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The role of zonal flows in reactive fluid closures

Plasma Science and Technology 2018年第7期20卷 41-49页

作者： jan weiland Chalmers University of Technology and EURATOM-VR Association SE-41296 GothenburgSweden

We will give an overview of results obtained by our reactive fluid model. It is characterised as a fluid model where all moments with sources in the experiment are kept. Furthermore, full account is taken for the highest moments appearing in unexpanded denominators also including full toroidicity. It has been demonstrated that the strength of zonal flows is dramatically larger in reactive fluid closures than in those which involve dissipation. This gives a direct connection between the fluid closure and the level of excitation of turbulence. This is because zonal flows are needed to absorb the inverse cascade in quasi 2D turbulence. This also explains the similarity in structure of the transport coefficients in our model with a reactive closure in the energy equation and models which have a reactive closure because of zero ion temperature such as the Hasegawa-Wakatani model. Our exact reactive closure unifies several well-known features of tokamak experiments such as the L-H transition, internal transport barriers and the nonlinear Dimits upshift of the critical gradient for onset of transport. It also gives transport of the same level as that in nonlinear gyrokinetic codes. Since these include the kinetic resonance this confirms the validity of the thermodynamic properties of our model. Furthermore, we can show that while a strongly nonlinear model is needed in kinetic theory a quasilinear model is sufficient in the fluid description. Thus our quasilinear fluid model will be adequate for treating all relevant problems in bulk transport. This is finally confirmed by the reproduction by the model of the experimental power scaling of the confinement time Te ~ P-2/3. This confirms the validity of our reactive fluid model. This also gives credibility to our ITER simulations including the H-mode barrier. A new result is here, that alpha heating strongly reduces the slope of the H-mode barrier. This should significantly reduce the effects of ELM＇s.

关键词： turbulent transport improved mixing length estimates zonal flows toroidicity

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Measurement of zonal flows in a Tokamak Using Langmuir Probe Array

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Plasma Science and Technology 2006年第1期8卷 10-13页

作者：徐国盛万宝年 Institute of Plasma Physics Chinese Academy of SciencesHefei 230031China

zonal flows have been measured with several novel Langmuir probe array on the HT-7 tokamak since 2002. The forked probe and its improved generation, the farmer forked probe were designed based on the original idea of the Reynolds stress triple tips array. In order to measure the radial shearing rate of zonal flows, the pyramid probe was also designed. This paper addresses the technical aspects regarding this new application. Since the zonal flows dynamics is now widely believed crucial to plasma confinement physics, an important application field of Langmuir probe was opened up.

关键词： zonal flows tokamak Langmuir probe reynolds stress

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Properties of Density Fluctuations Induced by Geodesic Acoustic Mode in the Edge of HT-7 Tokamak

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Plasma Science and Technology 2010年第3期12卷 262-267页

作者：赵海林兰涛刘阿娣孔德峰谢锦林刘万东俞昌旋张炜常加峰万宝年李建刚 CAS Key Laboratory of Basic Plasma Physics and Department of Modern PhysicsUniversity of Science and Technology of China Institute of Plasma Physics Chinese Academy of Sciences

Properties of the geodesic acoustic mode （GAM） density fluctuations are studied using two toroidally separated Langnmir triple-probe arrays on the top of HT-7 tokamak. The GAM scenario is identified in the potential fluctuations with the toroidally symmetric structure （n =0） and satisfying the temperature scaling of GAM mode frequency. Some theoretical predictions about the mode features of GAM density fluctuations are verified in our experiments： the toroidal mode number of GAM density fluctuations is n = 0; its amplitude is consistent with the theoretical prediction in a factor of 2; the density and potential fluctuations of GAM is in anti-phase at the top of plasma cross-section. Strong nonlinear interactions are found between GAM density fluctuations and ambient turbulence （AT）. The results support the conclusions that the envelope modulation of potential fluctuations is dominantly caused by the direct regulation of GAM in the generation processing, and the envelope modulation of density fluctuation is due to the GAM shear effect.

关键词： zonal flows geodesic acoustic mode (GAM) density fluctuations

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Understanding L-H transition in tokamak fusion plasmas

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Plasma Science and Technology 2017年第3期19卷 4-14页

作者： Guosheng XU Xingquan WU Institute of Plasma Physics Chinese Academy of Sciences Hefei 230031 People's Republic of China

This paper reviews the current state of understanding of the L-H transition phenomenon in tokamak plasmas with a focus on two central issues：（a） the mechanism for turbulence quick suppression at the L-H transition; （b） the mechanism for subsequent generation of sheared flow. We briefly review recent advances in the understanding of the fast suppression of edge turbulence across the L-H transition. We uncover a comprehensive physical picture of the L-H transition by piecing together a number of recent experimental observations and insights obtained from 1D and 2D simulation models. Different roles played by diamagnetic mean flow, neoclassical-driven mean flow, turbulence-driven mean flow, and turbulence-driven zonal flows are discussed and clarified. It is found that the L-H transition occurs spontaneously mediated by a shift in the radial wavenumber spectrum of edge turbulence, which provides a critical evidence for the theory of turbulence quench by the flow shear. Remaining questions and some key directions for future investigations are proposed.

关键词： L-H transition ExB flow shear turbulence suppression zonal flows Reynolds stress

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Wave-Number Spectral Characteristics of Drift Wave Micro-Turbulence with Large-Scale Structures

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Plasma Science and Technology 2011年第3期13卷 297-301页

作者：李继全 Y.KISHIMOTO Graduate School of Energy Science Kyoto University 611-0011 Gokasho Uji Japan

Wave-number spectral characteristics of drift wave micro-turbulence with large-scale structures （LSSs） including zonal flows （ZFs） and Kelvin-Holmheltz （KH） mode are investigated based on three dimensional gyrofluid simulations in a slab geometry. The focus is on the property of the wave-number spectral scaling law of the ambient turbulence under the back reaction of the self-generated LSSs. A comparison of the spectral scaling laws between ion/electron temperature gradient （ITG/ETG） driven turbulences is presented. It is shown that the spectral scaling of the ITG turbulence with robust ZFs is fitted well by an exponential-law function （Φ^2/2）E∝e^-λkx in kx and a power-law one in （Φ^2/2）p∝ky^-β in ky. However, the ETG turbulence is characterized by a mixing Kolmogorov-like power-law and exponential-law （Φ^2/2）E∝e-λkx＇yk^-3x,y/（1 ＋ k^2x,y）^2 scaling for both kx and ky spectra due to the ZFs and KH mode dynamics, with λ and β the slope index factors. The underlying physical mechanism is understood as the spectral scattering caused by the back-reaction of the LSSs on the ambient turbulence. These findings may provide helpful guideline to diagnose the plasma fluctuations and flow structures in experiments.

关键词： spectral scaling law zonal flows micro-turbulence gyrofluid simulation

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