Turbulence and zonal flow structures in the core and L-mode pedestal of tokamak plasmas

Villard, L.; McMillan, B. F.; Sauter, O.; Hariri, Farah; Dominski, J.; Merlo, Gabriele; Brunner, S.; Tran, T. M.

doi:10.1088/1742-6596/561/1/012022

Cited by 8 publications

(10 citation statements)

References 21 publications

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“…A first attempt to study profile stiffness with global simulations is presented in Ref. [5], where for a given TCV plasma shape the relation between increased heating and profiles evolution was studied in the ITG regime, obtaining a qualitative agreement with the experimental findings present in Ref. [4].…”

Section: Introductionsupporting

confidence: 57%

Investigating profile stiffness and critical gradients in shaped TCV discharges using local gyrokinetic simulations of turbulent transport

Merlo

Brunner

Sauter

et al. 2015

Plasma Phys. Control. Fusion

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Abstract. The experimental observation made on the TCV tokamak of a significant confinement improvement in plasmas with negative triangularity ( δ < 0) compared to those with standard positive triangularity has been interpreted in terms of different degrees of profile stiffness (Sauter et al. Phys. Plasmas 21 055906, 2014) and/or different critical gradients. Employing the Eulerian gyrokinetic code GENE (Jenko et al. Phys. Plasmas 7 1904, 2000, profile stiffness and critical gradients are studied under TCV relevant conditions. For the considered experimental discharges, trapped electron modes (TEMs) and electron temperature gradient (ETG) modes are the dominant microinstabilities, with the latter providing a significant contribution to the non-linear electron heat fluxes near the plasma edge. Two series of simulations with different levels of realism are performed, addressing the question of profile stiffness at various radial locations. Retaining finite collisionality, impurities and electromagnetic effects, as well as the physical electron-to-ion mass ratio are all necessary in order to approach the experimental flux measurements. However, flux-tube simulations are unable to fully reproduce the TCV results, pointing towards the need to carry out radially nonlocal (global) simulations, i.e. retaining finite machine size effects, in a future study. Some conclusions about the effect of triangularity can nevertheless be drawn based on the flux-tube results. In particular, the importance of considering the sensitivity to both temperature and density gradient is shown. The flux tube results show an increase of the critical gradients towards the edge, further enhanced when δ < 0, and they also appear to indicate a reduction of profile stiffness towards plasma edge.

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Section: Introductionsupporting

confidence: 57%

Investigating profile stiffness and critical gradients in shaped TCV discharges using local gyrokinetic simulations of turbulent transport

Merlo

Brunner

Sauter

et al. 2015

Plasma Phys. Control. Fusion

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“…In the near-marginal Dimits state, turbulence is suppressed by a quasi-static 'zonal staircase' arrangement of the ZFs and zonal temperature perturbations. This structure is reminiscent of the 'E × B staircase' observed in global GK simulations (Dif-Pradalier et al 2010Villard et al 2013Villard et al , 2014. The zonal staircase consists of interleaved regions of strong zonal shear that suppresses the ITG turbulence in those regions, and localised turbulent patches at the turning points of the ZF velocity.…”

Section: Nonlinear Saturation and Zonal Staircasementioning

confidence: 73%

“…2010, 2015, 2017; Villard et al. 2013, 2014). The zonal staircase consists of interleaved regions of strong zonal shear that suppresses the ITG turbulence in those regions, and localised turbulent patches at the turning points of the ZF velocity.…”

Section: Nonlinear Saturation and Zonal Staircasementioning

confidence: 99%

Zonally dominated dynamics and Dimits threshold in curvature-driven ITG turbulence

et al. 2020

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The saturated state of turbulence driven by the ion-temperature-gradient instability is investigated using a two-dimensional long-wavelength fluid model that describes the perturbed electrostatic potential and perturbed ion temperature in a magnetic field with constant curvature (a $Z$ -pinch) and an equilibrium temperature gradient. Numerical simulations reveal a well-defined transition between a finite-amplitude saturated state dominated by strong zonal-flow and zonal temperature perturbations, and a blow-up state that fails to saturate on a box-independent scale. We argue that this transition is equivalent to the Dimits transition from a low-transport to a high-transport state seen in gyrokinetic numerical simulations (Dimits et al., Phys. Plasmas, vol. 7, 2000, 969). A quasi-static staircase-like structure of the temperature gradient intertwined with zonal flows, which have patch-wise constant shear, emerges near the Dimits threshold. The turbulent heat flux in the low-collisionality near-marginal state is dominated by turbulent bursts, triggered by coherent long-lived structures closely resembling those found in gyrokinetic simulations with imposed equilibrium flow shear (van Wyk et al., J. Plasma Phys., vol. 82, 2016, 905820609). The breakup of the low-transport Dimits regime is linked to a competition between the two different sources of poloidal momentum in the system – the Reynolds stress and the advection of the diamagnetic flow by the $\boldsymbol {E}\times \boldsymbol {B}$ flow. By analysing the linear ion-temperature-gradient modes, we obtain a semi-analytic model for the Dimits threshold at large collisionality.

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“…These corrugation patterns are distributed with a characteristic length scale of the radial spacing. As will be discussed in the forthcoming paragraphs, the corrugation patterns resemble the staircase structures appearing in global ITG turbulence simulations [18,49,50]. We choose two elongation values κ = 1.0 and 2.0 to investigate the elongation effects on the zonal flows, corrugation patterns, and transport avalanches.…”

Section: Profile Corrugations and Large Scale Transport Events: The E...mentioning

confidence: 97%

Characteristics of trapped electron transport, zonal flow staircase, turbulence fluctuation spectra in elongated tokamak plasmas

Kwon

Hahm

et al. 2019

Nucl. Fusion

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The effects of plasma elongation on trapped electron mode (TEM) driven turbulence and transport are investigated with detailed analyses of fluctuation, zonal flow and zonal pressure. For the study, global nonlinear TEM simulations with different elongations () are performed by employing a bounce-averaged gyrokinetic code gKPSP. From the simulations, -scalings of the electron heat conductivity (), ion heat conductivity (), and particle diffusivity () are found as , and , respectively. Higher plasma elongation is found to enhance zonal flows and their shearing rate at short radial scales, which in turn reduce the radial correlation length of turbulence and limit avalanche-like large scale transport events. Radial corrugations of density and temperature profiles also emerge due to quasi-stationary zonal flows at short spatial scales. In addition, it is found that the Doppler shift caused by the quasi-stationary zonal flows can modify the phase velocity of fluctuation in the laboratory frame and result in turbulence spectra consisting of components propagating in both electron and ion diamagnetic directions, while the transport is mainly governed by TEM.

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Turbulence and zonal flow structures in the core and L-mode pedestal of tokamak plasmas

Cited by 8 publications

References 21 publications

Investigating profile stiffness and critical gradients in shaped TCV discharges using local gyrokinetic simulations of turbulent transport

Investigating profile stiffness and critical gradients in shaped TCV discharges using local gyrokinetic simulations of turbulent transport

Zonally dominated dynamics and Dimits threshold in curvature-driven ITG turbulence

Characteristics of trapped electron transport, zonal flow staircase, turbulence fluctuation spectra in elongated tokamak plasmas

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