Effect of Triangularity on Ion-Temperature-Gradient-Driven Turbulence*

Transport driven by ion temperature gradient (ITG) turbulence is an important loss channel in tokamaks. We model how changing triangularity δ, both negative and positive, affects ITG linear growth rates and turbulent saturation. Linear and nonlinear properties of an ITG scenario with adiabatic electrons are analyzed using the gyrokinetic code GENE. Peak linear growth rates decrease with negative δ but increase in finite radial wavenumber k_x with positive δ. The growth-rate spectrum broadens in k_x with negative δ and significantly narrows with positive δ. The effect of δ on linear instability properties can be explained through its impact on field line bending and curvature. Nonlinear heat flux is weakly dependent on triangularity for |δ| ≤ 0.5, decreasing significantly with extreme δ, regardless of sign. Zonal modes play an important role in nonlinear saturation in the configurations studied, and artificially suppressing zonal modes increased nonlinear heat flux by a factor of about four for negative δ, increasing with positive δ to almost a factor of 20. Proxies for zonal-flow damping and drive suggest that zonal flows are enhanced with increasing positive δ.