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Sasaki, Akira; Kato, Susumu*; Takahashi, Eiichi*; Kishimoto, Yasuaki*; Fujii, Takashi*; Kanazawa, Seiji*
Japanese Journal of Applied Physics, 55(2), p.026101_1 - 026101_10, 2016/02
Times Cited Count：2 Percentile：9.68(Physics, Applied)A percolation model of discharge, which can reproduce stochastic behaviors of initial partial discharge to the growth of a stepped leader, is presented. The model uses macroscopic cells, from which a network of electric circuits is defined, and the spatial and temporal evolutions of the electric field and current in the discharge medium are calculated. For each cell, one of two states, either insulator or conductor, which corresponds to neutral gas or ionized plasmas, respectively, is decided. The decision is made on the basis of probability for each calculation cell at each time step, taking the effects of local electric field and current, which enhance ionization and sustain the discharge channel, respectively, into account. The stochastic behavior of discharge is discussed, in conjunction with the characteristic feature of ionization, that is, the ionization occurs not only ahead of the streamer tip where the electric field is enhanced but randomly in the discharge medium.
Kikuchi, Mitsuru; Medvedev, S.*; Takizuka, Tomonori*; Fasoli, A.*; Wu, Y.*; Diamond, P. H.*; Duan, X.*; Kishimoto, Yasuaki*; Hanada, Kazuaki*; 41 of others*
Europhysics Conference Abstracts (Internet), 39E, p.P4.179_1 - P4.179_4, 2015/06
Power and particle control in fusion reactor is quite a challenge and we have studied the negative triangularity tokamak (NTT) as an innovative concept to reduce the transient ELM heat load and the quasi steady-state heat load. A double-null NTT is stable to ideal MHD modes for a reactor relevant bN 
3 while it is a magnetic hill configuration. In this paper, we report the configuration study of single-null NTT and its ideal MHD stability.
Kikuchi, Mitsuru; Fasoli, A.*; Takizuka, Tomonori*; Diamond, P. H.*; Medvedev, S.*; Wu, Y.*; Duan, X.*; Kishimoto, Yasuaki*; Hanada, Kazuaki*; Pueschel, M. J.*; et al.
Proceedings of 8th IAEA Technical Meeting on Steady State Operation of Magnetic Fusion Devices (CD-ROM), 20 Pages, 2015/05
The standard D shaped H-mode operation showed excellent plasma confinement ut has important issues of transient and steady state heat flux. To solbe this issues, we proposed new scenario using plasma shaping as one of possible scenario of future tokamak reactor.
Takahashi, Eiichi*; Kato, Susumu*; Furutani, Hirohide*; Sasaki, Akira; Kishimoto, Yasuaki*; Takada, Kenji*; Matsumura, Satoshi*; Sasaki, Hiroyasu*
Journal of Physics D; Applied Physics, 44(30), p.302001_1 - 302001_4, 2011/08
Times Cited Count：7 Percentile：31.23(Physics, Applied)A recently developed ultrafast camera that can acquire 
frames per second was used to investigate positive streamer discharge. It enabled single-shot evaluation of streamer evolution without the need to consider shot-to-shot reproducibility. This camera was used to investigate streamers in argon. Growing branches, the transition when a streamer forms a return stroke, and related phenomena were clearly observed.
Matsumoto, Taro; Li, J.*; Kishimoto, Yasuaki*
Fusion Science and Technology, 60(1T), p.75 - 79, 2011/07
Times Cited Count：0 Percentile：0.01(Nuclear Science & Technology)no abstracts in English
Takahashi, Eiichi*; Kato, Susumu*; Sasaki, Akira; Kishimoto, Yasuaki*; Furutani, Hirohide*
Journal of Physics D; Applied Physics, 44(7), p.075204_1 - 075204_6, 2011/02
Times Cited Count：20 Percentile：62.85(Physics, Applied)Positive streamer branching in atmospheric argon gas was controlled by a KrF laser irradiation. This laser irradiation changes the amount of background ionization before the streamer discharge. Initial electron density formed by the KrF laser was evaluated by measuring ionization current. Characteristic "feather like" branching structure was observed and was suppressed only for the irradiated region. The threshold of ionization density which can influence to the branching was evaluated to be 5
10
/cm
. This suppression behavior was explained by the relation between a size of avalanche head and mean initial electron distance. These experimental results support the origin of the feather like structure comes from the branching model of Loeb-Meek that is probabilistic merging of individual avalanches.
Sasaki, Akira; Kishimoto, Yasuaki*; Takahashi, Eiichi*; Kato, Susumu*; Fujii, Takashi*; Kanazawa, Seiji*
Physical Review Letters, 105(7), p.075004_1 - 075004_4, 2010/08
Times Cited Count：10 Percentile：55.33(Physics, Multidisciplinary)A three-dimensional simulation of laser guided-discharges based on the percolation is presented. The model includes both local growth of a streamer due to the enhanced electric field at the tip and propagation of a leader by remote ionization such that caused by runaway electrons. The stochastic behavior of the discharge through preformed plasma channel is reproduced by the calculation, which shows complex path with detouring and bifurcation. The probability of guiding is investigated with respect to the ionized, conductive fraction along the channel.
