Sheared flows and transition to improved confinement regime in the TJ-II stellarator

Estrada, T.; Happel, T.; Eliseev, L.G.; López‐Bruna, D.; Ascasíbar, E.; Blanco, E.; Cupido, L.; Fontdecaba, J. M.; Hidalgo, C.; Jiménez‐Gómez, R.; Krupnik, L. I.; Liniers, M.; Manso, M. E.; McCarthy, K. J.; Medina, F.; Melnikov, A. V.; Milligen, B. Ph. van; Ochando, M. A.; Pastor, I.; Pedrosa, M. A.; Tabarés, F.L.; Tafalla, D.; Team, Tj‐Ii

doi:10.1088/0741-3335/51/12/124015

Cited by 110 publications

(162 citation statements)

References 29 publications

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“…Due to the temporal and spatial scales involved in the L−H transition physics, specific experimental techniques are required to experimentally investigate the turbulence and flows dynamics [16]. In TJ−II, a two−channel Doppler reflectometer is used that allows the measurement of the radial electric field, E r and density fluctuations at two radial positions simultaneously with good spatial and temporal resolution [17].…”

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Spatiotemporal Structure of the Interaction between Turbulence and Flows at the L-H Transition in a Toroidal Plasma

Estrada¹,

Hidalgo²,

et al. 2011

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The spatio-temporal behavior of the interaction between turbulence and flows has been studied close to the L−H transition threshold conditions in the edge region (ρ ≥ 0.7) of TJ−II plasmas. The temporal dynamics of the interaction displays an oscillatory behaviour with a characteristic predator−prey relationship. The spatial evolution of this turbulence−flow oscillation−pattern has been measured, for the first time, showing both, radial outward and inward propagation velocities of the turbulence−flow front. The results indicate that the edge shear flow linked to the L−H transition can behave either as a slowing−down, damping mechanism of outward propagating turbulent−flow oscillating structures, or as a source of inward propagating turbulence-flow events.The High confinement mode (H−mode) regime has been extensively studied since its discovery in the AS-DEX tokamak [1]. Although significant progress has been made in describing the transition, the physical mechanism triggering the H-mode has still not been clearly identified. Bifurcation theory models based on the coupling between turbulence and radially sheared E×B flows (sheared flows) describes the Low to High confinement mode transition (L−H transition) passing through an intermediate, oscillatory transient stage [2,3]. These models consist of coupled evolution equations for turbulence, sheared flow and pressure gradient. Using the input power as a control parameter for the pressure gradient, these dynamical systems evolve from L− to H−mode. By increasing the pressure gradient, the instability grows until it is damped by the self-generated sheared flows. The transition occurs when the turbulence driven sheared flow is high enough to overcome the flow damping. As it is discussed in Ref. [3], by including the evolution of zonal flows self-consistently, the critical input power for the transition is lowered. Further studies [4] show that zonal flows are a necessary step for the transition. Zonal flows trigger the transition by regulating the turbulence until the mean shear flow is high enough to suppress turbulence effectively, which in turn subsequently impedes the zonal flow generation. Due to the self-regulation between turbulence and flows, the transition is marked by an oscillatory behavior with a characteristic predator−prey relationship and EAST [12].In these experiments, as in the predator−prey theory model [3], only the temporal dynamics of the turbulence−flow interaction is studied. Even though the spatial structure of the oscillating flow in the plasma edge region is shown in TJ−II [10] and AUG [11] and in a wider radial region in H−1 [13,14], no information is given on its spatial evolution or spatial propagation. However, as it has been pointed out in Ref. [15], where the 0−dimensional predator−prey theory model is upgraded toward a 1−dimensional one, the spatial evolution should also be taken into account as a necessary step to go towards the L−H transition model.The present work addresses for the first time this fundamental issue from the experime...

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confidence: 99%

Spatiotemporal Structure of the Interaction between Turbulence and Flows at the L-H Transition in a Toroidal Plasma

Estrada¹,

Hidalgo²,

et al. 2011

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“…It is generally believed that edge ExB shear, through mean or zonal flows, is important in turbulence suppression that can lead to the L-H transition 13,12 [and references therein]. There has also recently been some more evidence of the importance of zonal flows in the L-H transition, [14][15][16][17] but this is still an area of active experimental research and validation of theory.…”

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confidence: 99%

L–H threshold studies in NSTX

et al. 2011

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Recent experiments in the low aspect ratio National Spherical Torus Experiment (NSTX) have been run in support of the high priority ITER and ITPA issue of access to the H-mode. Specifically, a series of experiments showed reduced power threshold values for deuterium vs helium plasmas, and for plasmas with lower current, lower triangularity and with lithium conditioning. Application of n=3 fields at the plasma edge resulted in higher power thresholds. To within the constraints of temporal and spatial resolutions, no systematic difference in T e , n e , p e , T i , v or their derivatives was found in discharges that transitioned into the H-mode versus those at slightly lower power that did not. Finally, H 98y,2 ∼1 confinement quality could be achieved for powers just above the threshold power in ELM-free conditions.

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“…38. The turbulence induced flow shear is generated causing a reduction in the density fluctuations and a subsequent drop in the flow shear, which is followed by the increase in the density fluctuations [90][91][92]. These results have many of the characteristics of the predicted dynamics of L to H transition models based on turbulence driven flows.…”

Section: To H-mode Transitionmentioning

confidence: 79%

“…The increase in the low frequency E r fluctuations and the reduction in the high frequency density fluctuations are measured a few ms before the E r field shear development [91]. Analysis of the Doppler reflectometer signals in discharges with a slow L to H transition enables the study of the coupling between the sheared flows and turbulence.…”

Section: To H-mode Transitionmentioning

confidence: 99%

23rd IAEA Fusion Energy Conference: summary of sessions EX/C and ICC

Hawryluk¹

2011

Nucl. Fusion

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Abstract:An overview is given of recent experimental results in the areas of innovative confinement concepts, operational scenarios and confinement experiments as presented at the 2010 IAEA Fusion Energy Conference. Important new findings are presented from fusion devices worldwide, with a strong focus towards the scientific and technical issues associated with ITER and W7-X devices, presently under construction.

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Sheared flows and transition to improved confinement regime in the TJ-II stellarator

Cited by 110 publications

References 29 publications

Spatiotemporal Structure of the Interaction between Turbulence and Flows at the L-H Transition in a Toroidal Plasma

Spatiotemporal Structure of the Interaction between Turbulence and Flows at the L-H Transition in a Toroidal Plasma

L–H threshold studies in NSTX

23rd IAEA Fusion Energy Conference: summary of sessions EX/C and ICC

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