J. Q. Dong

Shear flows induced by nonlinear evolution of double tearing modes

Shear flows induced by nonlinear evolution of double tearing modes are investigated in a resistive magnetohydrodynamic model with slab geometry. It is found that intensive and thin poloidal shear flow layers are generated in the magnetic island region driven by coupled reconnection process at both rational surfaces. The structure of the flow layers keeps evolving after the merging of magnetic separatrices and forms a few narrow vortices along the open field lines in the final stage of magnetic reconnection. The effects of the distance between both rational surfaces and the initial magnetic shear on the nonlinear evolution of the plasma flows are also taken into consideration and the relevant mechanism is discussed.

Magnetic-island-induced ion temperature gradient mode

Characteristics of ion temperature gradient (ITG) instability in the presence of a magnetic island are investigated numerically using a gyrofluid model. It is shown that when the magnetic island is wide enough to produce a broad distribution of rational surfaces near the O-point region, the ITG perturbations at these rational surfaces form a radially global-type eigenmode with a fast growth rate, which is referred to as the magnetic-island-induced ITG mode. Moreover, the magnetic island also causes both radial and poloidal mode couplings, which play a stabilizing role.

Electromagnetic effects of kinetic geodesic acoustic mode in tokamak plasmas

Electromagnetic effects of the kinetic geodesic acoustic modes (KGAMs) are numerically studied in low β(= plasma pressure/magnetic pressure) tokamak plasmas. The parallel component of the perturbed vector potential is considered along with the electrostatic potential perturbation. The finite Larmor radius and finite orbit width of the ions as well as electron parallel dynamics are all taken into account. Systematic harmonic and ordering analysis is performed for collisionless damping of the KGAMs, assuming β~(κρi)2, where κand ρiare the radial component of the KGAM wave vector and the Larmor radius of the ions, respectively. It is found that the electron parallel dynamics enhances the damping of the electrostatic KGAM modes when the safety factor q is high. In addition, the electromagnetic (finite β effect is revealed to enhance and weaken the damping of the modes in plasmas of low and high safety factor q~2.0 and 5.5, respectively. The harmonic features of the KGAMs are discussed as well.

Double tearing mode induced by parallel electron viscosity in tokamak plasmas

The linear behaviors of the double tearing mode (DTM) mediated by parallel electron viscosity in cylindrical plasmas with reversed magnetic shear and thus two resonant rational flux surfaces is numerically investigated. The distance between the two surfaces is found to play an important role for modes with poloidal mode number m>1. Two modes, one of which is centered at the inner rational surface and the other is located between the two surfaces, are simultaneously unstable and the growth rates show the standard single tearing mode (STM) scaling as γ∝R−1/3 when the distance is large (here, the Reynolds number R≡τυ/τh, τυ, and τh are, respectively, the viscosity penetration time of the magnetic field and the Alfven time for a plasma sheet of width a). The latter is unstable only and the growth rate transits to the standard DTM scaling as γ∝R−1/5 for low-m (e.g., m<4) modes and keeps the STM scaling γ∝R−1/3 for high-m (e.g., m∼10) modes, which are found dominant, when the distance is decreased. In contrast,...

Gyrokinetic simulation of turbulence driven geodesic acoustic modes in edge plasmas of HL-2A tokamak

Strong correlation between high frequency microturbulence and low frequency geodesic acoustic mode (GAM) has been observed in the edge plasmas of the HL-2A tokamak, suggesting possible GAM generation via three wave coupling with turbulence, which is in turn modulated by the GAM. In this work, we use the gyrokinetic toroidal code to study the linear and nonlinear development of the drift instabilities, as well as the generation of the GAM (and low frequency zonal flows) and its interaction with the turbulence for realistic parameters in the edge plasmas of the HL-2A tokamak for the first time. The simulation results indicate that the unstable drift wave drives strong turbulence in the edge plasma of HL-2A. In addition, the generation of the GAM and its interaction with the turbulence are all observed in the nonlinear simulation. The simulation results are in reasonable agreement with the experimental observations.

