Finite beta effects on an ITG turbulence-zonal mode system in tokamak plasmas

Miyato, Naoaki; Kishimoto, Yasuaki; Li, J.*; Tokuda, Shinji

Finite beta effects on a coupled system of ion temperature gradient (ITG) driven turbulence and zonal modes are investigated by global fluid simulations of electromagnetic ITG turbulence. Finite beta has stabilizing effect on the linear ITG mode, while kinetic ballooning mode (KBM) is destabilized by beta and becomes dominant at high beta. In the ITG dominant regime zonal flows are driven by turbulence via the Reynolds stress and the geodesic transfer is a sink for the zonal flows. The Maxwell stress contribution to zonal flow generation is not so large even at the beta at which the most linearly unstable mode is changing from the ITG mode to the KBM. In electrostatic turbulence simulations, coarse choice of toroidal modes is often used to reduce computational time. But all toroidal modes should be included in numerical turbulence simulations for correct zonal mode generation.