A comparison of turbulent transport in a quasi-helical and a quasi-axisymmetric stellarator

McKinney, I. J.; Pueschel, M. J.; Faber, B. J.; Hegna, C. C.; Talmadge, J. N.; Anderson, David T.; Mynick, H.; Xanthopoulos, P.

doi:10.1017/s0022377819000588

Cited by 19 publications

(34 citation statements)

References 55 publications

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“…The discrepancy between the linear growth rates and the full nonlinear heat flux is consistent with trends found in McKinney et al. (2019). The results presented here also indicate that linear growth rates can be a misleading indicator for stellarator turbulence and turbulent transport.…”

Section: Performance Evaluationsupporting

confidence: 90%

“…The configuration presented here is not explicitly optimized for turbulent transport. However, previous gyrokinetic calculations indicate that QHS configurations demonstrate enhanced nonlinear energy transfer properties over other optimized configurations (Plunk, Xanthopoulos & Helander 2017;McKinney et al 2019). It is anticipated that future iterations will include optimization of nonlinear turbulent energy transfer using a novel metric modelling turbulent energy transfer to stable modes (Hegna et al 2018;Faber 2020).…”

Section: Turbulent Transportmentioning

confidence: 99%

“…Nonlinear flux-tube gyrokinetic calculations were performed to describe ion temperature gradient (ITG) turbulence at the s = 0.5 surface using the GENE code (Jenko et al 2000) for various values of the ion temperature scale length a/L Ti assuming adiabatic electrons. In figure 6, the heat flux in dimensionless gyro-Bohm units for WISTELL-A is compared to two other configurations, the HSX configuration, which has been well analysed for turbulent transport (Faber et al 2015;Pueschel et al 2016;Faber et al 2018;McKinney et al 2019), and a third 'turbulent reduced' configuration, which was the result of a separate optimization calculation. The turbulence-reduced configuration is identical to the configuration in Bader et al (2019) labelled 'Opt.…”

Section: Turbulent Transportmentioning

confidence: 99%

“…In Hegna et al (2018), the dominant saturation mechanism was shown to be energy transfer to stable modes through non-zonal, marginally stable drift waves. As such, the analysis presented here focuses on assessing the energy transfer through non-zonal modes as opposed to energy transfer through zonal flows, which is shown to be the dominant saturation mechanism in tokamaks (Terry et al 2018), the NCSX stellarator (Hegna et al 2018;McKinney et al 2019) and the W7-X stellarator (Plunk et al 2017). In figure 9, the triplet correlation lifetimes are shown for the HSX and the turbulence optimized configuration.…”

Section: Turbulent Transportmentioning

confidence: 99%

“…2018; McKinney et al. 2019), and a third ‘turbulent reduced’ configuration, which was the result of a separate optimization calculation. The turbulence-reduced configuration is identical to the configuration in Bader et al.…”

Section: Performance Evaluationmentioning

confidence: 99%

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Advancing the physics basis for quasi-helically symmetric stellarators

et al. 2020

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A new optimized quasi-helically symmetric configuration is described that has the desirable properties of improved energetic particle confinement, reduced turbulent transport by three-dimensional shaping and non-resonant divertor capabilities. The configuration presented in this paper is explicitly optimized for quasi-helical symmetry, energetic particle confinement, neoclassical confinement and stability near the axis. Post optimization, the configuration was evaluated for its performance with regard to energetic particle transport, ideal magnetohydrodynamic stability at various values of plasma pressure and ion temperature gradient instability induced turbulent transport. The effects of discrete coils on various confinement figures of merit, including energetic particle confinement, are determined by generating single-filament coils for the configuration. Preliminary divertor analysis shows that coils can be created that do not interfere with expansion of the vessel volume near the regions of outgoing heat flux, thus demonstrating the possibility of operating a non-resonant divertor.

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Section: Performance Evaluationsupporting

confidence: 90%

Section: Turbulent Transportmentioning

confidence: 99%

Section: Turbulent Transportmentioning

confidence: 99%

Section: Turbulent Transportmentioning

confidence: 99%

Section: Performance Evaluationmentioning

confidence: 99%

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Advancing the physics basis for quasi-helically symmetric stellarators

et al. 2020

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Kinetic-ballooning-mode turbulence in low-average-magnetic-shear equilibria

et al. 2021

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Kinetic-ballooning-mode (KBM) turbulence is studied via gyrokinetic flux-tube simulations in three magnetic equilibria that exhibit small average magnetic shear: the Helically Symmetric eXperiment (HSX), the helical-axis Heliotron-J and a circular tokamak geometry. For HSX, the onset of KBM being the dominant instability at low wavenumber occurs at a critical value of normalized plasma pressure $\beta ^{\rm KBM}_{\rm crit}$ that is an order of magnitude smaller than the magnetohydrodynamic (MHD) ballooning limit $\beta ^{\rm MHD}_{\rm crit}$ when a strong ion temperature gradient (ITG) is present. However, $\beta ^{\rm KBM}_{\rm crit}$ increases and approaches the MHD ballooning limit as the ITG tends to zero. For these configurations, $\beta ^{\rm KBM}_{\rm crit}$ also increases as the magnitude of the average magnetic shear increases, regardless of the sign of the normalized magnetic shear. Simulations of Heliotron-J and a circular axisymmetric geometry display behaviour similar to HSX with respect to $\beta ^{\rm KBM}_{\rm crit}$ . Despite large KBM growth rates at long wavelengths in HSX, saturation of KBM turbulence with $\beta > \beta _{\rm crit}^{\rm KBM}$ is achievable in HSX and results in lower heat transport relative to the electrostatic limit by a factor of roughly five. Nonlinear simulations also show that KBM transport dominates the dynamics when KBMs are destabilized linearly, even if KBM growth rates are subdominant to ITG growth rates.

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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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A comparison of turbulent transport in a quasi-helical and a quasi-axisymmetric stellarator

Cited by 19 publications

References 55 publications

Advancing the physics basis for quasi-helically symmetric stellarators

Advancing the physics basis for quasi-helically symmetric stellarators

Kinetic-ballooning-mode turbulence in low-average-magnetic-shear equilibria

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

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