Intrinsic trapping of stochastic sheared magnetic field lines

Negrea, Marian; Petrișor, Iulian; Balescu, Radu

doi:10.1103/physreve.70.046409

Cited by 14 publications

(24 citation statements)

References 10 publications

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“…In [13] this method was applied to calculate the diffusion coefficients in the slab geometry. It is interesting that for a relatively small magnetic turbulence (Kubo number = 0.1) and a shearless situation, the results obtained by the authors of [13] are similar with those obtained by us. The influence of the magnetic shear on the thermal diffusivity should be explored.…”

Section: Discussionmentioning

confidence: 99%

Diffusion in a stochastic magnetic field in ASDEX Upgrade

Dumbrājs¹,

Igochine²,

Zohm³

2008

Nucl. Fusion

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Heat diffusion coefficients in a stochastic magnetic field are determined in the case of frequently interrupted regime of neoclassical tearing modes and of incomplete sawtooth reconnection in ASDEX Upgrade tokamak. Here the experimentally measured perturbations and profiles are used and the mapping technique is applied. With the derived diffusion coefficients the nonstationary diffusion equation is solved, making it possible to study time evolution of fast MHD phenomena in ASDEX Upgrade. The proposed phenomenological approach relies heavily on experimental information and requires very moderate computing resources.

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Section: Discussionmentioning

confidence: 99%

Diffusion in a stochastic magnetic field in ASDEX Upgrade

Dumbrājs¹,

Igochine²,

Zohm³

2008

Nucl. Fusion

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“…is taken as a Gaussian random field, spatially homogeneous (in the statistical sense) in the (X, Y )-plane, and stationary (again in the statistical sense). The Eulerian auto-correlation function is taken as [23] M(X; Z) = (0; 0) (X;…”

Section: B(x; Z)mentioning

confidence: 99%

“…the appearance of spatially coherent structures in the radial or poloidal direction, such as zonal flows or streamers (see, e.g., [3]). We thus determine the diffusion coefficients for different levels of magnetic turbulence, whose intensity can be measured by the dimensionless magnetic Kubo number K m (see, e.g., [19,20,[22][23][24]), different plasma temperatures (expressed in terms of the dimensionless drift Kubo number K dr ) and different degrees of anisotropy in the magnetic fluctuations (described by the stochastic anisotropy parameter ). The impurities considered here are He, Be, Ne and W, all assumed to be fully ionized.…”

Section: Introductionmentioning

confidence: 99%

Ion and impurity transport in turbulent, anisotropic magnetic fields

Negrea

Petrișor

Isliker

et al. 2011

Plasma Phys. Control. Fusion

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We investigate ion and impurity transport in turbulent, possibly anisotropic, magnetic fields. The turbulent magnetic field is modeled as a correlated stochastic field, with Gaussian distribution function and prescribed spatial auto-correlation function, superimposed onto a strong background field. The (running) diffusion coefficients of ions are determined in the three-dimensional environment, using two alternative methods, the semi-analytical decorrelation trajectory (DCT) method, and test-particle simulations. In a first step, the results of the test-particle simulations are compared with and used to validate the results obtained from the DCT method. For this purpose, a drift approximation was made in slab geometry, and relatively good qualitative agreement between the DCT method and the test-particle simulations was found. In a second step, the ion species He, Be, Ne and W, all assumed to be fully ionized, are considered under ITER-like conditions, and the scaling of their diffusivities is determined with respect to varying levels of turbulence (varying Kubo number), varying degrees of anisotropy of the turbulent structures and atomic number. In a third step, the test-particle simulations are repeated without drift approximation, directly using the Lorentz force, first in slab geometry, in order to assess the finite Larmor radius effects, and second in toroidal geometry, to account for the geometric effects. It is found that both effects are important, most prominently the effects due to toroidal geometry and the diffusivities are overestimated in slab geometry by an order of magnitude.

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“…The stabilizing effect produced by a sheared magnetic field on bound eigenmodes [44], or, alternatively, the instability for wavelengths below the ion Larmor scale [45] are not considered here. The effects of the magnetic shear on transport were studied using DTM in [46], which enables the extension of the present study to more complex confining configuration.…”

Section: Test Modesmentioning

confidence: 99%

Random and quasi-coherent aspects in particle motion and their effects on transport and turbulence evolution

Vlad

Spineanu

2017

New J. Phys.

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The quasi-coherent effects in two-dimensional incompressible turbulence are analyzed starting from the test particle trajectories. They can acquire coherent aspects when the stochastic potential has slow time variation and the motion is not strongly perturbed. The trajectories are, in these conditions, random sequences of large jumps and trapping or eddying events. Trapping determines quasicoherent trajectory structures, which have a micro-confinement effect that is reflected in the transport coefficients. They determine non-Gaussian statistics and flows associated to an average velocity. Trajectory structures also influence the test modes on turbulent plasmas. Nonlinear damping and generation of zonal flow modes is found in drift turbulence in uniform magnetic field. The coupling of test particle and test mode studies permitted to evaluate the self-consistent evolution of the drift turbulence in an iterated approach. The results show an important nonlinear effect of ion diffusion, which can prevent the transition to the nonlinear regime at small drive of the instability. At larger drive, quasi-coherent trajectory structures appear and they have complex effects on turbulence.eddying. The general conclusion of these studies is that trajectory trapping or eddying determines non-standard statistics of trajectories: non-Gaussian distributions, long time Lagrangian correlations (memory), strongly modified transport coefficients and an increased degree of coherence. The trapped trajectories form quasicoherent structures similar to fluid vortices.Very recent results [15] have shown that these methods could be the basis for the development of a theoretical approach for the study of test modes in turbulent plasmas. It is similar to the Lagrangian approach initiated by Dupree [16,17] and developed in the 1970s. The assumption of random trajectories with Gaussian distribution limited the application of Dupree's method to the quasilinear regime. The DTM and NSA enable the evaluation of the average propagator of the test modes in the nonlinear regime and extend the theoretical procedure to the complex processes that are generated in these conditions. The tendency of drift turbulence evolution beyond the quasilinear regime was obtained in [15].We present a theoretical approach to the study of turbulence evolution, the iterated self-consistent method (ISC). It is based on the analysis of the test particles and test modes in turbulent plasmas. Both test particle and test mode studies start from given statistical description of the turbulence. However, the coupling of these problems can lead to the evaluation of turbulence evolution and to the understanding of the nonlinear processes that are generated. The ISC is applied here to the drift turbulence in plasmas confined in uniform magnetic fields.The paper is organized as follows. The physical processes analyzed in this paper and the main ideas that are followed in this study are presented in section 2. Section 3 contains the test particle studies of transport, the DTM and the NS...

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Intrinsic trapping of stochastic sheared magnetic field lines

Cited by 14 publications

References 10 publications

Diffusion in a stochastic magnetic field in ASDEX Upgrade

Diffusion in a stochastic magnetic field in ASDEX Upgrade

Ion and impurity transport in turbulent, anisotropic magnetic fields

Random and quasi-coherent aspects in particle motion and their effects on transport and turbulence evolution

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