Turbulent-driven low-frequency sheared<i>E</i>×<i>B</i>flows as the trigger for the H-mode transition

Tynan, G. R.; Xu, M.; Diamond, P. H.; Boedo, J. A.; Cziegler, I.; Fedorczak, N.; Mänz, P.; Miki, K.; Thakur, Saikat Chakraborty; Schmitz, L.; Zeng, L.; Doyle, E. J.; McKee, G.; Yan, Z.; Xu, G. S.; Wan, Baonian; Wang, Huiqian; Guo, H.Y.; Dong, J. Q.; Zhao, K.J.; Cheng, J.; Hong, W.Y.; Yan, L.W.

doi:10.1088/0029-5515/53/7/073053

Cited by 107 publications

(157 citation statements)

References 15 publications

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“…Results from several recent experiments have reinforced the suggestion that turbulence-driven flows trigger the H-mode transition. [4][5][6][7] A limit cycle oscillation (LCO) regime is observed before the transition, implying the existence of selfregulating interaction between the turbulence and the flow. 8,9 A quantitative transition criterion has been suggested.…”

Section: Introductionmentioning

confidence: 99%

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Flux-driven simulations of turbulence collapse

et al. 2015

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Using three-dimensional nonlinear simulations of tokamak turbulence, we show that an edge transport barrier (ETB) forms naturally once input power exceeds a threshold value. Profiles, turbulence-driven flows, and neoclassical coefficients are evolved self-consistently. A slow power ramp-up simulation shows that ETB transition is triggered by the turbulence-driven flows via an intermediate phase which involves coherent oscillation of turbulence intensity and E Â B flow shear. A novel observation of the evolution is that the turbulence collapses and the ETB transition begins when R T > 1 at t ¼ t R (R T : normalized Reynolds power), while the conventional transition criterion (x EÂB > c lin where x EÂB denotes mean flow shear) is satisfied only after t ¼ t C ( >t R ), when the mean flow shear grows due to positive feedback. V C 2015 AIP Publishing LLC.

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Section: Introductionmentioning

confidence: 99%

“…H transition. While some experiments 4,5,7,9 suggest that significant zonal flow growth by Reynolds work precedes, and in fact triggers, the L ! H transition, others 19,20 suggest that the mean E Â B flow evolution leads that of the zonal flow.…”

Section: Introductionmentioning

confidence: 99%

Flux-driven simulations of turbulence collapse

et al. 2015

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“…Recent studies of the edge plasma using Langmuir probes have provided evidence that the immediate trigger of the transition is the nonlinear exchange of kinetic energy between small scale turbulence and edge zonal flows. [10,11] Due to the potential difficulties which arise in interpreting probe signals in an environment of fluctuating plasma temperature, it is very important to ascertain the validity of these measurements via independent diagnostics. This paper presents the first gas-puff-imaging based study of the nonlinear interaction between the small scale edge turbulence and zonal flows in a time resolved sense in typical fast L-H transitions in Alcator C-Mod.…”

Section: Introductionmentioning

confidence: 99%

Zonal flow production in the L–H transition in Alcator C-Mod

Cziegler¹,

Tynan²,

Diamond³

et al. 2014

Plasma Phys. Control. Fusion

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Abstract. Transitions of tokamak confinement regimes from low-to highconfinement are studied on Alcator C-Mod [1] tokamak using gas-puff-imaging (GPI) with a focus on the interaction between the edge drift-turbulence and the local shear flow. Results show that the nonlinear turbulent kinetic energy transfer rate into the shear flow becomes comparable to the estimated value of the drift turbulence growth rate at the time the turbulent kinetic energy starts to drop, leading to a net energy transfer that is comparable to the observed turbulence losses. A corresponding growth is observed in the shear flow kinetic energy. The above behavior is demonstrated across a series of experiments. Thus both the drive of the edge zonal flow and the initial reduction of turbulence fluctuation power are shown to be consistent with a lossless kinetic energy conversion mechanism, which consequently mediates the transition into H-mode. The edge pressure gradient is then observed to build on a slower (1ms) timescale, locking in the H-mode state. These results unambiguously establish the time sequence of the transition as: first the peaking of the normalized Reynolds power, then the collapse of the turbulence, and finally the rise of the diamagnetic electric field shear as the L-H transition occurs.

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“…In this article blob refers to a coherent structure with high density with respect to the surrounding fluctuation level. It is widely accepted that poloidal shear flows increase the particle and energy confinement in fusion devices [2][3][4][5][6]. Recently, the extraordinary role of shear layers for blob generation has been identified [7][8][9][10][11][12].…”

Section: Introductionmentioning

confidence: 99%

Turbulent transport across shear layers in magnetically confined plasmas

et al. 2014

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Shear layers modify the turbulence in diverse ways and do not only suppress it. A spatialtemporal investigation of gyrofluid simulations in comparison with experiments allows to identify further details of the transport process across shear layers. Blobs in and outside a shear layer merge, thereby exchange particles and heat and subsequently break up. Via this mechanism particles and heat are transported radially across shear layers. Turbulence spreading is the immanent mechanism behind this process.

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Turbulent-driven low-frequency shearedE×Bflows as the trigger for the H-mode transition

Cited by 107 publications

References 15 publications

Flux-driven simulations of turbulence collapse

Flux-driven simulations of turbulence collapse

Zonal flow production in the L–H transition in Alcator C-Mod

Turbulent transport across shear layers in magnetically confined plasmas

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