Multi-machine analysis of turbulent transport in helical systems via gyrokinetic simulation

Abstract

We have investigated drift-wave instability and nonlinear turbulent transport in two configurations with different magnetic field structures by means of electromagnetic gyrokinetic simulations. Here, one is the neoclassically optimized Large Helical Device (LHD) plasma and the other is the Heliotron J (HJ) plasma. First, we show that the validation against the turbulent transport in the LHD plasma is successful, and that the neoclassically optimized configuration has smaller turbulent transport. Second, the neoclassical optimization through an enhanced toroidal mirror ratio, which is a capability of non-axisymmetric plasma, is found to improve the turbulent transport in the HJ plasma, which is qualitatively consistent with the observation in the HJ. Hence, the neoclassical optimization reduces the turbulent transport in both the LHD and HJ plasmas. Third, as a trial in evaluating the performance of a helical system designed with different concepts for stability, we compared turbulent transport in these plasmas and found that both the mixing-length-estimated diffusion and nonlinear turbulent transport of the HJ plasma are smaller than those of the LHD plasma in gyro-Bohm units. The significant difference is stronger zonal flows in the HJ plasma than in the LHD plasma.