Zonal flow triggers the L-H transition in the Experimental Advanced Superconducting Tokamak

Abstract

The kinetic energy transfer between shear flows and the ambient turbulence is investigated in the Experimental Advanced Superconducting Tokamak during the L-H transition. As the rate of energy transfer from the turbulence into the shear flow becomes comparable to the energy input rate into the turbulence, the transition into the H-mode occurs. As the observed behavior exhibits several predicted features of zonal flows, the results show the key role that zonal flows play in mediating the transition into H-mode. V C 2012 American Institute of Physics. [http://dx.doi.org/ 10.1063/1.4737612] INTRODUCTION The transition from low (L-mode) to high confinement (H-mode) regime in magnetized confined fusion devices occurs very rapidly at a critical condition, similar, in a sense, to leaning slightly over the side of a canoe causing only a small tilt of the craft, but leaning slightly more may roll you and the craft into the lake. Rapid threshold transitions between distinctly different stable states then require a triggering event, akin to leaning out too far from the canoe. 1 However, the physics that triggers the transition into Hmode is not understood, and thus predictions of the conditions for the transition into the H-mode regime-which are critical for the operation of ITER in the burning plasma regime-are based on empirical scalings with a wide range of uncertainty. Relative to the conditions found in low confinement regimes, H-mode plasmas are characterized by a reduced turbulence level and strong radial electric field (E r ) shear. 2,3 Azimuthally symmetric, bandlike, time-varying, turbulent generated shear flows called zonal flows (ZFs) also appear to be associated with the L-H transition. 4,5 Therefore, the interaction between micro and macroscale turbulent fluctuations has developed into one of the most active research topics in the physics of magnetized plasmas. The main focus has been placed on the generation of zonal flows and the reduction of the ambient turbulence via the nonlinear exchange, or transfer, of energy from the smaller scaled higher frequency turbulent fluctuations into the large scale, low frequency ordered zonal flow. 6-14 SELF-REGULATION OF TURBULENCETheory predicts that the L-H transition can be explained by an intermediate, quasi-periodic transient stage, where turbulence, zonal flow, mean shear flow, and the pressure gradient are coupled. 15,16 In this model, as the input power increases the pressure gradient also increases, resulting in stronger instabilities and fluctuation levels. The turbulence level grows and begins to nonlinearly drives the zonal flow until the zonal flow drive can overcome the flow damping. A finite zonal flow then begins to grow and extract kinetic energy from the turbulence and thereby acts to suppress the turbulence amplitude. Zonal flows can trigger the transition by regulating the turbulence level and associated transport until the mean shear flow is high enough to suppress the remaining turbulence and associated transport, causing the pressure gradient to i...

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

“…Consistent with the expectation from probe measurements [10] on other devices and the Kim-Diamond predator-prey model [18,9], the production term P = ṽ rṽθ ∂ r v θ exhibits a rapidly growing peak in time during these changes (Fig. 7e).…”

“…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.…”

Flux-driven simulations of turbulence collapse

“…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.…”

“…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.…”

Turbulent transport across shear layers in magnetically confined plasmas

“…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].…”