a critical role in stimulated transitions.
Miki, Kazuhiro
no journal, ,
External heating of magnetic confined plasmas can trigger an improvement of edge plasma confinement, i.e. L-H transition. Recent phenomenological progress on the transition has mainly focused on the identification of quasi-periodic oscillation of turbulence fluctuations, i.e. limit-cycle oscillation. We here report the role of zonal flows on triggering L-H transition, using a developed one-dimensional reduced predator-prey model. We also propose transient particle injection can be another trigger of the L-H transition. Besides, we introduce other recent theoretical and numerical studies of the L-H transition.
Miki, Kazuhiro; Diamond, P. H.*; Hahn, S.-H.*; Xiao, W. W.*; Grcan, . D.*; Tynan, G. R.*
no journal, ,
A central issue in H-mode physics is to achieve control, not only understanding. Work in control has focused mainly on fueling by pellet injection at the near edge. We here introduce a developed one-dimensional mesoscale model, to report on the L-H transition stimulated by pellet injection. For highly heated plasmas, transition occurs spontaneously; when heat flux increases, a strong zonal flow is sufficiently excited, leading to I-phase oscillations or a single burst. When the zonal flow is excited, turbulence is reduced, allowing an ion pressure gradient to steepen. Then, mean flow shear increases to lock in the H-mode. On the other hand, studies reveal that L-H transition can be triggered by particle injection into a subcritical state. We also observe that zonal flows do not play a critical role in stimulated transitions.
Miki, Kazuhiro; Diamond, P. H.*; Schmitz, L.*; McDonald, D. C.*; Gurcan, O.*; Tynan, G. R.*
no journal, ,
Since ITER will operate close to threshold and with limited control, the H-L back transition is a topic important for machine operations as well as physics. Using a reduced mesoscale model, we investigate ELM-free H-L back transition dynamics. Model studies indicate that turbulence spreading is the key process which triggers the back transition. The transition involves a feedback loop linking turbulence and profiles. The I-phase appears during the back transition following a slow power ramp down, while fast ramp-down reveal a single burst of zonal flow during the back transition. The I-phase nucleates at the pedestal shoulder, as this is the site of the residual turbulence in H-mode. Hysteresis in profile gradient is characterized by the Nusselt number. Relative hysteresis of temperature gradient vs density gradient is sensitive to the pedestal Prandtl number. We expect the H-mode to be somewhat more resilient in density than in temperature.
Miki, Kazuhiro; Idomura, Yasuhiro
no journal, ,
Studies of modes driven by energetic particles are of critical importance in burning plasmas. Experimental studies showed that the energetic particle could excite a zonal flow with having a finite frequency, i.e. EGAM. The EGAM exhibits a different frequency from that in the standard geodesic acoustic mode (GAM). A recent electrostatic gyrokinetic simulation showed a possible reproduction of the EGAM interacting with turbulence. Now, using the full-f gyrokinetic simulation code (GT5D), we would promote a study of the EGAM interacting with turbulence. As a first step, we investigate linear properties of the EGAM. Applying an initial particle distribution consisting of the bulk and beam ones, we can reproduce the excitation of the EGAM. The observed EGAM have about a half frequency of that in the standard GAM. The EGAM is excited by an amount of beam density exceeding about 10% of the bulk one. The observed threshold is consistent with theory.
Miki, Kazuhiro
no journal, ,
This is the award lecture of the 8th Young Scientist Award of the Physical Society of Japan. I have investigated various plasma critical phenomena dominated by nonlinear dynamics. First, I constructed a predator-prey model interacting with turbulence and zonal flow and GAM. The model was motivated by fluid simulations in the critical region of the ITG. Next, I have derived a one-dimensional reduced model for the L-H transition, based on the two predator-one prey model. Numerical studies revealed the possible trigger for the L-H transition, i.e. the bursty behavior of the zonal flows. Finally, the one-dimensional model was applied to the stimulated L-H transition induced by particle injection. The trigger for the transition is a change of edge radial electric field. Here, the zonal flow does not play a critical role to trigger the transition.
Miki, Kazuhiro; Idomura, Yasuhiro
no journal, ,
Energetic particles physics is of importance in the community of transport and confinement towards the future burning plasmas. We here introduce energetic particles in a full-f gyrokinetic code (GT5D). We find linear dynamics of the EGAM driven by bump-on-tail particle distributions. We examine flat-q, homogeneous, axisymmetric, electrostatic gyrokinetic simulations. Above a certain level of the beam intensity, an oscillatory mode grows with about a half of the standard GAM. The observed frequencies are consistent with the eigenmode analyses derived from the perturbed gyrokinetic equations. The theoretical analyses also indicate a bifurcation of the excited modes depending on q-value. Estimation of the finite-orbit-width effects can provide a size dependency of the EGAM growth rate. We find linear and nonlinear dynamics of the EGAM driven by slowing-down distributions. We examine the axisymmetric gyrokinetic simulations with DIII-D-like parameters. The observed growth rates and frequencies are consistent with results of other hybrid code.
Miki, Kazuhiro; Idomura, Yasuhiro
no journal, ,
Understanding of the energetic particles physics is of great interest in the future burning plasmas. We identify linear dynamics of energetic-particle-induced geodesic acoustic mode (EGAM). Representating bump-on-tail distributions as the energetic particles, we calculate real frequencies and linear growth rates, based on eigenmode analyses. Taking into account of finite-orbit-width (FOW) effects, we estimate the reduction of growth rates of the EGAM due to machine size. Results indicate that the reduction is small, within several percent, for experimentally relevant parameters. Next, applying more general distributions, i.e. slowing down, we examine linear evolution of the EGAM. Here the same q-profile is applied as the DIII-D experiments. Results on the gyrokinetic simulations are compared with the hybrid code ones. Obtained frequencies are similar to those obtained from the hybrid one. Linear growth rates are smaller than those obtained from the hybrid one.
Miki, Kazuhiro; Idomura, Yasuhiro
no journal, ,
Understanding of the energetic particles physics is of great interest in the future burning plasmas. Particularly, particle loss in the presence of EGAM may be critical for ITER. We here introduce energetic particles in a full-f gyrokinetic code (GT5D). (1) We find linear dynamics of the EGAM driven by bump-on-tail particle distributions. We examine flat-q, homogeneous, axisymmetric, electrostatic gyrokinetic simulations. We note that a sufficient resolution on the parallel velocity space is essential to reproduce the EGAM excitation. Above a certain level of the beam intensity, an oscillatory mode grows with about a half of the standard GAM. The observed frequencies are consistent with the eigenmode analyses derived from the perturbed gyrokinetic equations. The theoretical analyses also indicate a bifurcation of the excited modes depending on q-value. An estimation of the finite-orbit-width effects leads us to the reduction of the growth rate. (2) We find linear dynamics of the EGAM driven by slowing-down distributions. We examine the axisymmetric gyrokinetic simulations with DIII-D-like parameters. The observed growth rates and frequencies are consistent with results of other hybrid code.
