Role of zonal flow predator-prey oscillations in triggering the transition to H-mode confinement.
ABSTRACT Direct evidence of zonal flow (ZF) predator-prey oscillations and the synergistic roles of ZF- and equilibrium E×B flow shear in triggering the low- to high-confinement (L- to H-mode) transition in the DIII-D tokamak is presented. Periodic turbulence suppression is first observed in a narrow layer at and just inside the separatrix when the shearing rate transiently exceeds the turbulence decorrelation rate. The final transition to H mode with sustained turbulence and transport reduction is controlled by equilibrium E×B shear due to the increasing ion pressure gradient.
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ABSTRACT: To understand the L–H transition, one has to identify the modes to be stabilized at the edge of L-mode plasmas, roughly from ρ = 0.7 to the last closed flux surface. To address this issue, realistic edge tokamak parameters inspired by three different L-modes from DIII-D and Tore Supra have been investigated with a gyrokinetic code GENE (Jenko et al 2000 Phys. Plasmas 7 1904). Former fluid theories for such parameters predict resistive ballooning modes (RBMs) to be unstable (Rogers et al 1998 Phys. Rev. Lett. 81 4396). In this paper, linear gyrokinetic simulations demonstrate that, under realistic L-mode conditions, RBMs are linearly unstable at every edge, i.e. ρ ≥ 0.93. These modes predominantly drift in the electron diamagnetic direction at low wave numbers and are destabilized by higher collisionality. They are further destabilized by higher normalized temperature gradient and higher q. The magnetic shear and the density gradient length have a weaker impact.Plasma Physics and Controlled Fusion 10/2012; 54(11):115003. · 2.37 Impact Factor
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ABSTRACT: The consequences of vorticity conservation on the spatio-temporal interaction of a E × B zonal shear with a generic pattern of plasma potential modes are investigated in a magnetized plasma environment. Eddies organized on a chain along the zonal direction are locally depleted, resulting in what appears to be a radial decorrelation by the shear flow in the absence of dissipation. The eddy depletion occurs due to a transfer of enstrophy from the chain to the shear flow during the progressive growth in the chain anisotropy. The rate of zonal shear acceleration is derived analytically and its expression is validated by numerical simulations. The rate is proportional to the chain amplitude in the weak shear regime and to the shearing rate in the strong shear regime. Basic properties of the model are validated with fast visible imaging data collected on a magnetized plasma column experiment. A characteristic vorticity flux across the edge shear layer of tokamak plasmas is associated with the model predictions. The dependence of the interaction rate with turbulence amplitude and shearing rate could be an important ingredient of the low to high confinement mode transition.Plasma Physics and Controlled Fusion 01/2013; 55(2):025011. · 2.37 Impact Factor
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ABSTRACT: The edge and scrape-off layer (SOL) region of a tokamak plasma is considered, with emphasis on sheared flow generation and the dynamics of blob-filaments. Both numerical simulations and experimental data analysis are employed. The simulations use the fluid-based two-dimensional (2D) curvature-interchange model embedded in the SOLT code. A blob-tracking algorithm based on 2D time-resolved images from the gas puff imaging diagnostic has also been developed and applied to NSTX, Alcator C-Mod and simulation data. The algorithm is able to track the blob motion and changes in blob structure, such as elliptical deformations, that can be affected by sheared flows. Results of seeded blob simulations and quasi-steady turbulence simulations are compared with the experimental data to determine the role of plasma parameters on the blob tracks and to evaluate the exchange of momentum between the blobs and flows. The simulations are shown to reproduce many qualitative and quantitative features of the data including size, scale-length and direction of perpendicular (approximately poloidal) flows, the inferred Reynolds acceleration and residual stress, poloidal reversal of blob tracks, and blob trapping and/or ejection. Mechanisms related to blob motion, SOL currents and radial inhomogeneity are shown to be sufficient to explain the presence or absence of mean and oscillating zonal sheared flows in selected shots.Nuclear Fusion 06/2013; 53(7):073013. · 2.73 Impact Factor