Parallel flow driven instability due to toroidal return flow in high-confinement mode plasmas

We theoretically investigate turbulence in high-confinement mode (H-mode) plasmas with the pressure gradient and the mean flow. The toroidal flow, which is induced by the poloidal mean flow so as to satisfy the divergence free condition, exists in the H-mode, thus the effect of the toroidal return flow on instabilities is considered. The proposed model self-consistently includes not only the destabilization of the drift wave and the parallel flow shear instability, called the D'Angelo mode, but also the stabilization due to the poloidal flow shear. Depending on the strength of the flow shear or on the magnetic geometrical parameter, we obtain the stabilization of the drift wave and the destabilization of the D'Angelo mode. The competition between different instabilities through coupling of the poloidal flow with the toroidal return flow could be a key concept for understanding the turbulence in the H-mode. The characteristics of the instabilities are similar to the observations of the precursor of the type-III edge-localized mode. (paper)