Fixed-flux (FF), fixed-gradient (FG) and local fluxtube (FT) gyrokinetic simulations are systematically compared for ion-temperature-gradient (ITG)-driven turbulence. The collisionality (ν*) dependence of ion heat diffusivity is verified through the inter-model comparisons. When the temperature gradient is far from the nonlinear critical value, the FF and FT models give a weak ν*-dependence, while the FG model shows a strong ν*-dependence. The entropy transfer analysis on the zonal-flow saturation mechanisms in the quasi-steady state of the FT simulation provides clear insights on the different ν*-dependence of the turbulent transport and zonal-flow shearing rate in the far-above- and near-critical cases. It has also been revealed that the FG model provides the strong ν*-dependence through the change of ITG-mode stability due to ν*-dependent heating/sink by the adaptive heat source, where the velocity distribution function is deformed. The plasma size (ρ*) scan in the FF simulations show a Bohm-like transport scaling even in a local limit regime, ρ*−1 ⩾ 300, where profile-shear effects are weak. It has been clarified that the transient variations of local power balance are essential mechanisms leading to the Bohm-like heat transport even at similar mean temperature gradients, where the burst amplitude and its frequency increase with the plasma size and the heating power. The mechanism is unique to the FF model. Comparisons of statistical characteristics in the local limit regime show differences in frequency spectra and probability density functions of the heat flux, while zonal-flow structures and avalanche propagations properties are similar among these models.
Understanding nonlinear saturation in zonal-flow-dominated ion temperature gradient turbulence