Flow Shear Stabilization of Ion Temperature Gradient Modes in an Internal Transport Barrier

Ion temperature gradient (ITG) driven instability is studied withgyro-kinetic theory in an internal transport barrier (ITB) of tokamak plasmas. The stabilizationeffects of a parallel velocity shear on the modes are investigated. It is found that the modestructures and stability properties, as well as the effects of a velocity shear, in an ITB aresignificantly different from that in off-ITB regions.

Impact of T_(i)/T_(e )ratio on ion transport based on EAST H-mode plasmas

At the EAST tokamak, the ion temperature(T_(i)) is observed to be clamped around 1.25 keV in electron cyclotron resonance(ECR)-heated plasmas, even at core electron temperatures up to 10 keV(depending on the ECR heating power and the plasma density). This clamping results from the lack of direct ion heating and high levels of turbulence-driven transport. Turbulent transport analysis shows that trapped electron mode and electron temperature gradient-driven modes are the most unstable modes in the core of ECR-heated H-mode plasmas. Nevertheless, recently it was found that the T_(i)/T_(e)ratio can increase further with the fraction of the neutral beam injection(NBI) power, which leads to a higher core ion temperature(Ti0). In NBI heating-dominant H-mode plasmas, the ion temperature gradient-driven modes become the most unstable modes.Furthermore, a strong and broad internal transport barrier(ITB) can form at the plasma core in high-power NBI-heated H-mode plasmas when the T_(i)/T_(e)ratio approaches ~1, which results in steep core Teand Tiprofiles, as well as a peaked neprofile. Power balance analysis shows a weaker Teprofile stiffness after the formation of ITBs in the core plasma region, where Ticlamping is broken,and the core Tican increase further above 2 keV, which is 80% higher than the value of Ticlamping in ECR-heated plasmas. This finding proposes a possible solution to the problem of Ticlamping on EAST and demonstrates an advanced operational regime with the formation of a strong and broad ITB for future fusion plasmas dominated by electron heating.

Stability of tokamak plasmas with internal transport barriers against high n ideal magnetohydrodynamic ballooning mode

A ballooning mode equation for tokamak plasma, with the toroidicity and the Shafranov shift effects included, is derived for a shift circular flux tokamak configuration. Using this equation, the stability of the plasma configuration with an internal transport barrier （ITB） against the high n （the toroidal mode number） ideal magnetohydrodynamic （MHD） ballooning mode is analysed. It is shown that both the toroidicity and the Shaftanov shift effects are stabilizing. In the ITB region, these effects give rise to a low shear stable channel between the first and the second stability regions. Out of the ITB region towards the plasma edge, the stabilizing effect of the Shaftanov shift causes the unstable zone to be significantly narrowed.

Recent results of fusion triple product on EAST tokamak

High fusion triple product has been obtained in the advanced scenarios with high normalized beta(βN)on the Experimental Advanced Superconducting Tokamak(EAST).A record value of ni0Ti0τE1.0×1019m^(-3)ke V s for EAST deuterium plasma has been achieved,which is due to the formation of strong and broad internal transport barriers(ITBs)in ne,Teand Tiprofiles.Analysis shows that the strong ITB formation could be attributed to the reduction of transport from ITG modes.Based on the analysis,the physical mechanisms and methods to furtherimprove the plasma performance are discussed.