Semiquantitative estimates of currents generated by monochromatic waves in toroidal plasmas

Landau damping and transit time damping are analyzed from the point of view of toroidal electric current generation. Steady current density and energy absorption are estimated. The importance of momentum transfer to non-resonant electrons via Coulomb collisions is pointed out. If the bounce period of trapped electrons is less than their collisional relaxation time, the most part of the toroidal current is generated due to collisional dragging of non-resonant electrons.The author gratefully acknowledges valuable discussions with K.Jungwirth, V.Kopecký, V. L.Sizonenko and K. N.Stepanov.     

Nonlinear Landau damping of high frequency waves in non-Maxwellian plasmas

Space plasmas often possess non-Maxwellian distribution functions which have a significant effect on the plasma waves.When a laser or electron beam passes through a dense plasma,hot low density electron populations can be generated to alter the wave damping/growth rate.In this paper,we present theoretical analysis of the nonlinear Landau damping for Langmuir waves in a plasma where two electron populations are found.The results show a marked difference between the Maxwellian and non-Maxwellian instantaneous damping rates when we employ a non-Maxwellian distribution function called the generalized(r,q)distribution function,which is the generalized form of the kappa and Maxwellian distribution functions.In the limiting case of r=0 and q→∞,it reduces to the classical Maxwellian distribution function,and when r=0 and q→κ+1,it reduces to the kappa distribution function.     

Self-reversal phenomena of toroidal current by reversing the external toroidal field in helicity-driven toroidal plasmas

In order to understand self-organization in helicity-driven systems, we have investigated the dynamics of low-aspect-ratio toroidal plasmas by decreasing the external toroidal field and reversing its sign in time. Consequently, we have discovered that the helicity-driven toroidal plasma relaxes towards the flipped state. Surprisingly, it has been observed that not only toroidal flux but also poloidal flux reverses sign spontaneously during the relaxation process. The self-reversal of the magnetic fields is attributed to the nonlinear growth of the n=1 kink instability of the central open flux.     

Minimum dissipative relaxed states in toroidal plasmas

Relaxation of toroidal discharges is described by the principle of minimum energy dissipation together with the constraint of conserved global helicity. The resulting Euler-Lagrange equation is solved in toroidal coordinates for an axisymmetric torus by expressing the solutions in terms of Chandrasekhar-Kendall (C-K) eigenfunctions analytically continued in the complex domain. The C-K eigenfunctions are obtained as hypergeometric functions that are solutions of scalar Helmholtz equation in toroidal coordinates in the large aspect-ratio approximation. Equilibria are constructed by assuming the current to vanish at the edge of plasma. For the m=0, n=0 (m and n are the poloidal and toroidal mode numbers respectively) relaxed states, the magnetic field, current, q (safety factor) and pressure profiles are calculated for a given value of aspect-ratio of the torus and for different values of the eigenvalue λ _{r 0}. The new feature of the present model is that solutions allow for both tokamak as well as RFP-like behaviour with increase in the values of λ _{r 0}, which is related directly to volt-sec in the experiment.     

Enhanced damping of electrostatic waves due to combination scattering from density oscillation in plasmas

Experiments demonstrating the combination scattering of the electrostatic wave from the electron density oscillation in plasmas are described. The spatially periodic energy transfer among the satellite modes and the primary mode is observed, which results in an enhanced damping of the primary mode.     

Zonal flows in tokamak plasmas with toroidal rotation

Zonal flows in tokamak plasmas with toroidal rotation are theoretically investigated. It is found that the low-frequency branch of zonal flows, which is linearly stable in a nonrotating system, becomes linearly unstable in a rotating tokamak, and that the high-frequency branch of zonal flows, the geodesic acoustic mode, can propagate in the poloidal direction with the frequency significantly lower than the frequency of the standing wave geodesic acoustic mode in the nonrotating system.     

Electron cyclotron heating in high density toroidal plasmas

Wave trajectories of the ordinary mode as well as the extraordinary one injected obliquely into a magnetic field in high density toroidal plasmas, are studied theoretically. Both waves are finally converted into the electron Bernstein mode and cyclotron damped.     

