Formation of coherent vortex streets and transport reduction in electron temperature gradient driven turbulence

Nakata, M.; Watanabe, T.‐H.; Sugama, H.; Horton, W.

doi:10.1063/1.3356048

Cited by 13 publications

(22 citation statements)

References 31 publications

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“…7,8 Thus, the zonal-flow generation in the ETG turbulence is relatively weaker than that in the ITG turbulence so that the ETG turbulence involves not only zonal flows, but also various vortex structures, of which the appearance strongly depends on geometrical and plasma parameters. [8][9][10][11][12] From the aspect of regulating the turbulent transport in future burning plasmas, it is worthwhile to understand fundamental physics behind the formation of vortex and zonalflow structures and their nonlinear interactions as well as the related transport properties.…”

Section: Introductionmentioning

confidence: 99%

Nonlinear entropy transfer via zonal flows in gyrokinetic plasma turbulence

2012

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Nonlinear entropy transfer processes in toroidal ion temperature gradient (ITG) and electron temperature gradient (ETG) driven turbulence are investigated based on the gyrokinetic entropy balance relations for zonal and non-zonal modes, which are coupled through the entropy transfer function regarded as a kinetic extension of the zonal-flow production due to the Reynolds stress. Spectral analyses of the "triad" entropy transfer function introduced in this study reveal not only the nonlinear interactions among the zonal and non-zonal modes, but also their effects on the turbulent transport level. Different types of the entropy transfer processes between the ITG and ETG turbulence are found: the entropy transfer from non-zonal to zonal modes is substantial in the saturation phase of the ITG instability, while, once the strong zonal flow is generated, the entropy transfer to the zonal modes becomes quite weak in the steady turbulence state. Instead, the zonal flows mediate the entropy transfer from non-zonal modes with low radial-wavenumbers (with contribution to the heat flux) to the other non-zonal modes with higher radial-wavenumbers (but with less contribution to the heat flux) through the triad interaction. The successive entropy transfer processes to the higher radial-wavenumber modes are associated with transport regulation in the steady turbulence state. In contrast, in both the instability-saturation and steady phases of the ETG turbulence, the entropy transfer processes among low-wavenumber non-zonal modes are dominant rather than the transfer via zonal modes.

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Section: Introductionmentioning

confidence: 99%

Nonlinear entropy transfer via zonal flows in gyrokinetic plasma turbulence

2012

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“…Thus, the transitional behavior is expected to appear not only for the heat diffusivity, but also for potential and temperature fluctuation profiles in the real space. 23 On the other hand, no significant change in the spectrum is observed for the two phases in the case of ⌰ = 0.150 ͓Fig. 5͑b͔͒.…”

Section: A Vortex Structures and Transport Propertiesmentioning

confidence: 83%

“…23 Time evolution of the perturbed gyrocenter distribution function ␦f k Ќ ͑g͒ is numerically solved, where the equilibrium part is assumed to be the local Maxwellian. Also, we assume that Ќ -dependence of ␦f k Ќ ͑g͒ is given by the local…”

Section: Simulation Model and Linear Analysismentioning

confidence: 99%

“…In our earlier work, 23 we have investigated vortex structures in the slab ETG turbulence as well as velocity-space structures of the distribution function by means of the gyrokinetic Vlasov simulations with high phase-space resolution and have found the formation of coherent vortex streets accompanied with the significant transport reduction in the nonlinear phase. Detailed analysis of the distribution function clarified that the transport reduction is associated with the phase matching between the potential and temperature fluctuations rather than the reduction of the fluctuation amplitude.…”

Section: Introductionmentioning

confidence: 98%

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Effects of parallel dynamics on vortex structures in electron temperature gradient driven turbulence

et al. 2011

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Vortex structures and related heat transport properties in slab electron temperature gradient ͑ETG͒ driven turbulence are comprehensively investigated by means of nonlinear gyrokinetic Vlasov simulations, with the aim of elucidating the underlying physical mechanisms of the transition from turbulent to coherent states. Numerical results show three different types of vortex structures, i.e., coherent vortex streets accompanied with the transport reduction, turbulent vortices with steady transport, and a zonal-flow-dominated state, depending on the relative magnitude of the parallel compression to the diamagnetic drift. In particular, the formation of coherent vortex streets is correlated with the strong generation of zonal flows for the cases with weak parallel compression, even though the maximum growth rate of linear ETG modes is relatively large. The zonal flow generation in the ETG turbulence is investigated by the modulational instability analysis with a truncated fluid model, where the parallel dynamics such as acoustic modes for electrons is incorporated. The modulational instability for zonal flows is found to be stabilized by the effect of the finite parallel compression. The theoretical analysis qualitatively agrees with secondary growth of zonal flows found in the slab ETG turbulence simulations, where the transition of vortex structures is observed.

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“…Thus, generally speaking, the choice of the initial condition may affect the details of the zonal flow generation process at least in the early saturation phase of the instability growth, and hence, the resultant zonal flow amplitude. Recent slab ETG turbulence simulation [9] showed that the tilt angle of the magnetic field, Θ, influences the zonal flow amplitude. The dependence of the zonal flow production on Θ and the initial perturbations is left for future studies.…”

Section: Summary and Discussionmentioning

confidence: 99%

Gyrokinetic Simulations of Slab Ion Temperature Gradient Turbulence with Kinetic Electrons

Ishizawa

Watanabe

Nakajima

2011

Plasma and Fusion Research

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Ion temperature gradient (ITG) driven turbulence is investigated by means of gyrokinetic simulations which include both kinetic ions and electrons in slab geometry with uniform equilibrium magnetic field. The study confirms that numerical results satisfy the entropy balance equation including both ions and electrons. The entropy variable is transferred from ions to electrons through the perturbation of electrostatic potential, and the transferred fluctuation is diffused by the electron collision dissipation. In the ITG turbulence with kinetic electrons ion heat diffusion is larger than that with adiabatic electrons. The former is close to the latter when the ion mass is comparable to or larger than the hydrogen one. The difference between the transport coefficients does not originate from the suppression of turbulence by zonal flow, but stems from the difference between their spectra. The lowest wavenumber mode dominates the coefficient in the adiabatic electron case, while the transport is caused not only by the the lowest mode but by higher wavenumbers in the kinetic electron cases.

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Formation of coherent vortex streets and transport reduction in electron temperature gradient driven turbulence

Cited by 13 publications

References 31 publications

Nonlinear entropy transfer via zonal flows in gyrokinetic plasma turbulence

Nonlinear entropy transfer via zonal flows in gyrokinetic plasma turbulence

Effects of parallel dynamics on vortex structures in electron temperature gradient driven turbulence

Gyrokinetic Simulations of Slab Ion Temperature Gradient Turbulence with Kinetic Electrons

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