Experimental Evidence of Intrinsic Current Generation by Turbulence in Stationary Tokamak Plasmas

High-beta(theta e) (a ratio of the electron thermal pressure to the poloidal magnetic pressure) steady-state long pulse plasmas with steep central electron temperature gradient are achieved in the Experimental Advanced Superconducting Tokamak. An intrinsic current is observed to be modulated by turbulence driven by the electron temperature gradient. This turbulent current is generated in the countercurrent direction and can reach a maximum ratio of 25% of the bootstrap current. Gyrokinetic simulations and experimental observations indicate that the turbulence is the electron temperature gradient mode (ETG). The dominant mechanism for the turbulent current generation is due to the divergence of ETG-driven residual flux of current. Good agreement has been found between experiments and theory for the critical value of the electron temperature gradient triggering ETG and for the level of the turbulent current. The maximum values of turbulent current and electron temperature gradient lead to the destabilization of an m/n = 1/1 kink mode, which by counteraction reduces the turbulence level (m and n are the poloidal and toroidal mode number, respectively). These observations suggest that the self-regulation system including turbulence, turbulent current, and kink mode is a contributing mechanism for sustaining the steady-state long-pulse high-beta(theta e) regime.

Automated summary

Experimental achievement of high-beta steady-state long pulse plasmas with steep central electron temperature gradient is reported. Turbulent current, driven by electron temperature gradient mode (ETG), is observed and can reach a significant ratio of the bootstrap current. The maximum values of turbulent current and electron temperature gradient lead to the destabilization of an m/n=1/1 kink mode.