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: Dedicated experiments have been carried out at the TEXTOR tokamak in biasing-induced improved confinement regimes using Langmuir probe and GPI measurements. The results show the first evidence for the intimate interaction among sheared flows, eddy structures, Reynolds stress, zonal flows, and ambient fluctuations during a transition to an improved confinement. The sheared flows impose dynamic effects on turbulence structures, resulting in generation of Reynolds stress and zonal flows and eventually the suppression of background fluctuation levels via nonlinear energy transfer processes. The latter may therefore trigger a transition to the H mode.Physical Review Letters 08/2013; 111(5). · 7.73 Impact Factor
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ABSTRACT: In this Letter, we report analyses of spatiotemporal dynamics of turbulence and structure in the limit-cycle oscillation (LCO) that precedes an L-to-H transition. Zonal flows are not observed during LCO, and the oscillation is the periodic generations or decays of barrier with edge-localized mean flow. Oscillatory Reynolds stress is found to be too small to accelerate the LCO flow, by considering the dielectric constant in magnetized toroidal plasmas. Propagation of changes of the density gradient and turbulence amplitude into the core is also observed.Physical Review Letters 07/2013; 111(3):035002. · 7.73 Impact Factor
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ABSTRACT: An intermediate phase (labeled I phase) with dithering cycles between the L-mode and H-mode has been observed and experimentally characterized on Experimental Advanced Superconducting Tokamak (EAST). A typical characteristics of the I phase is that the DαDα signal, edge density fluctuation level and edge radiation show several kHz periodical oscillation. The analysis shows that the dithering event is at least 2 cm inside the separatrix and extends into the scrape-off layer (SOL) region. It is found that this dithering occurs in plasma with double null (DN) or upper single null (USN) configuration and cannot be observed in plasma with lower single null (LSN) configuration where the ion B×∇BB×∇B drift direction is ‘unfavorable’, i.e. away from the X-point, in this device. The dithering cycle length (ΔtditherΔtdither) has no clear dependence on the heating power. Both stored energy and density increase during the dithering phase and the increasing rates decrease with ΔtditherΔtdither. The evolution of density profiles during the L–I–H transition is analyzed and presented.Physics Letters A 10/2013; 377(s 28–30):1725–1735. · 1.77 Impact Factor