Convective Cell Modes Research Articles

Interplay between fluctuation driven toroidal axisymmetric flows and resistive ballooning mode turbulence

An interplay between fluctuation driven toroidal axisymmetric flows (convective cell modes) and resistive ballooning mode turbulence after the pedestal collapse is numerically studied by a four-field reduced MHD model in the BOUT++ framework. The strong flow shear suppresses the radial transport of pressure filaments, and the pressure profile in the pedestal region is partially recovered. As a result, a secondary instability is quasilinearly excited, which yields a secondary collapse. The subsequent damped oscillation is also analyzed by phase diagram analysis.

Nonlinear Electrostatic Waves in PIE and PI Plasmas With Field-Aligned Shear Flow

Nonlinear low-frequency ion acoustic (IA) wave and pair plasma convective cell (PPCC) mode are investigated in pair-ion (PI) plasmas with and without electrons in the presence of sheared flows. IA wave forms both KdV solitons and dipolar vortices in PI-electron plasma, and their amplitude depends upon electron density and sheared flow. But when electron density is too low, the structures disappear, because IA does not exist in pure PI plasma. On the other hand, PPCC mode cannot form pulse-type solitary structures in the pair plasma, while it forms dipolar vortices due to finite vorticity. The nonlinear analysis has been performed assuming the presence of electrons in fullerene PI plasma and limit of zero electron density is also considered for a comparison.

Simulation Study of Nonlocal Transport from Edge to Core in Tokamak Plasmas

AbstractThe simulation study of nonlocal transport from edge to core in tokamak plasmas is performed using the 4‐field reduced MHD model. A toroidally‐elongated cylindrical particle source is applied in the plasma edge, after saturation of the resistive ballooning turbulence is attained. After a short time, the source is switched off and plasma response is investigated in detail. The nonlocal transport appears at the location far from the edge source. It is found that the particle source induces (0,0) and (±1, 0) modes of density fluctuations, where (m, n) indicates the set of poloidal mode number m and the toroidal mode number n. These modes interact with each other by the nonlinear and/or toroidal couplings. The symmetry of (±1, 0) modes breaks after switching‐off the source and the formation of the spiral structure with poloidal rotation is observed, which yields a connection between core and edge regions. In this simulation, the convective cell mode such as (1,0) mode contributes the nonlocal transport. The simulation result indicates that two dimensional transport plays an essential role to produce the nonlocal transport. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Shear flow driven counter rotating vortices in an inhomogeneous dusty magnetoplasma

The coupling of Shukla-Varma (SV) and convective cell modes is discussed in the presence of non-Boltzmannian electron response and parallel equilibrium shear flow. In the linear case, a new dispersion relation is derived and analyzed. It is found that the coupled SV and convective cell modes destabilize in the presence of electron shear flow. On the other hand, in the nonlinear regime, it is shown that Shukla-Varma mode driven counter rotating vortices can be formed for the system under consideration. It is found that these vortices move slowly by comparison with the ion acoustic or electron drift-wave driven counter rotating vortices. The relevance of the present investigation with regard to space plasmas is also pointed out.

Cylindrically confined pair-ion-electron and pair-ion plasmas having axial sheared flow and radial gradients

The linear and nonlinear dynamics of pair-ion (PI) and pair-ion-electron plasmas (PIE) have been investigated in a cylindrical geometry with a sheared plasma flow along the axial direction having radial dependence. The coupled linear dispersion relation of low frequency electrostatic waves has been presented taking into account the Guassian profile of density and linear gradient of sheared flow. It is pointed out that the quasi-neutral cold inhomogeneous pure pair ion plasma supports only the obliquely propagating convective cell mode. The linear dispersion relation of this mode has been solved using boundary conditions. The nonlinear structures in the form of vortices formed by different waves have been discussed in PI and PIE plasmas.

Erratum: “Electron thermal effect on linear and nonlinear coupled Shukla–Varma and convective cell modes in dust-contaminated magnetoplasma” [Phys. Plasmas 17, 113702 (2010)

Erratum: “Electron thermal effect on linear and nonlinear coupled Shukla–Varma and convective cell modes in dust-contaminated magnetoplasma” [Phys. Plasmas 17, 113702 (2010)

Electron thermal effect on linear and nonlinear coupled Shukla–Varma and convective cell modes in dust-contaminated magnetoplasma

Linear and nonlinear properties of coupled Shukla–Varma (SV) and convective cell modes in the presence of electron thermal effects are studied in a nonuniform magnetoplasma composed of electrons, ions, and extremely massive and negatively charged immobile dust grains. In the linear case, the modified dispersion relation is given and, in the nonlinear case, stationary solutions of the nonlinear equations that govern the dynamics of coupled SV and convective cell modes are obtained. It is found that electrostatic dipolar and vortex street type solutions can appear in such a plasma. The relevance of the present investigation with regard to the Earth’s mesosphere as well as in ionospheric plasmas is also pointed out.

