Effects of multi-helicity confinement fields on zonal flows and ion temperature gradient turbulence

Effects of multi-helicity confinement magnetic fields on zonal flows and ion temperature gradient (ITG) turbulence are investigated by means of the gyrokinetic-Vlasov simulation code (GKV code). Detailed magnetic field parameters for the standard and inward-shifted configurations of the Large Helical Device (LHD) experiments are introduced in the simulation model. By the linear GKV simulations, it has been shown that, in the inward-shifted configuration, the maximum ITG mode growth rate slightly increases while the zonal flow is maintained for a longer time. Correspondingly, the nonlinear GKV simulations of the ITG turbulence in simple model configurations are extended to the more elaborate ones in the present study. The obtained results show effective regulation of the ITG turbulence by the zonal flows in the inward-shifted case. The zonal flows generated with three-times larger averaged amplitudes than those in the standard configuration lead to a lower ion heat transport level. The obtained results confirm the theoretical prediction that the neoclassical optimization of helical systems contributes to reduction of the anomalous transport by enhancing the zonal-flow level. (author)