Impact of magnetic islands on plasma flow and turbulence in W7-X

Estrada, T.; Maragkoudakis, E.; Carralero, D.; Windisch, T.; Velasco, J. L.; Killer, C.; Andreeva, T.; Geiger, J.; Dinklage, A.; Krämer‐Flecken, A.; Wurden, G. A.; Beurskens, M.; Bozhenkov, S.; Damm, H.; Fuchert, G.; Pasch, E.

doi:10.1088/1741-4326/ac146f

Cited by 12 publications

(12 citation statements)

References 55 publications

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“…Experiments performed in several devices have demonstrated that this assumption is, in general, valid [39][40][41][42], and only in high collisionality plasmas, the contribution of v ph to v ⊥ may become relevant [43,44]. Besides, previous results obtained in W7-X show a good agreement between the experimental E r profiles and those obtained from neoclassical predictions [18,45]. Thus, it is sensible to assume that this condition holds also in the present experiments.…”

Section: Doppler Reflectometry Measurementssupporting

confidence: 77%

“…As a consequence, it is difficult to know if the k ⊥ range of fluctuations observed by the DR in these discharges is below or above the spatial scale which should be decorrelated by the shear. As well, other examples exist of instances in which a strong radial gradient of E r seems to be followed by a strong local reduction of the fluctuation amplitude, as measured by the DR. One such case is the strong sheared structure caused in u ⊥ when an island chain of sufficient width is found inside the LCFS [45]. Therefore, while no clear evidence of shear-suppression effect on the turbulence has been found in this work, this analysis will need to be refined before it can be safely stated that there is no such mechanism at W7-X.…”

Section: Discussion and Summarymentioning

confidence: 80%

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On the interaction between the island divertor heat fluxes, the scrape-off layer radial electric field and the edge turbulence in Wendelstein 7-X plasmas

Maragkoudakis¹,

Carralero²,

Estrada³

et al. 2022

Nucl. Fusion

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The formation of the radial electric field, Er in the SOL has been experimentally studied for attached divertor conditions in stellarator W7-X. The main objective of this study is to test the validity in a complex 3D island divertor of simple models, typically developed in tokamaks, relating Er in the SOL to the sheath potential drop gradient at the target. Additionally, we investigate the effect of the edge Er shear on the reduction of density fluctuation amplitude, a well-established phenomenon according to the existing bibliography. The main diagnostic for measurements in the SOL is a V-band Doppler reflectometer that can provide the measurement of the Er and density fluctuations with good spatial resolution. Three-dimensional measurements of divertor parameters have been carried out using infrared cameras, with the exponential decay length of the divertor heat flux (λq) resulting a suitable proxy for the model-relevant λT, the exponential decay length of the temperature at the divertor. In the investigated attached regimes, it is shown for the first time that the formation of the Er in the SOL depends on parameters at the divertor, following a Er ∝ Te/λq qualitatively similar to that found in a tokamak. Then, from the analyzed plasmas, the observed Er shear at the edge is linked to a moderate local reduction of the amplitude of density fluctuations.

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Section: Doppler Reflectometry Measurementssupporting

confidence: 77%

Section: Discussion and Summarymentioning

confidence: 80%

On the interaction between the island divertor heat fluxes, the scrape-off layer radial electric field and the edge turbulence in Wendelstein 7-X plasmas

Maragkoudakis¹,

Carralero²,

Estrada³

et al. 2022

Nucl. Fusion

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“…These observations are interpreted in terms of local changes in E r induced by the rational surface that could facilitate or even trigger the L-H transition when the spin-up of the E × B flow takes place at the relevant edge layer. This interpretation is supported by experimental results showing the impact of the magnetic islands on flows and turbulence in TJ-II [67,68] as well as in other devices [69][70][71][72][73] and in numerical simulations [74].…”

Section: (D) Impact Of Magnetic Topologysupporting

confidence: 64%

H-mode transition in the TJ-II stellarator plasmas

Estrada¹,

Hidalgo²

2023

Phil. Trans. R. Soc. A.