Dynamics of large-scale structure and electron transport in tokamak microturbulence simulations

The important issue of whether zonal flows or streamers are preferentially formed in plasma turbulence with electron gyroradius scale is studied based on a gyrofluid model of electron temperature gradient (ETG) driven turbulence. Results from three approaches are presented. It is analytically derived first that the secondary generation of different large-scale structures is determined by the spectral anisotropy of turbulent fluctuation in two-dimensional Charney–Hasegawa–Mima turbulence. This is verified subsequently using three-dimensional simulations of sheared slab ETG turbulence, which show that the magnetic shear governs the pattern selection. It is found that a weak shear favours the enhancement of zonal flows so that the electron transport is strongly suppressed. In contrast, radially elongated streamers are formed nonlinearly in stronger-shear ETG turbulence. Finally, three-dimensional toroidal ETG simulations show that streamers are excited in the linearly stable region along the field (i.e. good curvature region) through a modulation instability after initial saturation of ETG modes. Although the electron transport at the quasi-steady state becomes higher than the initial saturation level, which is dominated by fluctuations with a peaked spectrum, the averaged value is still low at around the gyro-Bohm level. Furthermore, it is shown that the enhanced zonal flows in weak shear ETG turbulence may be limited by a Kelvin–Helmholtz instability. Also, it is found that the electromagnetic effects reduce the generation of zonal flows and reverse the so-called Okawa-scaling of electron transport on the β dependence.

Magnetohydrodynamic flow layer formation in development of resistive double tearing mode

Quasilinear development of double tearing modes induced by plasma resistivity is numerically analyzed. The spatiotemporal characteristics of the modes are analyzed in detail. Magnetohydrodynamic flow layers are demonstrated to merge in the development of the modes. The sheared flows lie just at the boundaries of the magnetic islands in the quasilinear stage. The flows have sufficient levels of shear required for turbulence suppression. Possible correlation between the layer formation and triggering of experimentally observed internal transport barriers, preferentially formed in the proximity of rational flux surfaces of low safety factors, is discussed.

Impurity effects on trapped electron mode in tokamak plasmas

The effects of impurity ions on the trapped electron mode (TEM) in tokamak plasmas are numerically investigated with the gyrokinetic integral eigenmode equation. It is shown that in the case of large electron temperature gradient ( ηe), the impurity ions have stabilizing effects on the TEM, regardless of peaking directions of their density profiles for all normalized electron density gradient R/Lne. Here, R is the major radius and Lne is the electron density gradient scale length. In the case of intermediate and/or small ηe, the light impurity ions with conventional inwardly (outwardly) peaked density profiles have stabilizing effects on the TEM for large (small) R/Lne, while the light impurity ions with steep inwardly (outwardly) peaked density profiles can destabilize the TEM for small (large) R/Lne. Besides, the TEM driven by density gradient is stabilized (destabilized) by the light carbon or oxygen ions with inwardly (outwardly) peaked density profiles. In particular, for flat and/or moderate R/Lne, ...

Gyrokinetic study of ion temperature gradient instability in vicinity of flux surfaces with reversed magnetic shear

Ion temperature gradient (ITG) driven instability is investigated in the vicinity of a flux surface where the magnetic shear reverses. The generic properties of the profile of the magnetic shear are taken into account with gyrokinetic stability theory in the local sheared slab geometry integral equation. The stability analysis shows that there are four distinct unstable ITG branches with significantly different eigenvalues and mode structures existing simultaneously in the vicinity of the minimum q layer. The variation of eigenmode structures with magnetic shear is investigated in detail. The mixing length estimation of the induced plasma transport is performed. Detailed numerical results are presented and general correlations with simulations and experiments are noted.

Stabilization of ion temperature gradient driven instability by a vortex flow

Stabilization of ion temperature gradient (ITG) driven mode by a large-scale vortex flow is investigated using a gyrofluid model in slab geometry. Extending a recent work [Z. X. Wang et al., Phys. Rev. Lett. 103, 015004 (2009)], the focus in this work is put on the effect of magnetic shear. It is shown that the vortex flow can effectively stabilize the ITG mode by inducing both radial and poloidal mode couplings. Furthermore, decreasing magnetic shear is identified to weaken the stabilizing role of the vortex flow. The effects of the magnetic shear on the ITG mode structure and on the growth rate in the presence of various shear flows are obtained and the relevant mechanisms are discussed in detail.