Role of Zonal Flow Predator-Prey Oscillations in Triggering the Transition to H-Mode Confinement

University of California-Los Angeles, Los Angeles, California 90095, USA.
Physical Review Letters (Impact Factor: 7.51). 04/2012; 108(15):155002. DOI: 10.1103/PhysRevLett.108.155002
Source: PubMed


Direct evidence of zonal flow (ZF) predator-prey oscillations and the synergistic roles of ZF- and equilibrium E×B flow shear in triggering the low- to high-confinement (L- to H-mode) transition in the DIII-D tokamak is presented. Periodic turbulence suppression is first observed in a narrow layer at and just inside the separatrix when the shearing rate transiently exceeds the turbulence decorrelation rate. The final transition to H mode with sustained turbulence and transport reduction is controlled by equilibrium E×B shear due to the increasing ion pressure gradient.

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    • "Two types of ZFs have been observed in toroidal confined plasmas, i.e. a low-frequency zonal flow (LFZF) with a near zero frequency and a geodesic acoustic mode (GAM) with a higher frequency [5] [6]. Recently, a number of experiments in several fusion plasma devices have shown the important role of ZFs in the L–H transition, including both LFZF and GAM [7] [8] [9] [10] [11]. These results are qualitatively consistent with the predator–prey model of the L–H transition, which includes the interactions between two competing predators (the mean shear flow and ZFs) and one prey (turbulence) [12]. "
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    ABSTRACT: Geodesic acoustic mode (GAM) and low-frequency zonal flow (LFZF) are both observed through Langmuir probe arrays during electron cyclotron resonance heating (ECRH) on the HL-2A tokamak edge. The radial distributions of the amplitude and peak frequency of GAM in floating potential fluctuations are investigated through rake probe arrays under different ECRH powers. It is observed that the GAM frequency would decrease and the intensity of carbon line emission would increase as the ECRH power exceeds a certain threshold. The analyses suggest that the impurity ions may play an important role in the GAM frequency at the edge region. It is also found that during the ECRH phase besides the mean flow, both GAM and LFZF are strengthened. The total fluctuation power and the fraction of that power associated with zonal flows both increase with the ECRH power, consistent with a predator-prey model. The auto- and cross-bicoherence analyses show the coupling between GAM and its second harmonic during the ECRH phase. Moreover, the results also suggest that the couplings between GAM and the components with multiple GAM frequency are strengthened. These couplings may be important for GAM saturation during the ECRH phase.
    Full-text · Article · Dec 2013 · Nuclear Fusion
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    • "Standard or fast L–H transitions are observed in the magnetic configuration having ι(a)/2π = 1.630 [6], while slow transitions are observed in the configuration having ι(a)/2π = 1.553 and a low order rational (3/2) at ρ = r/a ≃ 0.72. In the latter configuration, the socalled intermediate phase (I), characterized by a predator-prey type interaction between turbulence and flows, appears between the L and H phases [9] [10] [11], which has also been seen in models [12] [13] and on other devices [14] [15]. In both scenarios, spatiotemporal and scale resolved Doppler reflectometry measurements were performed in series of repetitive discharges [6] [7]. "
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    ABSTRACT: Plasma turbulence is studied using Doppler reflectometry at the TJ-II stellarator. By scanning the tilt angle of the probing beam, different values of the perpendicular wave numbers are probed at the reflection layer. In this way, the interaction between zonal flows and turbulence is reported with (a) spatial, (b) temporal, and (c) wavenumber resolution for the first time in any magnetic confinement fusion device. We report measurements of the bicoherence across the Low to High (L--H) confinement transition at TJ-II. We examine both fast transitions and slow transitions characterized by an intermediate (I) phase. The bicoherence, understood to reflect the non-linear coupling between the perpendicular velocity (zonal flow) and turbulence amplitude, is significantly enhanced in a time window of several tens of ms around the time of the L--H transition. It is found to peak at a specific radial position (slightly inward from the radial electric field shear layer in H mode), and is associated with a specific perpendicular wave number ($k_\perp \simeq 6-12$ cm$^{-1}$, $k_\perp \rho_s \simeq 0.8-2$). In all cases, the bicoherence is due to the interaction between high frequencies ($\simeq 1$ MHz) and a rather low frequency ($\lesssim 50$ kHz), as expected for a zonal flow.
    Full-text · Article · Nov 2012 · Nuclear Fusion
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    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. (C) 2012 American Institute of Physics. []
    Full-text · Article · Jul 2012 · Physics of Plasmas
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