Lagrangian dynamics of drift-wave turbulence

The statistical properties of Lagrangian particle transport are investigated in dissipative drift-wave turbulence modelled by the Hasegawa-Wakatani system. By varying the adiabaticity parameter c, the flow regime can be modified from a hydrodynamic limit for c=0 to a geostrophic limit for c→∞. For c of order unity the quasi-adiabatic regime is obtained, which might be relevant to describe the edge turbulence of fusion plasmas in tokamaks. This particularity of the model allows one to study the change in dynamics when varying from one turbulent flow regime to another. By means of direct numerical simulation we consider four values for c and show that the Lagrangian dynamics is most intermittent in the hydrodynamic regime, while the other regimes are not or only weakly intermittent. In both quasi-adiabatic and quasi-geostrophic regimes the PDFs of acceleration exhibit exponential tails. This behaviour is due to the pressure term in the acceleration and not a signature of intermittency.     

Anomalous transport due to drift-wave turbulence

The anomalous transport of particles and thermal energy is analyzed for the electron and ion temperature gradient driven drift-wave turbulence. The entropy production from the anomalous flows is calculated. Approximate formulas for the fluctuation spectra are briefly compared with a microwave scattering experiment in a low beta tokamak.     

Origin of lagrangian intermittency in drift-wave turbulence

The Lagrangian velocity statistics of dissipative drift-wave turbulence are investigated. For large values of the adiabaticity (or small collisionality), the probability density function of the Lagrangian acceleration shows exponential tails, as opposed to the stretched exponential or algebraic tails, generally observed for the highly intermittent acceleration of Navier-Stokes turbulence. This exponential distribution is shown to be a robust feature independent of the Reynolds number. For small adiabaticity, algebraic tails are observed, suggesting the strong influence of point-vortex-like dynamics on the acceleration. A causal connection is found between the shape of the probability density function and the autocorrelation of the norm of the acceleration.     

Multiscaling dynamics of impurity transport in drift-wave turbulence

Intermittency effects and the associated multiscaling spectrum of exponents are investigated for impurities advection in tokamak edge plasmas. The two-dimensional Hasagawa-Wakatani model of resistive drift-wave turbulence is used as a paradigm to describe edge tokamak turbulence. Impurities are considered as a passive scalar advected by the plasma turbulent flow. The use of the extended self-similarity technique shows that the structure function relative scaling exponent of impurity density and vorticity follows the She-Leveque model. This confirms the intermittent character of the impurities advection in the turbulent plasma flow and suggests that impurities are advected by vorticity filaments.     

Scaling and suppression of anomalous heating in ion traps

We measure and characterize anomalous motional heating of an atomic ion confined in the lowest quantum levels of a novel rf ion trap that features moveable electrodes. The scaling of heating with electrode proximity is measured, and when the electrodes are cooled from 300 to 150 K, the heating rate is suppressed by an order of magnitude. This provides direct evidence that anomalous motional heating of trapped ions stems from microscopic noisy potentials on the electrodes that are thermally driven. These observations are relevant to decoherence in quantum information processing schemes based on trapped ions and perhaps other charge-based quantum systems.     

Nonlinear triad interactions and the mechanism of spreading in drift-wave turbulence

We present the results of a derivation of the fluctuation energy transport matrix for the two-field Hasegawa-Wakatani model of drift wave turbulence. The energy transport matrix is derived from a two-scale direct interaction approximation assuming weak turbulence. We examine different classes of triad interactions and show that radially extended eddies, as occurs in penetrative convection, are the most effective in turbulence spreading. We show that in the near-adiabatic limit internal energy spreads faster than the kinetic energy. Previous theories of spreading results are discussed in the context of weak turbulence theory.     

Observation of electrostatic instability driven by cross-field current and ion heating

Applying an external voltage perpendicularly to a static magnetic field, the electrostatic instability ascribed to the cross-field current due to the E × B drift of electrons could be excited near the lower hybrid frequency, and the ion heating was observed.     

Observation of quasi-two-dimensional nonlinear interactions in a drift-wave streamer

A streamer, which is a bunching of drift-wave fluctuations, and its mediator, which generates the streamer by coupling with other fluctuations, have been observed in a cylindrical magnetized plasma. Their radial structures were investigated in detail by using the biphase analysis. Their quasi-two-dimensional structures were revealed to be equivalent with a pair of fast and slow modes predicted by a nonlinear Schr?dinger equation based on the Hasegawa-Mima model.     

Intermittency in drift-wave turbulence: structure of the momentum flux probability distribution function

We analytically compute the probability distribution function (PDF) of the local Reynolds stress ( R) for forced Hasegawa-Mima turbulence. With the assumption that the PDF tail is due to an instanton with the spatial form given by the modon solution, the tail of the PDF of R is found to be a stretched, non-Gaussian exponential, with the specific form exp[-cR(3/2)] ( c is a constant). We relate the temporal localization of the instanton to the degree of "burstiness" of the momentum transport event.     

Ion-acoustic turbulence and anomalous transport

We present results for the dynamics of evolution of non-linear state plasmas in a d.c. electric field which causes ion-acoustic turbulence (IAT). We look at (1) the time variation of the drift electron velocity and of the effective collision frequency, (2) features of the redistribution and heating of resonance ions, (3) the evolution of the spectral and angular distribution of turbulent pulsations, (4) processes of heating of the bulk of the particles. The results of an analytical IAT theory are compared with computer simulations.Special attention is paid to the theory of inhomogeneous plasmas with IAT. A self-consistent theory of anomalous transport is presented. We discuss the anisotropy of anomalous transport and the influence of non-Maxwellian particle velocity distributions on the transport processes. The electromagnetic properties, self-organization and hydrodynamic instability of plasmas with IAT are discussed.     

Scattering of ion beam by the current-driven ion wave turbulence

The effective scattering of an ion beam by the ion wave turbulence is investigated experimentally. The results are compared with the values predicted theoretically.     

On ion beam-plasma wave modes and ion beam deceleration by ion-acoustic turbulence

The properties of ion beam-plasma wave modes have been utilized to diagnose the ion beam deceleration by ion-acoustic turbulence in a multidipole plasma device.     

Selective ion heating by energetic ion beam injection into a plasma

High energy neutral beam injected into a plasma has been predicted to be susceptible to microinstabilities. If the growth time of these microinstabilities is shorter than the collisional relaxation time, then the energy transfer from the beam to the plasma may take place, in the initial phase, by collisionless wave-particle interaction. We report here an experiment where this effect has indeed been observed. Here ion Bernstein waves excited by an energetic ion beam lead to selective ion heating of the plasma in a collisionless manner.