Improved confinement of tokamak plasma with central negative shear is checked against the resistive ballooning mode. In the negative shear regime, the plasma is always unstable for purely growing resistive ballooning mode. For a simplest tokamak equilibrium model, the s-α model, characteristics of this kind of instability are fully clarified by numerically solving the high n resistive magnetohydrodynamic ballooning eigen-equation. Dependences of the growth rate on the resistivity, the absolute shear value, the pressure gradient are scanned in detail. It is found that the growth rate is a monotonically increasing function of a while it is not sensitive to the changes of the shear s, the initial phase θ0 and the resistivity parameter εR.
The dynamics of secondary large-scale structures in electron-temperature-gradient （ETG） turbulence is investigated based on gyrofluid simulations in sheared slab geometry. It is found that structural bifurcation to zonal flow dominated or streamer-like states depends on the spectral anisotropy of turbulent ETG fluctuation, which is governed by the magnetic shear. The turbulent electron transport is suppressed by enhanced zonal flows. However, it is still low even if the streamer is formed in ETG turbulence with strong shears. It is shown that the low transport may be related to the secondary excitation of poloidal long-wavelength mode due to the beat wave of the most unstable components or a modulation instability. This large-scale structure with a low frequency and a long wavelength may saturate, or at least contribute to the saturation of ETG fluctuations through a poloidal mode coupling. The result suggests a low fluctuation level in ETG turbulence.
Advanced operation modes with external and internal transport barriers （ETBs and ITBs） are desirable for magnetic fusion ignition in toroidal devices. Therefore, ITB formation mechanism is an important topic of magnetic fusion plasma physics and has attracted much attention in recent years. The ITBs are often found to form in vicinities of low safety factor q rational flux surfaces and to coincide with MHD activities in experimentsI1＇2]. On the other hand, the crucial role of sheared flows in formation of ETBs and ITBs has been demonstrated in recent decades.
DONG Jiaqi LONG Yongxing MOU Zhongze ZHANG Jinhua LI Jiquan
