Yue Liu

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.

Fast linear growth of collisionless double tearing modes in a cylindrical plasma

Scaling laws of fast growth of collisionless double tearing modes in the linear phase are studied numerically by a reduced magnetohydrodynamics model in a cylindrical plasma. It is found that in the case ?s de, with de the electron inertial skin depth and ?s the ion sound gyroradius, the scaling of the linear growth rate changes gradually from to as the rational surface separation ?rs increases. In the case de ?s, on the other hand, the scaling shifts gradually from or to or as ?rs increases. In the small ?rs regime, furthermore, it is shown that for short wavelength unstable modes, the scaling on de and ?s as their poloidal mode number varies is similar to that as ?rs does. In addition, ?s is found to play an important role in reducing the scaling dependence on de. These numerical scaling laws are testified to be reasonable in comparison with previous analytical theories, based on the similarity of physical characteristics of the same ? categories in tearing modes, where ? is the linear instability parameter for tearing modes. Finally, the characteristics of the second unstable eigenmode with different mode numbers, under the influence of de and ?s, are obtained and analysed.

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.

Dust-acoustic soliton in electronegative complex plasmas with streaming positive ions

The Sagdeev potential approach is applied to investigate theoretically and numerically the dust acoustic soliton in electronegative dusty plasmas with streaming positive ions. The self-consistent dust charge variation is also taken into account. The lower and upper limits of Mach numbers are then derived. Moreover, it is found that both the positive ion streaming velocity and negative ion density can lower the two limits. The soliton profiles, involving the spatial potential, particle densities, dust surface potential, and space net charge distributions in various cases, are plotted and discussed.

The dust acoustic solitary waves in dusty plasmas: Effects of ultraviolet irradiation

The effects of ultraviolet irradiation on small but finite amplitude dust-acoustic solitons, are studied by the reductive perturbation method. The self-consistent dust charge variation is taken into account. It is shown that the ultraviolet irradiation can significantly lower the magnitude of the dust negative charge, and even make the dust grains charged positively. With the growth of the dust charge, the phase velocity and the width of the solitary wave increase, whereas its amplitude decreases. The related physical mechanism is discussed.

The electrostatic sheath in an electronegative dusty plasma

Bohm criterion for the electrostatic sheath in electronegative dusty plasmas, which are composed of electrons, negative and positive ions, as well as dust grains, is investigated with Sagdeev potential, taking into account the self-consistent dust charge variation. The numerical solutions show that the dust and negative ion densities, as well as the positive ion and dust Bohm velocities, all have effects on the dust charge at the sheath edge. The positive ion and dust Bohm velocities increase with the growth of the dust density, while both of them decrease with the growth of negative ion density. Furthermore, the interactions between the two Bohm velocities are considered. The results are examined and found to be reliable by the quantitative analysis of Sagdeev potential.

Scalings of strongly coupled collisionless q = 2 triple tearing modes in a tokamak plasma

Within the framework of a reduced collisionless magnetohydrodynamics (MHD) model containing the effects of electron inertia and electron pressure gradient, scaling laws of the strongly coupled collisionless q=2 triple tearing instabilities with broad linear spectra in a tokamak plasma are studied numerically. It is found that as the poloidal mode number m increases, the scaling power laws of the linear growth rate of triple tearing modes (TTMs) on SHp, de, and ρs change gradually from the strongly coupled tearing mode scalings to the standard single tearing mode (STM) scalings, where SHp is the magnetic Reynolds number, de is the electron inertial skin depth, and ρs is the ion sound gyroradius. For example, de-scaling of the linear growth rate γ~deαd changes gradually from γ~de1 to γ~de3 as the m number increases in the case of ρs≪de, while ρs-scaling γ~deαρ shifts gradually from γ~ρs2/3 to γ~ρs1 as m increases in the case of de≪ρs. Furthermore, it is observed that the increase of ρs gradually weakens the...

Bohm criterion for dust-electron plasmas induced by UV irradiation

Bohm criterion for dust-electron plasmas induced by UV irradiation is investigated by Sagdeev potential, taking into account the self-consistent dust charge variation. In the collisionless limit, the Boltzmann electron distribution and dust cold fluid in the sheath are adopted. It is found that the dust Bohm velocity decreases with increasing dust density in the bulk plasma. The results are examined to be reliable by the quantitative analysis of Sagdeev potential.

Combined effects of trapped energetic ions and resistive layer damping on the stability of the resistive wall mode

A dispersion relation is derived for the stability of the resistive wall mode (RWM), which includes both the resistive layer damping physics and the toroidal precession drift resonance damping from energetic ions in tokamak plasmas. The dispersion relation is numerically solved for a model plasma, for the purpose of systematic investigation of the RWM stability in multi-dimensional plasma parameter space including the plasma resistivity, the radial location of the resistive wall, as well as the toroidal flow velocity. It is found that the toroidal favorable average curvature in the resistive layer contributes a significant stabilization of the RWM. This stabilization is further enhanced by adding the drift kinetic contribution from energetic ions. Furthermore, two traditionally assumed inner layer models are considered and compared in the dispersion relation, resulting in different predictions for the stability of the RWM.

Screening of external magnetic perturbation fields due to sheared plasma flow

Within the single fluid resistive magnetohydrodynamic model, systematic toroidal modelling efforts are devoted to investigate the plasma response induced screening of the applied external 3D magnetic field perturbations in the presence of sheared toroidal flow. One particular issue of interest is addressed, when the local flow speed approaches zero at the perturbation rational surface inside the plasma. Subtle screening physics, associated with the favourable averaged toroidal curvature effect (the GGJ effect (Glasser et al 1975 Phys. Fluids 7 875)), is found to play an essential role during slow flow near the rational surface by enhancing the screening at reduced flow. A strong cancellation effect between different terms of Ohms law is discovered, leading to different screening physics in the GGJ regime, as compared to that of conventional screening of the typical resistive-inertial regime occurring at faster flow. These modelling results may be applicable to interpret certain mode locking experiments, as well as type-I edge localized mode suppression experiments, with resonant magnetic field perturbations being applied to tokamak plasmas at low input toroidal torque.