Gyrokinetic simulations of spherical tokamaks

Roach, C. M.; Abel, Ian; Akers, R.; Arter, W.; Barnes, Michael; Camenen, Y.; Casson, F. J.; Colyer, Greg; Connor, J. W.; Cowley, S. C.; Dickinson, David; Dorland, W.; Field, A. R.; Guttenfelder, W.; Hammett, G. W.; Hastie, R. J.; Highcock, E. G.; Loureiro, Nuno; Peeters, A. G.; Reshko, M.; Saarelma, S.; Schekochihin, A. A.; Valovič, M.; Wilson, H. R.

doi:10.1088/0741-3335/51/12/124020

Cited by 94 publications

(189 citation statements)

References 35 publications

(81 reference statements)

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“…1). It has been found in nonlinear simulations that such large E×B shear (γ E /γ lin,max~1 ) can suppress ITG/TEM [64], and even ETG [65,66] turbulence, and we find the same to hold true for these NSTX microtearing simulations. Fig.…”

Section: Exb Shearsupporting

confidence: 83%

Simulation Of Microtearing Turbulence In NSTX

Guttenfelder¹,

Kaye²,

Nevins³

et al. 2012

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Thermal energy confinement times in NSTX dimensionless parameter scans increase with decreasing collisionality. While ion thermal transport is neoclassical, the source of anomalous electron thermal transport in these discharges remains unclear, leading to considerable uncertainty when extrapolating to future ST devices at much lower collisionality.Linear gyrokinetic simulations find microtearing modes to be unstable in high collisionality discharges. First non-linear gyrokinetic simulations of microtearing turbulence in NSTX show they can yield experimental levels of transport. Magnetic flutter is responsible for almost all the transport (~98%), perturbed field line trajectories are globally stochastic, and a test particle stochastic transport model agrees to within 25% of the simulated transport. Most significantly, microtearing transport is predicted to increase with electron collisionality, consistent with the observed NSTX confinement scaling. While this suggests microtearing modes may be the source of electron thermal transport, the predictions are also very sensitive to electron temperature gradient, indicating the scaling of the instability threshold is important. In addition, microtearing turbulence is susceptible to suppression via sheared E×B flows as experimental values of E×B shear (comparable to the linear growth rates) dramatically reduce the transport below experimental values. Refinements in numerical resolution and physics model assumptions are expected to minimize the apparent discrepancy. In cases where the predicted transport is strong, calculations suggest that a proposed polarimetry diagnostic may be sensitive to the magnetic perturbations associated with the unique structure of microtearing turbulence.

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Section: Exb Shearsupporting

confidence: 83%

Simulation Of Microtearing Turbulence In NSTX

Guttenfelder¹,

Kaye²,

Nevins³

et al. 2012

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“…This is due to the strong low-k cutoff provided by the large E B shearing rate [79,80]. For physical accuracy, the simulations include kinetic deuterium and carbon ions consistent with the experimental Z eff =1.7, collisions, , A || , B || perturbations (although electromagnetic effects are not very important for ETG simulations [80,81]), and E B shear. Figure 6 shows that the predicted nonlinear ETG heat flux for the experimental parameters (Q e,sim~1 .5 MW) is a significant fraction of the experimental transport (Q e,exp~2 MW).…”

Section: Etg Turbulencementioning

confidence: 88%

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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“…Significant thermal transport is possible from turbulence driven by the ETG mode [5][6][7][8][9][10][11][12][13] .…”

Section: Introductionmentioning

confidence: 99%

Suppressing electron turbulence and triggering internal transport barriers with reversed magnetic shear in the National Spherical Torus Experiment

et al. 2012

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The National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] can achieve high electron plasma confinement regimes that are supercritically unstable to the electron temperature gradient driven (ETG) instability.These plasmas, dubbed electron internal transport barriers (e-ITBs), occur when the magnetic shear becomes strongly negative. Using the gyrokinetic code GYRO [J.Candy and R. E. Waltz, J. Comp. Phys. 186, 545 (2003)], the first nonlinear ETG simulations of NSTX e-ITB plasmas reinforce this observation. Local simulations identify a strongly upshifted nonlinear critical gradient for thermal transport that depends on magnetic shear. Global simulations show e-ITB formation can occur when the magnetic shear becomes strongly negative. While the ETG-driven thermal flux at the outer edge of the barrier is large enough to be experimentally relevant, the turbulence cannot propagate past the barrier into the plasma interior.

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Gyrokinetic simulations of spherical tokamaks

Cited by 94 publications

References 35 publications

Simulation Of Microtearing Turbulence In NSTX

Simulation Of Microtearing Turbulence In NSTX

Progress in simulating turbulent electron thermal transport in NSTX

Suppressing electron turbulence and triggering internal transport barriers with reversed magnetic shear in the National Spherical Torus Experiment

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