Role of stable modes in zonal flow regulated turbulence

Makwana, Kirit; Terry, P. W.; Kim, Juhyung

doi:10.1063/1.4729906

Cited by 30 publications

(48 citation statements)

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“…The transfer by weak zonal flow might be related to that in Ref. 22 and would be studied in detail in our future work. The quantitative value of the transfer function of the triad mode interaction is described by the numbers along the red arrows in Fig.…”

Section: -6mentioning

confidence: 99%

Electromagnetic gyrokinetic turbulence in finite-beta helical plasmas

et al. 2014

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A saturation mechanism for microturbulence in a regime of weak zonal flow generation is investigated by means of electromagnetic gyrokinetic simulations. The study identifies a new saturation process of the kinetic ballooning mode (KBM) turbulence originating from the spatial structure of the KBM instabilities in a finite-beta Large Helical Device (LHD) plasma. Specifically, the most unstable KBM in LHD has an inclined mode structure with respect to the mid-plane of a torus, i.e., it has a finite radial wave-number in flux tube coordinates, in contrast to KBMs in tokamaks as well as ion-temperature gradient modes in tokamaks and helical systems. The simulations reveal that the growth of KBMs in LHD is saturated by nonlinear interactions of oppositely inclined convection cells through mutual shearing as well as by the zonal flow. The saturation mechanism is quantitatively investigated by analysis of the nonlinear entropy transfer that shows not only the mutual shearing but also a self-interaction with an elongated mode structure along the magnetic field line.

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Section: -6mentioning

confidence: 99%

Electromagnetic gyrokinetic turbulence in finite-beta helical plasmas

et al. 2014

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“…While the former effect has been appreciated in the literature (Diamond et al 2005), the latter has largely gone unnoticed, and in some cases is entirely neglected (e.g. Holland et al 2003, Makwana, Terry & Kim 2012. To appreciate the importance of the E × B nonlinearity, consider the criterion given by (5.19) using the mode-coupling coefficients of the (m)HME where M Im p M Im r = 0 ((3.9) with δ k = 0) with a background drift wave at wavenumber p y .…”

Section: Zonal-mode Stability Analysis Of the Tertiary Instabilitymentioning

confidence: 99%

“…This process is dominated by the largest eddies of the system, and underlies the typical picture of shearing by large-scale zonal flows (Diamond et al 2005). More complex models also have an increased number of fields, leading to multiple branches (see the recent work of Makwana et al 2012Makwana et al , 2014 energy between branches, then, can lead to an efficient source of energy dissipation. This is exacerbated when moving to a kinetic system, where there could be an arbitrary number of stable branches.…”

Section: Zonal-mode Stability Analysis Of the Tertiary Instabilitymentioning

confidence: 99%

“…However, while the non-local contribution from adiabatic electrons in the E × B nonlinearity has been appreciated in some areas of the literature (see, for example, Rogers et al 2000, Guzdar et al 2001, it has gone unnoticed in other areas. As an example, two studies (Holland et al 2003;Makwana et al 2012) that used a two-field gyrofluid model of the ITG instability neglected the E × B nonlinearity altogether, and thus this effect was entirely absent. Similarly, the nonlinear contribution from proper adiabatic electron response is not mentioned in the work of Diamond et al (2005), Itoh et al (2006), while the linear contribution under the partial time derivative is considered.…”

Section: Zonal-mode Stability Analysis Of the Tertiary Instabilitymentioning

confidence: 99%

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On non-local energy transfer via zonal flow in the Dimits shift

