Regulation of Turbulent Transport in Neoclassically Optimized Helical Configurations with Radial Electric Fields

Gyrokinetic Vlasov simulations of the ion temperature gradient turbulence demonstrate reduction of the turbulent transport with enhanced zonal-flow generation in the neoclassically optimized helical configuration as predicted by the theoretical analysis of the zonal-flow response. The inward-shifted plasma of the Large Helical Device, thus, has better confinement than that with the standard magnetic axis position, which is consistent with the experimental results. The zonal-flow response is also investigated for case with the equilibrium radial electric field that can be generated by the ambipolar neoclassical particle transport in helical systems. The gyrokinetic theory and simulation clarify that the poloidal E ×B rotation of the helically trapped particles enhances the zonal flow response which can lead to further reduction of the turbulent transport.