Plasma size and collisionality scaling of ion-temperature-gradient-driven turbulence

Nakata, Motoki; Idomura, Yasuhiro   

Fixed-flux (FF), fixed-gradient (FG), and local fluxtube (FT) gyrokinetic simulations are systematically compared for ion temperature gradient (ITG) driven turbulence. The collisionality () 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 -dependence, while the FG model shows a strong -dependence. It has been revealed that the FG model provides such strong -dependence through the change of ITG-mode stability due to -dependent deformation of the velocity distribution function. The plasma size () scan in the FF simulations show a Bohm like transport scaling even in a local limit regime, , 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.