Comparisons and physics basis of tokamak transport models and turbulence simulations

The predictions of gyrokinetic and gyrofluid simulations of ion-temperature-gradient (ITG) instability and turbulence in tokamak plasmas as well as some tokamak plasma thermal transport models are compared. These comparisons provide information on effects of differences in the physics content of the various models and on fusion-relevant figures of merit. Many comparisons are undertaken for a simplified plasma model representing DIII-D H-mode conditions. Most models show good agreement in linear growth rates and frequencies, but significant differences exist in transport levels. The Dimits shift — an upshift of the nonlinear critical gradient relative to linear threshold — is identified and attributed to zonal flow generation.