Flux- and gradient-driven global gyrokinetic simulation of tokamak turbulence

Görler, T.; Lapillonne, Xavier; Brunner, S.; Dannert, T.; Jenko, F.; Aghdam, Sohrab Khosh; Marcus, Patrick; McMillan, B. F.; Merz, F.; Sauter, O.; Told, D.; Villard, L.

doi:10.1063/1.3567484

Cited by 61 publications

(88 citation statements)

References 30 publications

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“…9, which shows the heat conduction coefficient as a function of the time and the radial coordinate. Avalanches have been observed in turbulent simulations under various circumstances [18][19][20][21][22], and the behaviour here is similar to the one previously reported in connection with staircases [21]. Avalanches start in the region |ω ExB | < γ and propagate outward / inward across the staircase.…”

Section: Zonal Flow Evolutionsupporting

confidence: 69%

Gradient-driven flux-tube simulations of ion temperature gradient turbulence close to the non-linear threshold

et al. 2016

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It is shown that ITG turbulence close to the threshold exhibits a long time behaviour, with smaller heat fluxes at later times. This reduction is connected with the slow growth of long wave length zonal flows and, consequently, the numerical dissipation on these flows must be sufficient small. Close to the nonlinear threshold for turbulence generation, a relatively small dissipation can maintain a turbulent state with a sizeable heat flux, through the damping of the zonal flow. Lowering the dissipation causes the turbulence, for temperature gradients close to the threshold, to be subdued. The heat flux then does not go smoothly to zero when the threshold is approached from above. Rather, a finite minimum heat flux is obtained below which no fully developed turbulent state exists. The threshold value of the temperature gradient length at which this finite heat flux is obtained is up to 30% larger compared with the threshold value obtained by extrapolating the heat flux to zero, and the cyclone base case is found to be nonlinearly stable. Transport is subdued when a fully developed staircase structure in the ExB shearing rate forms. Just above the threshold, an incomplete staircase develops, and transport is mediated by avalanche structures which propagate through the marginally stable regions.

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Section: Zonal Flow Evolutionsupporting

confidence: 69%

Gradient-driven flux-tube simulations of ion temperature gradient turbulence close to the non-linear threshold

et al. 2016

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“…14,20 This is often assumed to be the case for modelling of experiments. For larger ρ * (or ρ * eff where the minor radius is replaced by the variation length of the gradient 13 ), however, including neoclassical effects in a turbulent simulation becomes strongly advisable.…”

Section: Discussionmentioning

confidence: 99%

“…The general trend that turbulent transport decreases with growing ρ * is a well established observation. 14,[19][20][21][22] Due to the narrow profile shape (see above) the convergence to the local limit occurs at relatively small ρ * . At this point it is important to remember that the fluxes are measured in units which imply the gyro-Bohm scaling, i.e.…”

Section: B System Size Scalingmentioning

confidence: 99%

Interaction between neoclassical effects and ion temperature gradient turbulence in gradient- and flux-driven gyrokinetic simulations

et al. 2016

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Neoclassical and turbulent transport in tokamaks have been studied extensively over the past decades, but their possible interaction remains largely an open question. The two are only truly independent if the length scales governing each of them are sufficiently separate, i.e. if the ratio ρ * between ion gyroradius and the pressure gradient scale length is small. This is not the case in particularly interesting regions such as transport barriers. Global simulations of collisional ion-temperature-gradient-driven microturbulence performed with the nonlinear global gyrokinetic code Gene are presented. In particular, comparisons are made between systems with and without neoclassical effects. In fixed-gradient simulations the modified radial electric field is shown to alter the zonal flow pattern such that a significant increase in turbulent transport is observed for ρ * 1/300. Furthermore, the dependency of the flux on the collisionality changes. In simulations with fixed power input we find that the presence of neoclassical effects decreases the frequency and amplitude of intermittent turbulent transport bursts (avalanches) and thus plays an important role for the self-organisation behaviour.

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“…However, due to the relatively small field strength in STs and corresponding larger values of * = s /a ( *,unit =0.0067 & 0.0075 at r/a=0.6 for 129016 & 120968, respectively), the computational domain width is often a significant fraction of the minor radius (r/a 0.3-0.9). It is very likely that non-local [97][98][99] (and possibly multi-scale) effects will be important for simulating the total transport across the radius of these discharges. For example, although the microtearing simulations at r/a=0.6 discussed in Sec.…”

Section: Profile Effectsmentioning

confidence: 99%

Progress in simulating turbulent electron thermal transport in NSTX

et al. 2013

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Abstract. Nonlinear simulations based on multiple NSTX discharge scenarios have progressed to help differentiate unique instability mechanisms and to validate with experimental turbulence and transport data. First nonlinear gyrokinetic simulations of microtearing (MT) turbulence in a high-beta NSTX H-mode discharge predict experimental levels of electron thermal transport that are dominated by magnetic flutter and increase with collisionality, roughly consistent with energy confinement times in dimensionless collisionality scaling experiments. Electron temperature gradient (ETG) simulations predict significant electron thermal transport in some low and high beta discharges when ion scales are suppressed by E B shear. Although the predicted transport in H-modes is insensitive to variation in collisionality (inconsistent with confinement scaling), it is sensitive to variations in other parameters, particularly density gradient stabilization. In reversed shear (RS) Lmode discharges that exhibit electron internal transport barriers, ETG transport has also been shown to be suppressed nonlinearly by strong negative magnetic shear, s<<0. In many high beta plasmas, instabilities which exhibit a stiff beta dependence characteristic of kinetic ballooning modes (KBM) are sometimes found in the core region. However, they do not have a distinct finite beta threshold, instead transitioning gradually to a trapped electron mode (TEM) as beta is reduced to zero. Nonlinear simulations of this "hybrid" TEM/KBM predict significant transport in all channels, with substantial contributions from compressional magnetic perturbations. As multiple instabilities are often unstable simultaneously in the same plasma discharge, even on the same flux surface, unique parametric dependencies are discussed which may be useful for distinguishing the different mechanisms experimentally.

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Flux- and gradient-driven global gyrokinetic simulation of tokamak turbulence

Cited by 61 publications

References 30 publications

Gradient-driven flux-tube simulations of ion temperature gradient turbulence close to the non-linear threshold

Gradient-driven flux-tube simulations of ion temperature gradient turbulence close to the non-linear threshold

Interaction between neoclassical effects and ion temperature gradient turbulence in gradient- and flux-driven gyrokinetic simulations

Progress in simulating turbulent electron thermal transport in NSTX

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