St-Onge

2017

J. Plasma Phys.

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The two-dimensional Terry-Horton equation is shown to exhibit the Dimits shift when suitably modified to capture both the nonlinear enhancement of zonal/drift-wave interactions and the existence of residual Rosenbluth-Hinton states. This phenomenon persists through numerous simplifications of the equation, including a quasilinear approximation as well as a four-mode truncation. It is shown that the use of an appropriate adiabatic electron response, for which the electrons are not affected by the flux-averaged potential, results in an E × B nonlinearity that can efficiently transfer energy non-locally to length scales of the order of the sound radius. The size of the shift for the nonlinear system is heuristically calculated and found to be in excellent agreement with numerical solutions. The existence of the Dimits shift for this system is then understood as an ability of the unstable primary modes to efficiently couple to stable modes at smaller scales, and the shift ends when these stable modes eventually destabilize as the density gradient is increased. This non-local mechanism of energy transfer is argued to be generically important even for more physically complete systems.

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“…Since turbulence clearly cannot be saturated by linear physics alone, this means that our estimate does not capture the correct nonlinear time. We conjecture that it is in fact the coupling to the zonal flows, invisible to BES (because their δn = 0), that dominates over the nonlinear interaction between the drift-wave-like fluctuations represented by τ NZ nl [38,[40][41][42][43][44][45][46]. It has long been suspected that the relative amplitude of the zonal flows compared to that of the drift waves depends on the ion collisionality [38,[47][48][49].…”

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confidence: 99%

Experimental Signatures of Critically Balanced Turbulence in MAST

et al. 2013

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Beam Emission Spectroscopy (BES) measurements of ion-scale density fluctuations in the MAST tokamak are used to show that the turbulence correlation time, the drift time associated with ion temperature or density gradients, the particle (ion) streaming time along the magnetic field and the magnetic drift time are consistently comparable, suggesting a "critically balanced" turbulence determined by the local equilibrium. The resulting scalings of the poloidal and radial correlation lengths are derived and tested. The nonlinear time inferred from the density fluctuations is longer than the other times; its ratio to the correlation time scales as ν −0.8±0.1 * i , where ν * i = ion collision rate/streaming rate. This is consistent with turbulent decorrelation being controlled by a zonal component, invisible to the BES, with an amplitude exceeding the drift waves' by ∼ ν −0.8 * i .Introduction. Microscale turbulence hindering energy confinement in magnetically confined hot plasmas is driven by gradients of equilibrium quantities such as temperature and density. These gradients give rise to instabilities that inject energy into plasma fluctuations ("drift waves") at scales just above the ion Larmor scale. The most effective of these is believed to be the iontemperature-gradient (ITG) instability [1][2][3]. A turbulent state ensues, giving rise to "anomalous transport" of energy [4]. It is of interest, both for practical considerations of improving confinement and for the fundamental understanding of multiscale plasma dynamics, what the structure of this turbulence is and how its amplitude, scale(s) and resulting transport depend on the equilibrium parameters: ion and electron temperatures, density, angular velocity, magnetic geometry, etc.Fluctuations in a magnetized toroidal plasma are subject to a number of distinct physical effects, which can be thought about in terms of various time scales such as the drift times associated with the temperature and density gradients, the particle streaming time along the magnetic field as it takes them around the torus toroidally and poloidally, the magnetic (∇B and curvature) drift times of particles moving across the field, the nonlinear time of the fluctuations being advected across the field by the fluctuating E × B velocity, the time between collisions, the shear time associated with plasma rotation. Some of these time scales and, consequently, the corresponding physics may be irrelevant, while others play a crucial role for the saturation of the linearly unstable fluctuations. There has been a growing understanding [5], driven largely by theory [6][7][8][9], observations [10][11][12] and simulations of magnetohydrodynamic [13][14][15] and kinetic [7,16] plasma turbulence in space, that if a medium can

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Role of stable modes in zonal flow regulated turbulence

Cited by 30 publications

References 24 publications

Electromagnetic gyrokinetic turbulence in finite-beta helical plasmas

Electromagnetic gyrokinetic turbulence in finite-beta helical plasmas

On non-local energy transfer via zonal flow in the Dimits shift

Experimental Signatures of Critically Balanced Turbulence in MAST

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