Instability and turbulent relaxation in a stochastic magnetic field

Cao, Mingyun; Diamond, P. H.

doi:10.1088/1361-6587/ac47d7

Cited by 9 publications

(14 citation statements)

References 38 publications

(69 reference statements)

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“…q is safety factor, β is plasma beta defined as the ratio of plasma kinetic pressure to magnetic pressure, ϵ is inverse aspect ratio and δB r is a radial magnetic field perturbation. This is in accordance with recent theory and simulations that stochastic fields reduce the linear growth rate of turbulence [48,61,62]. The modulational growth coefficient of the zonal flow is modified as…”

Section: Scaling With Stochastic Fieldssupporting

confidence: 90%

Zonal shear layer collapse and the power scaling of the density limit: old L-H wine in new bottles

Singh¹,

Diamond²

2022

Plasma Phys. Control. Fusion

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Edge shear layer collapse causes edge cooling and aggravates radiative effects. This paper details on the microscopic dynamics of the emergence of power (Q) scaling of density limit from the shear layer collapse transport bifurcation scenario. The analysis is based on a novel 4-field model which evolves turbulence energy, zonal flow energy, temperature gradient and density, including the neoclassical screening of zonal flow response. Bifurcation analysis yields power scaling of critical density for shear layer collapse as ncrit∼Q1/3. The favorable Q scaling of the density limit emerges from the fact that the shear layer strength increases with Q, thus preventing shear layer collapse. This in turn reduces particle transport and improves particle confinement. RMP induced ambient stochastic fields degrade the shear layer by inducing decoherence in the Reynolds stress. As a result the particle transport increases and particle confinement degrades. This leads to the emergence of unfavorable stochastic field intensity (bst 2) scaling of the critical density as ncrit∼(1+bst 2)-5/3 . All fields, including zonal flow shear, exhibit hysteresis when the power Q is ramped cyclically across the bifurcation point. The hysteresis is due to dynamical delay in bifurcation on account of critical slowing down. Thus, the dynamical hysteresis here is fundamentally different from the hysteresis associated with the existence of bi-stable states.

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Section: Scaling With Stochastic Fieldssupporting

confidence: 90%

Zonal shear layer collapse and the power scaling of the density limit: old L-H wine in new bottles

Singh¹,

Diamond²

2022

Plasma Phys. Control. Fusion

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“…It can be observed that inside the braces of the scaling of ν T , there are two terms labeled as 'old' and 'new' respectively. If only the 'old' term is retained, the scaling of ν T reverts to that given by equation (38) in [10]. In this study, due to the changes in the mode structure and the spatial scaling ordering, both bx and by enter the calculation of the scaling of ν T .…”

Section: Analysis Of Resultsmentioning

confidence: 80%

“…must be satisfied for arbitrary k 1 , which is clearly impossible. This brings us back to the narrative we developed in our previous study on resistive interchange modes in a stochastic magnetic field [10], i.e. small-scale convective cells must be driven by the beat of stochastic magnetic field with quasimode, which further generate a current density fluctuation J⊥ so as to keep ∇ • ( J∥ + J⊥ ) = 0.…”

Section: Generation Of the Microturbulencementioning

confidence: 73%

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Quasi-mode evolution in a stochastic magnetic field

Cao,

Diamond

2024

Nucl. Fusion

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We present a multi-scale model of quasi-mode evolution in a stochastic magnetic field. The similarity between a quasi-mode and a ballooning mode enables us to address the challenges arising from the disparate geometries in the theories of ballooning modes in the presence of resonant magnetic perturbations. We obtain useful insights into our understanding of ballooning mode dynamics in a stochastic background. To maintain quasi-neutrality at all scales, the beat between the quasi-mode and the stochastic magnetic field drives microturbulence, which drives the turbulent background that promotes mixing and damps the quasi-mode. As a result of the broad mode structure of the quasi-mode, the turbulent viscosity and the turbulent diffusivity produced by the microturbulence are larger than those in our related study on resistive interchange modes. The stochastic magnetic field can also enhance the effective plasma inertia and reduce the effective drive, thereby slowing the mode growth. A nontrivial correlation between the microturbulence and the magnetic perturbations is shown to develop. This could account for the reduction in the Jensen-Shannon complexity of pedestal turbulence in the RMP ELM suppression phase observed in recent experiments. Directions for future experimental and theoretical studies are suggested.

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“…Based on the present analysis, one can also understand this damping as the fact that in the island topology, ZFs are not energetically favored due to field-line bending, hence the turbulence preferentially drives the island vortex-flow. Another possible explanation is the effect of stochastic magnetic perturbations [33], not taken into account here. Wilcox et al [12] showed that during ELM suppression due to RMP, the density profile as calculated by the two-fluid 3D MHD equilibrium shows a helical variation, is consistent with diagnostic measurements seeing a non-axisymmetric intensity of the turbulence, and thus suggests the presence of an island affecting the turbulence, consistent with the theory proposed here.…”

Section: Discussionmentioning

confidence: 99%

Turbulence-driven vortex-flow around a magnetic island

Leconte

Cho

2023

Nucl. Fusion

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We predict a new state of plasma self-organization, in presence of magnetic islands, namely turbulence-driven island Vortex-Flows, which are non-axisymmetric flows. The interaction between an $E \times B$ sheared Vortex-Flow and the Drift-wave turbulence (DW) driving it, is derived in presence of a coherent static magnetic island. The turbulence is driven by a 3D density profile due to quasi-linear island-induced profile flattening, and the drift-waves thus follow the local electron diamagnetic drift along the island. The metric tensor is introduced, making the analysis easier in island geometry. An extended Charney-Hasegawa-Mima equation describes the DW turbulence - Flow interaction, from which a wave-kinetic equation (WKE) is obtained in island geometry. This yields a turbulence - Vortex Flow predator-prey model which predicts a nonlinear threshold for island Vortex-Flow formation. The Vortex-Flow threshold decreases with increasing island-width as $\gamma_{\rm th} \sim \frac{1}{W^2}$, which shows that wider islands may more easily drive Vortex-Flows.

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Instability and turbulent relaxation in a stochastic magnetic field

Cited by 9 publications

References 38 publications

Zonal shear layer collapse and the power scaling of the density limit: old L-H wine in new bottles

Zonal shear layer collapse and the power scaling of the density limit: old L-H wine in new bottles

Quasi-mode evolution in a stochastic magnetic field

Turbulence-driven vortex-flow around a magnetic island

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