Turbulence In Toroidal Plasmas Research Articles

On the interplay between MHD instabilities and turbulent transport in magnetically confined plasmas

The interplay between MHD and turbulence is an interesting topic in magnetically confined plasma and solar plasma. The experimental observations made recently, shown below, suggest coupling and interplay between MHD and turbulence in magnetically confined toroidal plasmas. (1) Turbulence spreading into the magnetic island, (2) there is a self-organized change in topology and turbulence in the magnetic island, (3) the flow is damped by a stochastic magnetic field, (4) the trigger mechanism for the MHD bursts, (5) MHD bursts have an impact on the ion velocity distribution and potential, and (6) turbulence exhausts are created at the MHD burst event. In this paper, experimental evidence for the interplay between MHD and turbulence in toroidal plasmas is reviewed. The physics mechanism of the interplay and a possible link to astrophysical plasma physics are also discussed.

The scale-to-scale energy transfers correlated with the development of turbulence in toroidal plasmas

Two different directions of scale-to-scale energy transfer that corresponds to forward and inverse energy cascades, respectively, were clarified in toroidal plasma turbulence by means of experimental analysis. In the present work we first report the evidence that the turbulence dominated by broad wavenumber spectrum was developed under the action of forward (direct) energy cascade in a reversed field pinch (RFP) plasma, while the generation of GAM zonal flow (ZF) was related with the energy concentrating into low frequency region from higher frequency region due to inverse energy cascade in a tokamak plasma. The two evidences are compared. The results indicate a modulation mechanism in turbulence which is relative to the generation or development of specific turbulent flows. This hints a correlation between energy transfer and the development of toroidal plasma turbulence.

Heavy ion beam probing—diagnostics to study potential and turbulence in toroidal plasmas

Heavy ion beam probing (HIBP) is a unique diagnostics to study the core plasma potential and turbulence. Advanced HIBPs operate in the T-10 tokamak and TJ-II flexible heliac with fine focused (<1 cm) and intense (100 µA) beams. They provide measurements in the wide density interval = (0.3–5) × 1019 m−3, in a wide range of Ohmic and electron cyclotron resonance heated (ECRH) discharges with various currents at T-10, and in the wide range of magnetic configurations with ECR and neutral beam injection (NBI) heating at TJ-II. Time evolution of the radial profiles and/or local values of plasma parameters from high field side (HFS) to low field side (LFS), −1 < ρ < 1, is observed in TJ-II by 125 keV Cs+ ions in a single shot, while LFS (+0.2 < ρ < 1) is observed in T-10 by 300 keV Tl+ ions. Multi-slit energy analyzers provide simultaneously data on the plasma potential φ (by the beam extra energy), plasma density ne (by the beam current), poloidal magnetic field Bpol (by the beam toroidal shift), poloidal electric filed Epol that allows one to derive the electrostatic turbulent particle flux ΓE×B. The cross-phase of density oscillations produces the phase velocity of their poloidal propagation or rotation; also it gives the poloidal mode number. Dual HIBP, consisting of two identical HIBPs located ¼ torus apart provide the long-range correlations of core plasma parameters. Low-noise high-gain electronics allows us to study broadband turbulence and quasi-coherent modes like geodesic acoustic modes and Alfvén eigenmodes.

Flux tube train model for local turbulence simulation of toroidal plasmas

A new simulation method for local turbulence in toroidal plasmas is developed by extending the conventional idea of the flux tube model. In the new approach, a train of flux tubes is employed, where flux tube simulation boxes are serially connected at each end along a field line so as to preserve a symmetry of the local gyrokinetic equations for image modes in an axisymmetric torus. Validity of the flux tube train model is confirmed against the toroidal ion temperature gradient turbulence for a case with a long parallel correlation of fluctuations, demonstrating numerical advantages over the conventional method in the time step size and the symmetry-preserving property.

Complete multi-field characterization of the geodesic acoustic mode in the TCV tokamak

The geodesic acoustic mode (GAM) is a coherently oscillating zonal flow that may regulate turbulence in toroidal plasmas. Uniquely, the complete poloidal and toroidal structure of the magnetic component of the turbulence-driven GAM has been mapped in the TCV tokamak. Radially localized measurements of the fluctuating density, ECE radiative temperature and poloidal flow show that the GAM is a fully coherent, radially propagating wave. These observations are consistent with electrostatic, gyrokinetic simulations.

