Impact of T_i⁄T_e ratio on ion transport based on EAST H-mode plasmas

Abstract

At the EAST tokamak, the ion temperature (Ti) is observed to be clamped around 1.25 keV in ECR-heated plasmas, even at core electron temperatures up to 10 keV (depending on the electron cyclotron resonance heating (ECRH) 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 (TEM) and electron temperature gradient (ETG) driven modes are the most unstable modes in the core of ECR-heated H-mode plasmas. Nevertheless, recently it was found that the Ti/Te 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 (ITG) 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 Ti/Te ratio approaches ~1, which results in steep core Te and Ti profiles, as well as a peaked ne profile. Power balance analysis shows a weaker Te profile stiffness after the formation of ITBs in the core plasma region, where Ti clamping is broken, and the core Ti can increase further above 2 keV, which is 80% higher than the value of Ti clamping in ECR-heated plasmas. This finding proposes a possible solution to the problem of Ti clamping in EAST and demonstrates an advanced operational regime with formation of a strong and broad ITB for future fusion plasmas dominated by electron heating.