Geometry effects on zonal flow dynamics and turbulent transport in optimized stellarators.

With recent advances in optimized stellarator designs reducing neoclassical transport and energetic particle losses, it is now essential to assess their effectiveness in suppressing turbulent transport. Turbulence driven by ion temperature gradient (ITG) and trapped electron mode (TEM) instabilities plays a key role in energy loss and confinement. Due to the complexity of 3D magnetic fields, global gyrokinetic simulations are critical for quantifying ITG/TEM-driven transport and the resulting confinement in these configurations. Here, we present global gyrokinetic GTC simulations of electrostatic ITG and TEM turbulence in recently proposed quasi-axisymmetric (QA) and quasi-helical symmetric (QH) stellarators, compared with NCSX, W7-X, and the ITER tokamak. The simulations reveal strong suppression of ITG turbulence by zonal flows. Notably, the QH and quasi-isodynamic (QI) stellarators exhibit much greater ITG transport reduction than the QA or tokamak cases, owing to their higher linear zonal flow residuals and lower nonlinear zonal flow frequencies. As a result, the transport levels and energy confinement times in QH and QI are comparable to a tokamak of similar size and temperature gradient, despite the higher linear growth rates. TEM transport exhibits a similar zonal flow response, indicating a universal difference in zonal flow dynamics across configurations.