Zheng-Xiong Wang

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.

Sheath criterion for a collisional sheath

The sheath criterion in a collisional plasma sheath is investigated with a two-fluid model. It is shown that if neutral-ion collisionality in the sheath is taken into account, upper and lower limits for the sheath criterion exist.

The Bohm criterion for the dusty plasma sheath

The Bohm criterion for the dusty plasma sheath is investigated with a fluid model. It is shown that the presence of dust particles near the interface between the bulk plasma and the sheath should have effects upon the ion Mach number. The dust particles charging and density variation are taken into account, and therefore the self-consistent relation between the dust surface potential and ion Mach number at the sheath edge is derived. The numerical results reveal that both the ion critical Mach number and the dust critical Mach number with variation of density ratio of dust-to-electron begin to decline after a maximum.

Self-suppression of double tearing modes via Alfvén resonance in rotating tokamak plasmas

Reversed magnetic shear configuration, a key method for improving plasma confinement in advanced tokamaks, is prone to exciting double tearing modes (DTMs) that can severely degrade the plasma confinement. In this letter, we reveal a new mechanism of suppressing the DTM instability due to the self-induced Alfven resonance in rotating tokamak plasmas. The linear growth rate is reduced from ~SHp-1/3 of the fast DTM regime to ~SHp-3/5 of the slow single tearing mode regime, where SHp is magnetic Reynolds number. Instead of generating magnetic islands at the inner rational surface that can greatly enhance plasma transport in the core region, the formation of current sheets at resonance layers not only prevents the fast nonlinear DTM reconnection phase but also contributes to plasma heating.

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.

Interlocking and nonlinear saturation of double tearing modes in differentially rotating plasmas

Interlocking and nonlinear saturation of double tearing modes (DTMs) in rotating plasmas are investigated in a reduced magnetohydrodynamic model. Differential plasma rotation is found to have a significant stabilizing effect on the DTM. Analysis for the threshold island width and locking frequency is carried out. The effect of the viscosity on the mode is also discussed.

A mode transition in self-suppressing double tearing modes via Alfvén resonance in rotating tokamak plasmas

The rotation profile effects on self-suppressing double tearing modes (DTMs) via Alfv?n resonance in rotating tokamak plasmas with reversed magnetic shear are numerically investigated using a reduced magnetohydrodynamic model. The synergetic effects of Alfv?n resonance and flow shear on suppressing the DTMs are addressed. It is found that the Alfv?n resonances on both sides of the inner rational surface rs1 decouple the strongly coupled DTMs, and simultaneously the flow shear further stabilizes the tearing instability on the outer rational surface rs2. When the tearing instability on rs2 is stabilized so significantly that it becomes less unstable than the original one on rs1, a new mode transition occurs, in which the tearing instability excitation switches from rs2 to rs1; meanwhile, the Alfv?n resonances switch from both sides of rs1 to both sides of rs2. Moreover, the characteristics of the two eigenmode patterns of the DTM-induced Alfv?n resonances are analysed in detail. In addition, it is observed that the critical rotation frequency of the mode transition is almost independent of resistivity.

Dust-acoustic soliton in dust-electron plasmas induced by ultraviolet irradiation

Sagdeev potential approach is used to investigate the dust-acoustic soliton in dust-electron plasmas induced by ultraviolet irradiation, taking into account the self-consistent dust charge variation. It is shown that only the positive potential soliton can exist in the two-component plasmas. In addition, there exist both dust and electron density humps accompanying the soliton.

Finite frequency zonal flows in multi-scale plasma turbulence including resistive MHD and drift wave instabilities

The evolution of multi-scale plasma turbulence including resistive MHD and micro-instabilities is studied based on a 5-field slab gyrofluid simulation aiming to understand complex nonlinear interactions and turbulent transport. It is observed that the spatial structure of the mixed-scale electromagnetic turbulence is characterized by a power-law scaling spectrum typical of MHD perturbations, but the spectral amplitude is enhanced by the micro-instability at all scales. A robust oscillatory zonal flow (ZF) with finite frequency is created in slab geometry for the first time due to the multi-scale interaction so that the ion heat transport is not efficiently suppressed. It is identified that the finite frequency ZF results from a net oscillatory electromagnetic torque, which is sustained by micro-instability through multi-scale nonlinear interaction.

Low frequency dusty plasma modes in a uniform magnetic field

Low frequency electrostatic waves of dusty plasmas in a uniform magnetic field are investigated. Since in conventional experiment parameter ranges the dust particles can hardly be magnetized, a “moderately magnetized” assumption of magnetized electrons and ions and unmagnetized dust particles is applied to such dusty plasmas. Effects of the neutral gas damping and the ion drag force on dust particles are taken into account. In the frequency range of ion waves, both dust-ion-acoustic waves propagating parallel to the magnetic field and the ion cyclotron waves are damped due to the ion drag on dust particles. In the very low dust frequency range, a new mode of slow dust waves propagating across the field line is found. The mode is damped to zero frequency in the long wavelength range due to the neutral gas damping and ion drag.