High- and low-confinement modes in simple magnetized toroidal plasmas

Three-field simulations of interchange turbulence are presented for a simple magnetized toroidal plasma with a vertical magnetic field. The simulations show the presence of two turbulent regimes characterized by low (L) and high (H) confinement properties. We evaluate analytically the properties of the L regime, obtaining expressions for the plasma gradients and for the density and heat fluxes that agree well with the simulations. By increasing the plasma source strength or reducing the vertical magnetic field, a transition to a H regime occurs, in which a strong velocity shear limits the perpendicular transport with respect to the L scaling and the plasma profiles steepen. The analytic estimate of the transition condition is in accord with the simulations.     

Nondiffusive suprathermal ion transport in simple magnetized toroidal plasmas

We investigate suprathermal ion dynamics in simple magnetized toroidal plasmas in the presence of electrostatic turbulence driven by the ideal interchange instability. Turbulent fields from fluid simulations are used in the nonrelativistic equation of ion motion to compute suprathermal tracer ion trajectories. Suprathermal ion dispersion starts with a brief ballistic phase, during which particles do not interact with the plasma, followed by a turbulence interaction phase. In this one simple system, we observe the entire spectrum of suprathermal ion dynamics, from subdiffusion to superdiffusion, depending on beam energy and turbulence amplitude. We estimate the duration of the ballistic phase and identify basic mechanisms during the interaction phase that determine the dependencies of the character of suprathermal ion dispersion upon the beam energy and turbulence fluctuation amplitude.     

Dust-lower-hybrid waves in quantum plasmas

The dispersion relation of the dust-lower-hybrid wave has been derived using the quantum hydrodynamic model of plasmas in an ultracold Fermi dusty plasma in the presence of a uniform external magnetic field. The dust dynamics, electron Fermi temperature effect, and the quantum corrections give rise to significant effects on the dust-lower-hybrid wave of the magnetized quantum dusty plasmas.     

Self-phasing of drift waves

It is shown that self-phasing can be used to generate and control large-amplitude drift waves in a magneto-plasma.     

Landau damping in degenerate solid state plasmas

Helicon waves are found useful for studying Landau damping in degenerate plasmas. The damping is analyzed as the phase velocity of the wave is increased from ω/q v_F to ω/q v_F. There is no first-orderlike transition at ω/q = V_F. In the collisionless limit, the damping tends to zero as ω/q → v_F. For finite collision times τ it does not vanish for ω/q > v_F. Nonlocal corrections to the wavelength exhibit a peak at ω/q = V_F, which degenerates into a shoulder for ωτ 100.     

Landau-like damping in rotating pure electron plasmas

A numerical solution of the two-dimensional (2-D) linearized incompressible fluid equations has been used to study the response of a cylindrical pure electron plasma to an externally applied pulse. The plasma response to the pulse exhibits a nondissipative decay process. This decay is due to differential rotation of the plasma at different radii and is clearly shown by computer-generated displays of the perturbed density     

Grad-Shafranov equilibria with negative core toroidal current in Tokamak plasmas

Numerical Grad-Shafranov (GS) equilibria with negative current density in the plasma core are computed which do not impose any particularly chosen models for the pressure and current-density profiles. This flexibility allows the profiles to be tailored so that an island unfolds in the low-field side, even for elongated plasmas, thus sustaining the negative-current core against outward forces. Among other topological results, reversed GS equilibria are also shown to be necessarily non-nested, except for the cylindrical and other very special degenerate, hence structurally unstable cases.     

Role of pressure gradient on intrinsic toroidal rotation in tokamak plasmas

The toroidal plasma rotation generated by the external momentum input and by the plasma itself (intrinsic rotation) has been separated through a novel momentum transport analysis in the JT-60U tokamak device. The toroidal rotation, which is not determined by the momentum transport coefficients and the external momentum input, has been observed. It is found that this intrinsic rotation is locally determined by the local pressure gradient and increases with increasing pressure gradient. This trend is almost the same for various plasmas: low and high confinement mode, co and counterrotating plasmas.     

Observation of spontaneous toroidal rotation inversion in Ohmically heated Tokamak plasmas

Bulk plasma toroidal rotation is observed to invert spontaneously from counter to cocurrent direction in TCV (Tokamak à Configuration Variable) Ohmically heated discharges, in low confinement mode, without momentum input. The inversion occurs in high current discharges, when the plasma electron density exceeds a well-defined threshold. The transition between the two rotational regimes has been studied by means of density ramps. The results provide evidence of a change of the balance of nondiffusive momentum fluxes in the core of a plasma without an external drive.