On microinstabilities and turbulence in steep-gradient regions of fusion devices

Pueschel, M. J.; Hatch, D. R.; Ernst, D. R.; Guttenfelder, W.; Terry, P. W.; Citrin, J.; Connor, J. W.

doi:10.1088/1361-6587/aaf8c1

Cited by 16 publications

(16 citation statements)

References 95 publications

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“…The low background magnetic shear in the present scenario enables destabilization of these modes, termed tearing-parity KBMs, which had previously been conjectured not to exist based on a study involving high-shear equilibria (Pueschel et al. 2019). It is worth noting that it is unlikely that the tearing-parity KBMs discussed here are micro tearing modes because of the unique ITG dependence of the modes, the fact that the modes propagate in the ion diamagnetic direction and the relatively small electron thermal transport nonlinearly, three qualities that are not characteristic of micro tearing modes.…”

Section: Electromagnetic Itg and Kbm Turbulencementioning

confidence: 73%

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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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Section: Electromagnetic Itg and Kbm Turbulencementioning

confidence: 73%

“…As a consequence, KBMs can peak at finite ballooning angle, and even tearing-parity KBMs (TKBMs in the nomenclature of Pueschel et al. (2019)) are found. With respect to , increases monotonically, regardless of the sign of the average magnetic shear.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2021

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“…Although peak growth rates may increase with positive δ , at low wavenumbers k y ≲ 0.3, the linear growth rates decrease with δ . Because turbulent transport is typically dominated by modes at low wavenumbers, in many instances, only the k x = 0 modes are considered in linear analysis for tokamaks 17,22,62 . But, as shown in Fig.…”

Section: Iii1 Growth Rate Spectramentioning

confidence: 99%

Effect of triangularity on ion-temperature-gradient-driven turbulence

et al. 2022

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The linear and nonlinear properties of ion-temperature-gradient-driven turbulence with adiabatic electrons are modeled for axisymmetric configurations for a broad range of triangularities δ, both negative and positive. Peak linear growth rates decrease with negative δ but increase and shift toward a finite radial wavenumber kx with positive δ. The growth-rate spectrum broadens as a function of kx with negative δ and significantly narrows with positive δ. The effect of triangularity on linear instability properties can be explained through its impact on magnetic polarization and curvature. Nonlinear heat flux is weakly dependent on triangularity for [Formula: see text], decreasing significantly with extreme δ, regardless of sign. Zonal modes play an important role in nonlinear saturation in the configurations studied, and artificially suppressing zonal modes increased nonlinear heat flux by a factor of about four for negative δ, increasing with positive δ by almost a factor of 20. Proxies for zonal-flow damping and drive suggest that zonal flows are enhanced with increasing positive δ.

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“…2017; Xie, Xiao & Lin 2017 b ; Pueschel et al. 2019). In weak equilibrium gradient conditions, microinstabilities are often characterized by a conventional ballooning eigenmode function, with the electrostatic potential featuring an even mode parity around the outboard midplane position () and peaking at the same location with a well-defined mode propagation direction.…”

Section: Microinstabilities In Steep Pressure Gradient Conditionsmentioning

confidence: 99%

Moment-based approach to the flux-tube linear gyrokinetic model

Frei,

Hoffmann,

Ricci

et al. 2023

J. Plasma Phys.

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This work reports on the development and numerical implementation of the linear electromagnetic gyrokinetic (GK) model in a tokamak flux-tube geometry using a moment approach based on the expansion of the perturbed distribution function on a velocity-space Hermite–Laguerre polynomials basis. A hierarchy of equations of the expansion coefficients, referred to as the gyro-moments (GMs), is derived. We verify the numerical implementation of the GM hierarchy in the collisionless limit by performing a comparison with the continuum GK code GENE, recovering the linear properties of the ion temperature gradient, trapped electron, kinetic ballooning and microtearing modes, as well as the collisionless damping of zonal flows. An analysis of the distribution functions and ballooning eigenmode structures is performed. The present investigation reveals the ability of the GM approach to describe fine velocity-space-scale structures appearing near the trapped and passing boundary and kinetic effects associated with parallel and perpendicular particle drifts. In addition, the effects of collisions are studied using advanced collision operators, including the GK Coulomb collision operator. The main findings are that the number of GMs necessary for convergence decreases with plasma collisionality and is lower for pressure gradient-driven modes, such as in H-mode pedestal regions, compared with instabilities driven by trapped particles and magnetic gradient drifts often found in the core. The accuracy of approximations often used to model collisions (relative to the GK Coulomb operator) is studied in the case of trapped electron modes, showing differences between collision operator models that increase with collisionality and electron temperature gradient, consistent with the results of Pan et al. (Phys. Rev. E, vol. 103, 2021, L051202). Such differences are not observed in other edge microinstabilities, such as microtearing modes. The importance of a proper collision operator model is also confirmed by analysing the collisional damping of geodesic acoustic modes and zonal flows.

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On microinstabilities and turbulence in steep-gradient regions of fusion devices

Cited by 16 publications

References 95 publications

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

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

Effect of triangularity on ion-temperature-gradient-driven turbulence

Moment-based approach to the flux-tube linear gyrokinetic model

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