Nakamura, Tatsufumi; Fukuda, Yuji; Kishimoto, Yasuaki*
Physical Review A, 80(5), p.053202_1 - 053202_6, 2009/11
Times Cited Count：14 Percentile：56.33(Optics)Interactions of X-ray free electron laser (XFEL) light with a single cluster target are numerically investigated by using a three-dimensional Particle-in-Cell code. The plasma dynamics as well as relevant atomic processes are taken into account, such as photo-ionization, the Auger effect, collisional ionization/relaxation, and field ionization. It is found that as the XFEL intensity increases to as high as 
photons/pulse/mm
, the field ionization, which is the dominant ionization process over the other atomic processes, leads to rapid target ionization. The target damage due to the irradiation by XFEL light is numerically evaluated, which gives an estimation of the XFEL intensity so as to suppress the target damage within a tolerable range for imaging.
Sasaki, Akira; Takahashi, Eiichi*; Kato, Susumu*; Kishimoto, Yasuaki*
Denki Gakkai Parusu Pawa, Hoden Godo Kenkyukai Shiryo (PPT-09-95, ED-09-139), p.9 - 13, 2009/10
A new simulation model of the initiation of high pressure discharge is presented. The model reproduces branching and detouring properties of the discharge path. The model is based on combined random percolation and electric circuit models. We show that the idea of phase transition allows one to renormalize the effect of elementary atomic processes and to perform calculation of the discharge path with a realistic scale (mm to km). As a simple application of the model, discharge through a pre-ionized channel created by a laser irradiation, is investigated, which shows stochastic behaviors of the discharge guiding.
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 
.
Kato, Susumu*; Takahashi, Eiichi*; Sasaki, Akira; Kishimoto, Yasuaki*
Purazuma, Kaku Yugo Gakkai-Shi, 84(8), p.477 - 483, 2008/08
no abstracts in English
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.
surface due to zonal flow modification in reversed shear tokamaksMiyato, Naoaki; Kishimoto, Yasuaki; Li, J. Q.*
Nuclear Fusion, 47(8), p.929 - 935, 2007/08
Times Cited Count：10 Percentile：35.26(Physics, Fluids & Plasmas)Zonal flow behaviour and its effect on turbulent transport in tokamak plasmas with reversed magnetic shear are investigated by global fluid simulations of electrostatic ion temperature gradient (ITG) driven turbulence. It is found that for high (safety factor) turbulent transport is high even at a minimum region because oscillatory zonal flows called geodesic acoustic modes (GAMs) are dominant. The turbulent transport is reduced in the neighbourhood of a minimum surface when is low enough to damp the GAMs. The difference of zonal flow behaviour causes the difference of the turbulent transport and may trigger formation of ion internal transport barriers in the reversed shear tokamaks.
Matsumoto, Taro; Li, J.*; Kishimoto, Yasuaki*
Nuclear Fusion, 47(8), p.880 - 885, 2007/08
Times Cited Count：5 Percentile：19(Physics, Fluids & Plasmas)no abstracts in English
Doyle, E. J.*; Houlberg, W. A.*; Kamada, Yutaka; Mukhovatov, V.*; Osborne, T. H.*; Polevoi, A.*; Bateman, G.*; Connor, J. W.*; Cordey, J. G.*; Fujita, Takaaki; et al.
Nuclear Fusion, 47(6), p.S18 - S127, 2007/06
no abstracts in English
Miyato, Naoaki; Kishimoto, Yasuaki; Li, J. Q.*
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
Zonal flow behaviour and its effect on turbulent transport in tokamak plasmas are investigated by global fluid simulations of electrostatic ion temperature gradient (ITG) driven turbulence. It is found that oscillatory zonal flows called geodesic acoustic modes (GAMs) appearing in a high (safety factor) region have the same frequency in a certain radial region whose width is almost proportional to 
in positive shear tokamaks, and the radial wavelength of the GAMs is proportional to 
, where is an ion Larmor radius and 
is a minor radius of a plasma. The turbulent transport is affected by the nonlocal behaviour of the GAMs. It seems that the radial width of the region connecting a low tranport region where the stationary zonal flows are dominant with a high transport region where the GAMs are dominant is related with the nonlocal width of the GAMs. In reversed shear tokamaks with high , turbulent transport is high even in a minimum region because the GAMs are dominant. The turbulent transport is reduced in the minimum region when is enough low to damp the GAMs. The difference of zonal flow behaviour causes the difference of the turbulent transport and may trigger formation of ion internal transport barriers in both positive and reversed shear tokamaks.
Matsumoto, Taro; Kishimoto, Yasuaki; Li, J.*
Proceedings of 21st IAEA Fusion Energy Conference (FEC 2006) (CD-ROM), 8 Pages, 2007/03
Characteristics of ETG-driven turbulent transport dominated by zonal flows and nonlinearly generated large scale structures with low toroidal/poloidal wave numbers are investigated by gyro-fluid simulations in slab geometry. Main results found in this research are as follows. (1) The zonal flows change the characteristics of turbulence from "homogeneous" structure to "inhomogeneous" one, in which micro-scale vortices and nonlinearly generated macro-scale vortices coexist at different radial zones, exhibiting a two-scale nature in turbulence. (2) The fractal dimension is simultaneously reduced at any radial region with an increase of the ratio of zonal flow energy to that of total fluctuations, accompanied by the disappearance of exponential PDF tail of the heat flux. (3) The reduction of heat flux in strong zonal flow regime results from two mechanisms in the relation between poloidal electric field and pressure perturbation, i.e. the reduction of coherence in the zone of micro-scale vortices and the phase synchronization in that of macro-scale ones.
