Effects of three-dimensional geometry and collisions on zonal flows and ion temperature gradient modes in helical systems

Effects of three-dimensional geometry of the field configuration and collisions on the zonal flows and the ion temperature gradient (ITG) modes in the Large Helical Device (LHD) are investigated by using a newly developed gyrokinetic simulation code, GKV-X. The GKV-X incorporates full geometrical information such as the Jacobian and the metric tensor of the flux surface obtained from MHD equilibrium. The effects of the full geometry on the growth rate and real frequency of the ITG instability are clearly found in the large poloidal wavenumber region where the finite gyroradius effect is also important. The weak collision under the LHD experimental conditions reduces the residual zonal flow level, even if the geodesic acoustic mode oscillation as well as the growth rate and the real frequency of the ITG modes are not much affected. Furthermore, the simulation results for the linear ITG modes are compared with a high ion temperature discharge in the LHD experiments. It is found that the unstable ITG modes have the growth rates which peak at radial positions with the largest deviation of the temperature gradient from the critical values, where the peak positions of the growth rate are close to the regions where the density fluctuation peaks are measured in the experiment. (author)