Revisiting coupled Shukla–Varma and convective cell mode in classical and quantum dusty magnetoplasmas

AbstractThe coupled Shukla–Varma (SV) and convective cell mode is revisited in classical and quantum dusty magnetoplasmas. It is shown that the inclusion of electron thermal effects modifies the original coupled SV and convective cell mode. It is also discussed how the quantum effects can be incorporated in the coupled SV and convective cell mode.

Drift eigenmodes in plasmas with negative ions

AbstractThe instabilities associated with drift waves can be suppressed (or even removed) by introducing negative ions into the usual electron ion plasma. The presence of negative ions causes a decrease in the amplitude of the perturbed electrostatic fields which are mainly produced by the difference of electron and ion masses. It is shown that the presence of negative ions is a stabilizing effect for drift eigenmode in a slab geometry and the frequency of this electrostatic mode is reduced. In a pure pair-ion plasma the convective cell mode can become unstable because of shear flow under certain conditions.

Gyrofluid turbulence studies of the effect of the poloidal position of an axisymmetric Debye sheath

The field line connection of a tokamak sheared magnetic field ensures a finite parallel dynamical response for every degree of freedom available in the system. In the scrape-off layer (SOL) the flux surfaces are open, and the field line connection property is broken by the presence of a Debye sheath arising where the field lines strike boundary plates, hence allowing the existence of convective cell modes for which there is no dynamical parallel response. This leads to a major distinction in terms of turbulence character between closed and open flux surface regions. We study this using three-dimensional electromagnetic gyrofluid computations. The turbulence is found to change character from an ion temperature gradient to a generic interchange type, crossing the last closed flux surface (LCFS) radially outward. The width of the transition zone is about ten ion gyroradii. Various poloidal configurations of the Debye sheaths retain this interface property but affect the interaction between the turbulence and the slowly varying, self-consistent background. The strongest effect is found in a case with sheath plates at both the top and bottom of the SOL, allowing the high-field and low-field sides of the SOL to decouple. In these sides the curvature is favourable and unfavourable, respectively. The clear asymmetry observed between these sides of the plasma is consistent with previous experimental results and makes room for future experimental qualitative comparisons, for instance, on double null configurations of the tokamak ASDEX Upgrade.

A criterion for pure pair-ion plasmas and the role of quasineutrality in nonlinear dynamics

A criterion is presented to decide whether a produced plasma can be called a pure pair-ion plasma or not. The theory is discussed in the light of recent experiments which claim that a pure pair-ion fullerene (C60±) plasma has been produced. It is also shown that the ion acoustic wave is replaced by the pair ion convective cell (PPCC) mode as the electron density becomes vanishingly small in a magnetized plasma comprised of positive and negative ions. The nonlinear dynamics of pure pair plasmas is described by two coupled equations which have no analog in electron-ion plasmas. In a stationary frame, it becomes similar to the Hasegawa-Mima equation but does not contain drift waves and ion acoustic waves.

Tokamak turbulence computations on closed and open magnetic flux surfaces

We study turbulence on closed and open flux surfaces in a comparative manner using the three-dimensional electromagnetic gyrofluid turbulence code GEM. A magnetic field on a tokamak is doubly periodic and sheared. This leads to the so-called field line connection, which ensures a finite parallel response for every degree of freedom. In contrast, in the scrape-off layer (SOL), the field lines end on plates, breaking this constraint and allowing the existence of convective cell modes. Since the parallel electron response provides a path to dissipation, whether or not it is allowed to vanish is important. For the SOL case, a standard Debye sheath model is used to provide the parallel boundary conditions. A zero loss model (no fluxes into the plates) is also used to assess the importance of the Debye currents. Turbulence on closed and open flux surfaces at the same parameters is found to be very different, a property which basic transport models should take into account.

