Experimental observation of coupling between turbulence and sheared flows during L-H transitions in a toroidal plasma

Estrada, T.; Happel, T.; Hidalgo, C.; Ascasíbar, E.; Blanco, E.

doi:10.1209/0295-5075/92/35001

Cited by 92 publications

(120 citation statements)

References 30 publications

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“…Similar phenomena have been reported and discussed at JET and elsewhere [2,[5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20], usually concentrating on electrostatic fluctuation measurements in similar frequency ranges. This is a rapidly advancing field, references [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] may not include all the latest or relevant results, but are representative.…”

Section: Introduction and General Description Of M-modesupporting

confidence: 76%

Axisymmetric oscillations at L–H transitions in JET: M-mode

Solano¹,

Vianello

Delabie

et al. 2016

Nucl. Fusion

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Abstract. L to H transition studies at JET have revealed an n=0 m=1 magnetic oscillation starting immediately at the L to H transition (called M-mode for brevity). While the oscillation is present a weak ELM-less H-mode regime is obtained, with a clear increase of density and a weak T e pedestal, with medium confinement, between high (H-mode) and low (L-mode). In ICRH heated plasmas or low density NBI plasmas the mode and the pedestal pressure can remain steady for the duration of the heating phase, of order 10 s or more. The axisymmetric magnetic oscillation has period ~ 1-2 ms, and poloidal mode number m=1: it looks like a pedestal localised up/down oscillation, although it is clearly a natural oscillation of the plasma, not driven by the position control system. Electron Cyclotron Emission, interferometry, reflectometry and fast Li beam measurements locate the mode in the pedestal region. D α , fast infrared camera and Langmuir probe measurements show that the mode modulates heat and particle flux to the target. The mode frequency appears to scale with the poloidal Alfvén velocity, and not with sound speed (i.e., it is not a Geodesic Acoustic Mode). A heuristic model is proposed for the frequency scaling of the mode. We discuss the relationship between Mmode and other related observations near the L-H transition.

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Section: Introduction and General Description Of M-modesupporting

confidence: 76%

Axisymmetric oscillations at L–H transitions in JET: M-mode

Solano¹,

Vianello

Delabie

et al. 2016

Nucl. Fusion

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“…As reported in [10], close to the L−H transition threshold conditions pronounced oscillations in both, E r and density fluctuation level are measured at the plasma edge, right inside the E r −shear position but not outside. Depending on the NBI heating power and magnetic configuration, the oscillations can endure the whole NBI heating phase of the discharge.…”

supporting

confidence: 77%

“…A dynamic behaviour of plasma parameters occurring near the L−H transition threshold conditions was first observed in the H−1 stellarator [6] and later in the DIII−D tokamak [7,8] where experimental evidence was found supporting the predator−prey relationship between turbulence and flows as the basis for the L−H transition. Recently, this intermediate stage has also been observed in other devices: NSTX [9], TJ−II [10], AUG [11] and EAST [12].…”

mentioning

confidence: 66%

“…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.…”

mentioning

confidence: 99%

“…At about t ≈ 170 ms the H α drops, the plasma energy content and density increase and the spectrogram of the Doppler reflectometer signal shows an increase in its frequency and the onset of the oscillations. The oscillations appear as changes in the intensity and frequency of the Doppler reflectometry spectra, and show a predator−prey relationship between turbulence and flows, with the flow −the predator− following the turbulence −the prey− with a phase delay of 90 • in a limit−cycle way [10]. Contrary to DIII−D or AUG observations, in the TJ−II case no signatures of the oscillations appear in the H α signals or in the evolution of plasma density and temperature, indicating a very small effect, if any, on particle transport.…”

mentioning

confidence: 99%

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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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Turbulence as a Problem in Non-equilibrium Statistical Mechanics

Goldenfeld

Shih

2016

J Stat Phys

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The transitional and well-developed regimes of turbulent shear flows exhibit a variety of remarkable scaling laws that are only now beginning to be systematically studied and understood. In the first part of this article, we summarize recent progress in understanding the friction factor of turbulent flows in rough pipes and quasi-two-dimensional soap films, showing how the data obey a two-parameter scaling law known as roughness-induced criticality, and exhibit power-law scaling of friction factor with Reynolds number that depends on the precise form of the nature of the turbulent cascade. These results hint at a nonequilibrium fluctuation-dissipation relation that applies to turbulent flows. The second part of this article concerns the lifetime statistics in smooth pipes around the transition, showing how the remarkable super-exponential scaling with Reynolds number reflects deep connections between large deviation theory, extreme value statistics, directed percolation and the onset of coexistence in predator-prey ecosystems. Both these phenomena reflect the way in which turbulence can be fruitfully approached as a problem in non-equilibrium statistical mechanics.

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Experimental observation of coupling between turbulence and sheared flows during L-H transitions in a toroidal plasma

Cited by 92 publications

References 30 publications

Axisymmetric oscillations at L–H transitions in JET: M-mode

Axisymmetric oscillations at L–H transitions in JET: M-mode

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

Turbulence as a Problem in Non-equilibrium Statistical Mechanics

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