Global gyrokinetic simulation of turbulent transport in optimized stellarators

With recent progress on optimized stellarator designs leading to reduced neoclassical transport and energetic particle prompt loss, there is a pressing need to evaluate the effectiveness of the optimization for turbulent transport in these stellarators. Turbulence transport caused by ion temperature gradient (ITG) and trapped electron mode (TEM) instabilities play a major role in transport processes and strongly affects confinement properties. Given the three-dimensional magnetic fields, global gyrokinetic simulations [1] are crucial for determining the global ITG and TEM turbulence transport level and resulting confinement property in the neoclassically optimized stellarators. Here we present global gyrokinetic GTC [2] simulations of electrostatic ITG turbulence in recently proposed quasi-axisymmetric (QA) stellarator and quasi-helical-symmetric (QH) stellarator [3] in comparison with W7-X quasi-isodynamic (QA) stellarator and ITER tokamak [4]. An interesting finding from these simulations is steady-state turbulent transport in the QI and QH is much lower than that in the QA, although all have similar linear growth rates. This suggests that linear dispersion relation is not sufficient to assess turbulent transport level in the optimized stellarators. Effects of kinetic electrons [5] and electromagnetic turbulence will also be reported.