Properties of high-β microturbulence and the non-zonal transition

Pueschel, M. J.; Hatch, D. R.; Görler, T.; Nevins, W. M.; Jenko, F.; Terry, P. W.; Told, D.

doi:10.1063/1.4823717

Cited by 39 publications

(62 citation statements)

References 37 publications

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“…We have assumed that β crit = β KBM . However, an additional β-limit is apparent from numerical simulations -β N ZT (non-zonal-transition) -leading to a sharp rise in flux due to a reduction in zonal flow activity [29,30]. For highly driven systems, this occurs for β N ZT < β KBM .…”

Section: Parametrization Of Em-stabilizationmentioning

confidence: 99%

Electromagnetic stabilization of tokamak microturbulence in a high-βregime

Citrin

García

Görler

et al. 2014

Plasma Phys. Control. Fusion

146

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The impact of electromagnetic stabilization and flow shear stabilization on ITG turbulence is investigated. Analysis of a low-β JET L-mode discharge illustrates the relation between ITG stabilization, and proximity to the electromagnetic instability threshold. This threshold is reduced by suprathermal pressure gradients, highlighting the effectiveness of fast ions in ITG stabilization. Extensive linear and nonlinear gyrokinetic simulations are then carried out for the high-β JET hybrid discharge 75225, at two separate locations at inner and outer radii. It is found that at the inner radius, nonlinear electromagnetic stabilization is dominant, and is critical for achieving simulated heat fluxes in agreement with the experiment. The enhancement of this effect by suprathermal pressure also remains significant. It is also found that flow shear stabilization is not effective at the inner radii. However, at outer radii the situation is reversed. Electromagnetic stabilization is negligible while the flow shear stabilization is significant. These results constitute the high-β generalization of comparable observations found at low-β at JET. This is encouraging for the extrapolation of electromagnetic ITG stabilization to future devices. An estimation of the impact of this effect on the ITER hybrid scenario leads to a 20% fusion power improvement.

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Section: Parametrization Of Em-stabilizationmentioning

confidence: 99%

Electromagnetic stabilization of tokamak microturbulence in a high-βregime

Citrin

García

Görler

et al. 2014

Plasma Phys. Control. Fusion

146

View full text Add to dashboard Cite

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“…To understand the zonal flow dynamics quantitatively, however, both their drive and their depletion mechanisms have to be considered. The latter, in the form of nonlinear mode interaction [27][28][29] or the action of magnetic perturbations on the residual flow [5,30,31], is counteracted by the former: the energy transfer from the linear mode to the zonal mode via sidebands-a process often referred to as secondary instability, as zonal flows saturate the linear mode at the onset of turbulence. The growth rate of the secondary instability gives valuable insights into the zonal flow picture, and its dependence on β is one of the primary subjects of this paper.…”

Section: Introductionmentioning

confidence: 99%

“…[5,30], a brief overview of the NZT physics is given here for convenience. At sufficiently large amplitudes of the magnetic fluctuation level -and, by extension, at sufficiently large β -field lines start to decorrelate from the magnetic potential, bringing about a sudden increase in field line diffusivity.…”

Section: Introductionmentioning

confidence: 99%

“…Considering the potential impact of both tertiary and secondary instability on the physics of the non-zonal transition (NZT) [5,30], particular focus is put on the behavior near the NZT threshold β NZT crit . First described in Ref.…”

Section: Introductionmentioning

confidence: 99%

“…In the context of gyrokinetic simulations, the normalized electron plasma pressure β and related effects have been the subject of various avenues of study in recent years: codes agree on electromagnetic transport values [1][2][3][4][5], nonlinear stabilization of ion-temperaturegradient-driven (ITG) turbulence has been found to be related to an enhancement [6] of the usual nonlinear upshift of the critical gradient [7], and advances have been made regarding microtearing turbulence [8,9] as well as nonlinearly excited subdominant microtearing (SMT) [10,11]. In addition to radially local gyrokinetic simulations -on which the aforementioned studies are based -strides have been made in operating gyrokinetic codes with global profiles electromagnetically [12][13][14][15][16].…”

Section: Introductionmentioning

confidence: 99%

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On secondary and tertiary instability in electromagnetic plasma microturbulence

et al. 2013

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Zonal flows, widely accepted to be the secondary instability process leading to the nonlinear saturation of ion temperafture gradient modes, are shown to grow at higher rates relative to the linear mode amplitude as the plasma pressure β is increased-thus confirming that zonal flows become increasingly important in the turbulent dynamics at higher β. At the next level of nonlinear excitation, radial corrugations of the distribution function (zonal flow, zonal density, and zonal temperature) are demonstrated to modify linear growth rates moderately through perturbedfield, self-consistent gradients: on smaller scales, growth rates are reduced below the linear rate.In particular, excitation of kinetic ballooning modes well below their usual threshold is not to be expected under normal conditions. These findings strengthen the theory of the non-zonal transition [M.J. Pueschel et al., Phys. Rev. Lett. 110, 155005 (2013)].

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The development of magnetic field line wander in gyrokinetic plasma turbulence: dependence on amplitude of turbulence

Bourouaine

Howes

2017

J. Plasma Phys.

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The dynamics of a turbulent plasma not only manifests the transport of energy from large to small scales, but also can lead to a tangling of the magnetic field that threads through the plasma. The resulting magnetic field line wander can have a large impact on a number of other important processes, such as the propagation of energetic particles through the turbulent plasma. Here we explore the saturation of the turbulent cascade, the development of stochasticity due to turbulent tangling of the magnetic field lines and the separation of field lines through the turbulent dynamics using nonlinear gyrokinetic simulations of weakly collisional plasma turbulence, relevant to many turbulent space and astrophysical plasma environments. We determine the characteristic time $t_{2}$ for the saturation of the turbulent perpendicular magnetic energy spectrum. We find that the turbulent magnetic field becomes completely stochastic at time $t\lesssim t_{2}$ for strong turbulence, and at $t\gtrsim t_{2}$ for weak turbulence. However, when the nonlinearity parameter of the turbulence, a dimensionless measure of the amplitude of the turbulence, reaches a threshold value (within the regime of weak turbulence) the magnetic field stochasticity does not fully develop, at least within the evolution time interval $t_{2}<t\leqslant 13t_{2}$. Finally, we quantify the mean square displacement of magnetic field lines in the turbulent magnetic field with a functional form $\langle (\unicode[STIX]{x1D6FF}r)^{2}\rangle =A(z/L_{\Vert })^{p}$ ($L_{\Vert }$ is the correlation length parallel to the magnetic background field $\boldsymbol{B}_{\mathbf{0}}$, $z$ is the distance along $\boldsymbol{B}_{\mathbf{0}}$ direction), providing functional forms of the amplitude coefficient $A$ and power-law exponent $p$ as a function of the nonlinearity parameter.

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Properties of high-β microturbulence and the non-zonal transition

Abstract: The

Cited by 39 publications

References 37 publications

Electromagnetic stabilization of tokamak microturbulence in a high-βregime

Electromagnetic stabilization of tokamak microturbulence in a high-βregime

On secondary and tertiary instability in electromagnetic plasma microturbulence

The development of magnetic field line wander in gyrokinetic plasma turbulence: dependence on amplitude of turbulence

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