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Since the first H-mode transitions were observed in TJ-II plasmas in 2008, an extensive experimental effort has been done aiming at better physics understanding of confinement transitions. In this article, an overview of the main findings related to the L-H transition in TJ-II is presented including how the radial electric field is driven, what are the possible mechanisms for turbulence suppression and what are the related temporal and spatial scales that can impact the transition. The trigger of the L-H transition in TJ-II plasmas is found to be more correlated with the development of fluctuating E × B flows than with steady-state E r effects, pointing to the role played by zonal flows in mediating the transition. Experimental evidence supporting the predator–prey relationship between turbulence and flows as the basis for the L-H transition, found for the first time in TJ-II, reinforces this conclusion. Besides, the reduction in the turbulent transport at the transition is detected at the barrier region but also in a wider radial range with weak or even zero E × B flow shear, which points to other mechanisms beyond the turbulence suppression by local sheared flows. This article is part of a discussion meeting issue ‘H-mode transition and pedestal studies in fusion plasmas’.

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“…And third, a lower boundary is set to minimize errors in determining the turbulence heatflux Q turb = Q exp − Q NC by avoiding zero crossings of the ambipolar radial electric field E r, and thus ensuring that E r is negative across the entire profile (ion root). E r profile measurements with DR [56] and CXRS [] have confirmed the ion root conditions across the full profile for each power step.…”

Section: Conclusion and Discussionmentioning

confidence: 91%

“…The latter is of utmost important when considering avenues that can suppress ITG turbulence, as is done in e.g. post-pellet experiments [7,8,56]. In W7-X, with its beneficial 3D properties [5,6], a so-called stability valley with elevated a/L T i = a/L n can be achieved when ITGs and densitygradient-driven TEMs, or so-called iTEMs, are simultaneously suppressed [6,7].…”

Section: Modelling Of Ion Turbulent Heat Transportmentioning

confidence: 99%

Ion temperature clamping in Wendelstein 7-X electron cyclotron heated plasmas

et al. 2021

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The neoclassical transport optimization of the Wendelstein 7-X stellarator has not resulted in the predicted high energy confinement of gas fueled electron-cyclotron-resonance-heated (ECRH) plasmas as modelled in (Turkin et al 2011 Phys. Plasmas 18 022505) due to high levels of turbulent heat transport observed in the experiments. The electron-turbulent-heat transport appears non-stiff and is of the electron temperature gradient (ETG)/ion temperature gradient (ITG) type (Weir et al 2021 Nucl. Fusion 61 056001). As a result, the electron temperature T e can be varied freely from 1 keV-10 keV within the range of P ECRH = 1-7 MW, with electron density n e values from 0.1-1.5 × 10 20 m −3 . By contrast, in combination with the broad electron-to-ion energy-exchange heating profile in ECRH plasmas, ion-turbulent-heat transport leads to clamping of the central ion temperature at T i ∼ 1.5 keV ± 0.2 keV. In a dedicated ECRH power scan at a constant density of n e = 7 × 10 19 m −3 , an apparent 'negative ion temperature profile stiffness' was found in the central plasma for (r/a < 0.5), in which the normalized gradient ∇T i /T i decreases with increasing ion heat flux. The experiment was conducted in helium, which has a higher radiative density limit compared to hydrogen, allowing a broader power scan. This 'negative stiffness' is due to a strong exacerbation of turbulent transport with an increasing ratio of T e /T i in this electron-heated plasma. This finding is consistent with electrostatic microinstabilities, such as ITG-driven turbulence. Theoretical calculations made by both linear and nonlinear gyro-kinetic simulations performed by the GENE code in the W7-X three-dimensional geometry show a strong enhancement of turbulence with an increasing ratio of T e /T i . The exacerbation of turbulence with increasing T e /T i is also found in tokamaks and inherently enhances ion heat transport in electron-heated

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Impact of magnetic islands on plasma flow and turbulence in W7-X

Cited by 12 publications

References 55 publications

On the interaction between the island divertor heat fluxes, the scrape-off layer radial electric field and the edge turbulence in Wendelstein 7-X plasmas

On the interaction between the island divertor heat fluxes, the scrape-off layer radial electric field and the edge turbulence in Wendelstein 7-X plasmas

H-mode transition in the TJ-II stellarator plasmas

Ion temperature clamping in Wendelstein 7-X electron cyclotron heated plasmas

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