Dynamics of zonal-flow-like structures in the edge of the TJ-II stellarator

Alonso, A.; Velasco, J. L.; Arévalo, Juan Miguel Insúa; Hidalgo, C.; Pedrosa, M. A.; Carralero, D.; Silva, Carlos

doi:10.1088/0741-3335/55/1/014001

Cited by 12 publications

(18 citation statements)

References 23 publications

(48 reference statements)

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“…While the residual zonal flow level does not appear as a quantity susceptible of direct measurement [25,26,27], the oscillation frequency does, and very recently an oscillatory relaxation of a (zonal) potential perturbation was experimentally detected in pellet injection experiments carried out in the TJ-II stellarator [4]. The experimental measurements were compared to global gyrokinetic simulations finding a qualitative agreement between experimental and simulated oscillation frequencies and damping rates.…”

Section: Introductionmentioning

confidence: 86%

Oscillatory relaxation of zonal flows in a multi-species stellarator plasma

Sánchez¹,

Calvo²,

Velasco³

et al. 2018

Plasma Phys. Control. Fusion

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The low frequency oscillatory relaxation of zonal potential perturbations is studied numerically in the TJ-II stellarator (where it was experimentally detected for the first time). It is studied in full global gyrokinetic simulations of multi-species plasmas. The oscillation frequency obtained is compared with predictions based on single-species simulations using simplified analytical relations. It is shown that the frequency of this oscillation for a multi-species plasma can be accurately obtained from single-species calculations using extrapolation formulas. The damping of the oscillation and the influence of the different inter-species collisions is studied in detail. It is concluded that taking into account multiple kinetic ions and electrons with impurity concentrations realistic for TJ-II plasmas allows to account for the values of frequency and damping rate in zonal flows relaxations observed experimentally.

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Section: Introductionmentioning

confidence: 86%

Oscillatory relaxation of zonal flows in a multi-species stellarator plasma

Sánchez¹,

Calvo²,

Velasco³

et al. 2018

Plasma Phys. Control. Fusion

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“…Generally, this picture is only slightly modified by considering the measured turbulent momentum transport. Indeed, local measurements of Reynolds stress [6] show average events Γ RS /n 0.1 m/s (which would not be perceptible in the evolution of E r , see Fig. 3) and infrequent extreme events of Γ RS /n 10 m/s (which would not fundamentally modify the results).…”

mentioning

confidence: 92%

“…In non-quasisymmetric stellarators, the non-ambipolar radial fluxes, and consequently the radial electric field, are basically determined by the neoclassical theory of collisional transport in magnetized plasma [5]. Therefore, the interaction between neoclassical and turbulent processes, and concretely the extent to which turbulence can overcome the neoclassical viscosity and modify the E × B rotation through momentum transport, has been the subject of recent works [4][5][6][7]. In this Letter, we contribute to the above programme with the study of the so-called low density confinement transition [8][9][10][11][12] in the flexible heliac TJ-II [13] which, as we discuss below, shows similarities with other transitions described in the literature.…”

mentioning

confidence: 99%

Vanishing Neoclassical Viscosity and Physics of the Shear Layer in Stellarators

Velasco¹,

Alonso²,

Calvo³

et al. 2012

Phys. Rev. Lett.

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The drift kinetic equation is solved for low density TJ-II plasmas employing slowly varying, time-dependent profiles. This allows us to simulate density ramp-up experiments and describe from first principles the formation and physics of the radial electric field shear layer. The main features of the transition are perfectly captured by the calculation, and good quantitative agreement is also found. The results presented here, that should be valid for other nonquasisymmetric stellarators, provide a fundamental explanation for a wealth of experimental observations connected to the shear layer emergence in TJ-II. The key quantity is the neoclassical viscosity, which is shown to go smoothly to zero when the critical density is approached from below. This makes it possible for turbulence-related phenomena, and particularly zonal flows, to arise in the neighborhood of the transition.

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“…In some experiments, bursty transport, Reynolds accelerations and edge flows have been shown to be linked. [12,13,[27][28][29] The role of blob and eddy tilting, also recently investigated in other studies [30][31][32], will be examined as both as a theoretical mechanism and a diagnostic of Reynolds stress in particular experimental discharges.…”

Section: Introductionmentioning

confidence: 98%

Edge sheared flows and the dynamics of blob-filaments

et al. 2013

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The edge and scrape-off-layer (SOL) region of a tokamak plasma is considered, with emphasis on sheared flow generation and the dynamics of blob-filaments. Both numerical simulations and experimental data analysis are employed. The simulations use the fluid-based 2D curvature-interchange model embedded in the SOLT code. A blob-tracking algorithm based on 2D time-resolved images from the gas puff imaging diagnostic has also been developed and applied to NSTX, Alcator C-Mod and simulation data. The algorithm is able to track the blob motion and changes in blob structure, such as elliptical deformations, that can be affected by sheared flows. Results of seeded blob simulations and quasi-steady turbulence simulations are compared with the experimental data to determine the role of plasma parameters on the blob tracks and to evaluate the exchange of momentum between the blobs and flows. The simulations are shown to reproduce many qualitative and quantitative features of the data including size, scale-length and direction of perpendicular (approximately poloidal) flows, the inferred Reynolds acceleration and residual stress, poloidal reversal of blob tracks, and blob trapping and/or ejection. Mechanisms related to blob motion, SOL currents and radial inhomogeneity are shown to be sufficient to explain the presence or absence of mean and oscillating zonal sheared flows in selected shots.

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Dynamics of zonal-flow-like structures in the edge of the TJ-II stellarator

Cited by 12 publications

References 23 publications

Oscillatory relaxation of zonal flows in a multi-species stellarator plasma

Oscillatory relaxation of zonal flows in a multi-species stellarator plasma

Vanishing Neoclassical Viscosity and Physics of the Shear Layer in Stellarators

Edge sheared flows and the dynamics of blob-filaments

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