Turbulence Mechanisms of Enhanced Performance Stellarator Plasmas

Xanthopoulos, P.; Bozhenkov, S.; Beurskens, M.; Smith, H. M.; Plunk, G. G.; Helander, P.; Beidler, C. D.; Alcusón, J.; Alonso, A.; Dinklage, A.; Ford, O.; Fuchert, G.; Geiger, J.; Proll, J. H. E.; Pueschel, M. J.; Turkin, Y.; Warmer, F.

doi:10.1103/physrevlett.125.075001

Cited by 46 publications

(54 citation statements)

References 38 publications

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“…2015; Xanthopoulos et al. 2020) have hinted that this is indeed the case. The present paper investigates the question directly, demonstrating the fully nonlinear effect of trapped electron stabilisation on TEM turbulence, and also how the effect extends to ITG turbulence, by comparing simulation results obtained in the high-mirror configuration of W7-X, HSX and the DIII-D tokamak (Luxon 2002).…”

Section: Introductionmentioning

confidence: 94%

Turbulence mitigation in maximum-J stellarators with electron-density gradient

et al. 2022

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In fusion devices, the geometry of the confining magnetic field has a significant impact on the instabilities that drive turbulent heat loss. This is especially true of stellarators, where the density-gradient-driven branch of the ‘trapped electron mode’ (TEM) is predicted to be linearly stable if the magnetic field has the maximum-J property, as is very approximately the case in certain magnetic configurations of the Wendelstein 7-X experiment (W7-X). Here we show, using both analytical theory and simulations, that the benefits of the optimisation of W7-X also serve to mitigate ion-temperature-gradient (ITG) modes as long as an electron density gradient is present. We find that the effect indeed carries over to nonlinear numerical simulations, where W7-X has low TEM-driven transport, and reduced ITG turbulence in the presence of a density gradient, giving theoretical support for the existence of enhanced confinement regimes, in the presence of strong density gradients (e.g. hydrogen pellet or neutral beam injection).

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Section: Introductionmentioning

confidence: 94%

Turbulence mitigation in maximum-J stellarators with electron-density gradient

et al. 2022

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“…As both the density gradient and the ion root radial electric field rise after the pellet injection, a theoretical analysis using nonlinear gyro-kinetic simulations with the GENE code compared the effect of strong density gradients as well as a radial electric field on the turbulent transport. The results show that both can help to reduce the turbulent transport, but that in the case of the pellet experiments, the introduction of a strong density gradient is likely to be the main driving mechanism for the (ITG) turbulence suppression [7]. It is found that simultaneous TEM turbulence is not strongly induced by the enhanced density gradients, and instead, a so-called ion-TEM or iTEM is the dominant mode during the high-T i phase.…”

Section: Conclusion and Discussionmentioning

confidence: 90%

“…This neoclassical transport optimization has been experimentally demonstrated in plasmas in which the turbulent heat transport is suppressed by means of steep density gradients. Thanks to the beneficial 3D geometry of W7-X [5], both the ion temperature gradient (ITG) and trapped-electron-mode (TEM) turbulence may be reduced or even suppressed when the ion temperature and density gradients align [6,7] and the (ion) neoclassical transport becomes the more dominant transport mechanism. These conditions were achieved after a train of ice pellets transiently produced a peaked density profile.…”

Section: Introductionmentioning

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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“…However, it has recently been shown that the radial electric field itself can reduce turbulence by shifting the band of strong fluctuations out of the region of strong unfavourable curvature (Xanthopoulos et al. 2020).…”

Section: Discussionmentioning

confidence: 99%

Phase contrast imaging measurements and numerical simulations of turbulent density fluctuations in gas-fuelled ECRH discharges in Wendelstein 7-X

et al. 2021

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The fundamental nature of turbulent density fluctuations in standard Wendelstein 7-X (W7-X) stellarator discharges is investigated experimentally via phase contrast imaging (PCI) in combination with gyrokinetic simulations with the code GENE. We find that density fluctuations are ion-temperature-gradient-driven and radially localised in the outer half of the plasma. It is shown that the line-integrated PCI measurements cover the right range of wavenumbers and a favourable toroidal and poloidal location to capture some of the strongest density fluctuations in W7-X. Due to the radial localisation of fluctuations, measured wavenumber–frequency spectra exhibit a dominant phase velocity, which can be related to the $\boldsymbol {E\times B}$ rotation velocity at the radial position of a well in the neoclassical radial electric field. The match is robust against variations of heating power and line-integrated density, which is partly due to the localisation of fluctuations and partly due to effects of the radial gradient in the $\boldsymbol {E\times B}$ velocity profile on the wavenumber–frequency spectrum. The latter effect is studied with a newly built synthetic PCI diagnostic and global gyrokinetic simulations with GENE-3D.

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Turbulence Mechanisms of Enhanced Performance Stellarator Plasmas

Cited by 46 publications

References 38 publications

Turbulence mitigation in maximum-J stellarators with electron-density gradient

Turbulence mitigation in maximum-J stellarators with electron-density gradient

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

Phase contrast imaging measurements and numerical simulations of turbulent density fluctuations in gas-fuelled ECRH discharges in Wendelstein 7-X

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