HF Conductivity of Parametrically Unstable Magnetized Plasma

In this paper the absorption processes of LH and UH pump waves in magnetized homogeneous and inhomogeneous plasmas are investigated. We determine the RF power absorbed in the plasma when the external pump parametrically excites low-frequency plasma oscillations (purely growing convective cells, modified convective cell modes and electron drift waves). Expressions for the effective absorption length of the external RF wave as a function of various plasma parameters are derived. For typical parameters of a hot ionospheric plasma, absorption length values are calculated.

Convective cells in nonuniform dusty plasmas

It is shown that purely damped convective cell modes acquire a real frequency in the presence of static charged dust grains in a nonuniform magnetized dusty plasma. The frequency of the two-dimensional mode is induced by the plasma density gradient and corresponds to charge density waves arising out of the plasma non-neutrality. The dynamics of weakly interacting finite-frequency convective cell modes is governed by the generalized Navier–Stokes equation. The presence of the new term arising from the dust inhomogeneity in the latter provides the possibility of a two-dimensional dipolar vortex solution in dusty plasmas.

Alfvén vortices in nonuniform dusty magnetoplasmas.

The Alfv\'en waves are shown to be linearly coupled with finite-frequency convective cell modes in a nonuniform cold dusty magnetoplasma. The dynamics of weakly interacting finite-amplitude electromagnetic oscillations is governed by a pair of nonlinear equations. The latter admit dipolar vortices as possible stationary solutions. The present results can have relevance to the nonlinear electromagnetic wave propagation in a space environment such as cometary tails or interstellar clouds.

Parametric excitation of convective cell modes and nonlinear stabilization of purely-growing instability

Parametric coupling of lower-hybrid pump waves with purely-damped convective cell modes generates enhanced electrostatic fluctuations in a magnetized plasma. The threshold electric field for exciting a purely-growing instability is found. In order to consider the saturation of the parametric instability, the authors have invoked a nonlinear mechanism which involves scattering of charged particles by suprathermal fluctuations. An expression for the effective collision frequency is also obtained, which characterizes the HF power dissipation in the plasma as well as the heating. Attempts have been made to correlate the results with some experimental data.

Coupling between Varma, modified-convective-cell and Alfvén modes in an inhomogeneous plasma

The effect of parallel electron dynamics is incorporated in the study of the Varma mode, which is a flute-like electrostatic ion drift perturbation in an inhomogeneous magnetic field. Starting from the electron and ion continuity equations, an equation for the conservation of the ion magnetic moment, the generalized Ohm's law, and Faraday's and Ampère's laws, we derive a set of four nonlinearly coupled differential equations. The nonlinearities arising from the E × B0 convection of the density and magnetic-moment variations, nonlinear ion polarization drift and Lorentz force, and the coupling of the parallel electron flow to the perturbed magnetic field are included. In the linear limit we find that the Varma mode becomes coupled with the modified-convective-cell and Alfvén modes. The instability of the coupled system is analysed and an expression for the growth rate is obtained. In the nonlinear regime we present the localized dipole vortex solution of the coupled Varma and convective-cell modes as well as the coupled Varma and Alfvén modes. It is found that for the former case the nonlinear structure is a solution of a second-order partial differential equation, whereas for the coupled Varma and inertial Alfvén waves the dipole vortex belongs to a family of fourth-order differential equations. Our results can be useful in understanding the electrostatic and electromagnetic fluctuations in mirror reactors, tokamaks and astrophysical plasmas.

Critical fluctuations due to the parametric coupling of lower-hybrid waves with convective cells in plasmas

The fluctuation spectra of the magnetized plasma in the presence of parametric interaction of lower-hybrid waves with modified convective cell modes are analytically derived.

Formation of a spatially ordered structure in a finite- beta edge plasma.

A three-dimensional inhomogeneous magnetohydrodynamic equation is investigated numerically. It is found that a spatially ordered structure is formed at a saturated energy level by the excitation of the convective cell modes due to the three-dimensional nonlinear mode coupling. It is shown that the saturated energy level and radial plasma flux are proportional to the magnitude of the background density gradient. The experimental observation of a self-organized coherent structure in a finite-..beta.. collisional edge plasma is expected.

Electrostatic interchange modes with finite parallel wavelengths

A set of nonlinear equations is derived that includes the parallel electron dynamics in the study of the electrostatic interchange instability. In the linear limit, interchange modes are coupled with drift-modified convective cell modes. It is shown that a pressure gradient as well as a nonuniform magnetic field can induce a new class of instabilities.