Observation of turbulence suppression under low magnetic shear in EAST plasma

Dedicated transport experiments based on q-profile modulation have been performed in EAST with broadband turbulence dominated plasmas. Reduced broadband turbulence under lower internal inductance (li) is observed by a CO2 laser collective scattering system in a series of experiments across different current platforms. Experimental observations found that under higher current platforms, the internal inductance decreases with weaker magnetic shear, leading to a significant decrease in turbulence (for 1 ≤ kθρs ≤ 5) and improved confinement, which is consistent with power balance analysis. The stabilizing effect of low magnetic shear under low li on turbulence and thermal transport is also verified by using the gyrokinetic code NLT. Linear simulations show that both the growth rate and frequency of trapped electron mode under low magnetic shear are smaller than those under high magnetic shear. Nonlinear simulations are also conducted and compared to experimental results, which indicate that the reduction in electron turbulent transport with the low shear q profile may have contributed to improve Te. This work contributes to understanding the impact of magnetic configurations on turbulence transport.