Electron diffusion in a sheared unperturbed magnetic field and an electrostatic stochastic field

Petrișor, Iulian; Negrea, Marian; Weyssow, Boris

doi:10.1088/0031-8949/75/1/001

Cited by 18 publications

(11 citation statements)

References 30 publications

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“…The simulations and the DCT method yield diffusion coefficients that differ by 50% or less, they can thus be considered to be in good qualitative agreement. Another instance where the results obtained by the DCT method were compared with numerical test-particle simulation, and where also relatively good agreement was found, is given in [33,34].…”

Section: Test-particle Simulationsmentioning

confidence: 97%

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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Section: Test-particle Simulationsmentioning

confidence: 97%

Ion and impurity transport in turbulent, anisotropic magnetic fields

Negrea

Petrișor

Isliker

et al. 2011

Plasma Phys. Control. Fusion

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“…Various aspects of the transport in fusion plasmas were studied using the DTM, that was developed and adapted to the study of models with increased complexity [32][33][34][35][36][37][38][39][40].…”

Section: Trapping and Micro-confinement Description By Dtmmentioning

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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“…When the nonlinear stage is attaint for V > V * , the first effect is produced when the fraction of trapped trajectories is still small by the quasi-coherent component of ion motion. The structures of ion trajectories determine the F factor (17), which modifies the effective diamagnetic frequency (16) and the frequency ω. At this stage the flows can be neglected (n tr ∼ = 0, n f r ∼ = 1) and R ji ∼ = 0 in Eqs.…”

Section: Trapping Effects On the Test Modesmentioning

confidence: 99%

“…It is characterized by the trapping of the trajectories, which determines a strong influence on the transport coefficient and on the statistical characteristics of the trajectories. The transport induced by the E × B stochastic drift in electrostatic turbulence [12] (including effects of collisions [13], average flows [14], motion along magnetic field [15], effect of magnetic shear [16]) and the transport in magnetic turbulence [17], [18] were studied in a series of papers using the decorrelation trajectory method. It was also shown that a direct transport (an average velocity) appears in turbulent magnetized plasmas due to the inhomogeneity of the magnetic field [19]- [21].…”

Section: Anomalous Diffusion Regimesmentioning

confidence: 99%

Trapping, Anomalous Transport, and Quasi-coherent Structures in Magnetically Confined Plasmas

Vlad

Spineanu

2009

Plasma and Fusion Research

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Strong electrostatic turbulence in magnetically confined plasmas is characterized by trapping or eddying of particle trajectories produced by the E × B stochastic drift. Trapping is shown to produce strong effects on test particles and on test modes. It determines non-standard statistics of trajectories: non-Gaussian distribution, memory effects and coherence. Trapped trajectories form quasi-coherent structure. Trajectory trapping has strong nonlinear effects on the test modes on turbulent plasmas. We determine the growth rate of drift modes as function of the statistical characteristics of the background turbulence. We show that trapping provides the physical mechanism for the inverse cascade observed in drift turbulence and for the zonal flow generation.

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Electron diffusion in a sheared unperturbed magnetic field and an electrostatic stochastic field

Cited by 18 publications

References 30 publications

Ion and impurity transport in turbulent, anisotropic magnetic fields

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

Trapping, Anomalous Transport, and Quasi-coherent Structures in Magnetically Confined Plasmas

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