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

Abstract

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...

Flux-driven simulations of turbulence collapse

“…This has features of a limit cycle oscillation, as predicted by predator-prey type models 12,14,16 and subsequently observed in various experiments. 8,9 Moreover, Fig. 4(c) shows that there is a phase delay of $p=2 between turbulence intensity and the E Â B flow shear during the LCO-like phase.…”

“…[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. The criterion is that the normalized Reynolds power R T > 1.…”

Coherent structures in ion temperature gradient turbulence-zonal flow

“…However, numerical simulations and experiments show that drift wave turbulence exhibits several non-Gaussian features like intermittency, bursty transport, etc. [8][9][10][11][12][13][14][15][16][17][18][19][20] These characteristics are related to the presence of coherent structures in drift-wave turbulence [21][22][23] and cannot be described by renormalized weak turbulence theories. Nonlinear coherent structure formation in the zonal flow envelope in simple drift wave zonal flow system has been studied in Refs.…”

Experimental Evidence of Turbulent Transport Regulation by Zonal Flows

“…In this way, the ZF (predator) is able to reduce turbulence (prey) leading to an interplay between ZFs and turbulence amplitudes as it is described in several types of predator-prey models [13,14,15]. These models have attracted attention since they are able to describe many aspects of the experimentally observed [16,17,18] transition from low (L-mode) to high (H-mode) confinement regimes in fusion devices including an oscillatory intermediate phase [19].…”