Convective motion in collisionless trapped electron mode turbulence

Global gyrokinetic particle simulation of collisionless trapped electron mode turbulence in toroidal plasmas finds both diffusive and convective electron motion using a Lagrangian analysis. The convective motion is identified using simulation and analytic theory to arise from the conservation of the second invariant when resonant trapped electrons lose kinetic energy to the drift wave. A resonance broadening model fits well the diffusive and convective electron motion.

Turbulent Transport of Trapped-Electron Modes in Collisionless Plasmas

Global gyrokinetic particle simulations of collisionless trapped-electron mode turbulence in toroidal plasmas find that electron heat transport exhibits a device size scaling with a gradual transition from Bohm to gyro-Bohm scaling. A comprehensive analysis of spatial and temporal scales shows that the turbulence eddies are predominantly microscopic because of zonal flow shearing, but the presence of mesoscale structures drives a nondiffusive component in the electron heat flux due to the weak nonlinear detuning of the precessional resonance that excites the linear instability.

Investigation of the parallel dynamics of drift-wave turbulence in toroidal plasmas

The three-dimensional structure of drift-wave turbulence is studied in the core of the toroidal low-temperature plasma in the torsatron TJ-K. The results are compared with simulations from the GEM3 turbulence code. In experiment and simulation, the dimensionless parameters are similar to those of fusion edge plasmas. Arrays with 64 probes are used to measure parallel wavenumbers, propagation velocities and the tilt of the turbulent structures with respect to the field line. A parallel wavelength of 15 m and a parallel velocity in between the ion-sound and the Alfvén velocity confirm the three-dimensional nature of drift-wave turbulence. Quantitative agreement between experiment and simulation is found. The comparison with the drift-wave dispersion relation gives evidence for the coupling of the density perturbation to the shear-Alfvén wave.

Causal Relationship between Zonal Flow and Turbulence in a Toroidal Plasma

Electric field fluctuations are directly measured using twin heavy-ion-beam probes in Compact Helical System. The spectrum of the electric field fluctuation reveals the existence of a zonal flow at a frequency of less than 1 kHz, coherent modes with a long correlation length (conjectured as geodesic acoustic modes), and background turbulence. Analyses using a wavelet elucidate nonlinear interactions between these fluctuation components in disparate scales. The highlighted findings are that the turbulent fluctuations should be modulated in response to the direction of zonal flow, and should be suppressed, on average, by the zonal flow. This is the first observation that demonstrates the causal linkage between zonal flow and turbulence in toroidal plasmas at a high temperature.

Dynamic structure in self-sustained turbulence

A dynamical equation for self-sustained and pressure-driven turbulence in toroidal plasmas is derived. The growth rate of the dressed-test mode, which belongs to the subcritical turbulence, is obtained as a function of the turbulent transport coefficient. In the limit of a low fluctuation level, the mode has nonlinear instability and shows explosive growth. The growth rate vanishes when the driven transport reaches a stationary-turbulent level. The stationary solution is thermodynamically stable. The characteristic time, by which the stationary and self-sustained turbulence are established, scales with the ion-sound transit time and is accelerated by bad magnetic curvature. The influence of the pressure gradient as well as the radial electric field inhomogeneity are quantified.

Trapped-particle scattering by electrostatic turbulence in toroidal plasmas

This work is concerned with a study of possible effects of turbulent electrostatic waves on trapped particle motions in low-β toroidal plasmas. A mechanism of resonance between the frequencies of the waves in the turbulent spectrum and the harmonics of the particle bounce frequency in the magnetic wells is found to provide stochastic trapped particle diffusion. The effectiveness of this diffusive process in scattering trapped particles out of the trapping cone in velocity space has been investigated in detail as functions of the wave features and of the particle parallel energy for the cases of ion sound waves and of different types of drift waves. The conclusions are that an electrostatic turbulence of a relatively low level, with typical frequencies higher than the bounce frequency and wavelengths shorter than the connection length, can be effective in scattering trapped electrons out of the trapping cone in typical times longer than the bouncing time but still shorter than collisional times. On the other hand, similar effects cannot be obtained easily for the ions, owing to their greater inertia.