Effects of negative triangularity on zonal flow dynamics, ITG turbulence saturation and turbulent heat flux in tokamaks*

Gyrokinetic flux tube simulations are presented that clarify the physics mechanisms behind the beneficial effects of negative triangularity (NT) flux surface shapes over positive triangularity (PT) flux surface shapes with respect to the micro-stability and turbulent transport driven by collisionless ion temperature gradient (ITG) driven mode. Electron response is considered Boltzmann. The linear growth rates are lower for NT than for PT. The reduced linear growth rate for NT is linked to the reduced eigenmode averaged magnetic drift frequency and wider and stronger negative local magnetic shear region about the outboard mid-plane. The nonlinear heat flux is lower for NT than that for PT. The reduced nonlinear heat flux for NT is due to reduced radial turbulence correlation length and increased turbulence correlation time. This, in turn, is due to higher self-generated zero frequency EΧB zonal shearing rate for NT than that for PT. Interestingly, the self-generated zero frequency zonal potential is lower for NT than for PT, as expected from the linear zonal flow residual calculations, and yet the corresponding zonal EΧB shearing rate is higher for NT than for PT.