Effect of the helical ripple on the confinement via zonal flows in helical plasmas

The control of zonal flows is the key issue in fusion research. We have examined the reduction of the anomalous transport due to the excitation of zonal flows by the transport code analysis in the core region [1]. The electron Internal Transport Barrier in helical plasmas was shown to be formed by the mechanisms of (i) the bifurcation of the ambipolar electric field and (ii) the reduced damping of zonal flows which causes the suppression of the turbulent transport. In the collisionless plasmas of the Large Helical Device (LHD) (in the region of the negative electric field), if the helical ripple becomes smaller, the level of the anomalous transport gets lower. We focus on the effect of the helical ripple related with zonal flows on the confinement in the helical plasmas. The reduction of the effective helical ripple causes the smaller damping rate of zonal flows in the helical plasmas even in the branch of the ion root, which means that the reduction of the anomalous transport can be obtained. For the cases of the different values for the helical ripple, the calculation results including the effects of zonal flows will be shown. This study is to investigate the impact of shifting axis positions, because the value of the helical ripple changes due to the variation of the experimental magnetic axis positions in LHD. This calculation result will explain the observations on LHD.\\[4pt] [1] S. Toda et al., Nucl. Fusion Vol. 47 (2007) 914-919