Interplay between Gyrokinetic Turbulence, Flows, and Collisions: Perspectives on Transport and Poloidal Rotation

The impact of ion-ion collisions on confinement is investigated with the full-$f$ and global gyrokinetic Gysela code through a series of nonlinear turbulence simulations for tokamak parameters. A twofold scan in the turbulence drive and in collisionality is performed, highlighting (i) a heat transport expressed in terms of critical quantities—threshold and exponent, (ii) a first evidence of turbulent generation of poloidal momentum, and (iii) the dominance of mean flow shear, mediated through the turbulent corrugation of the mean profiles, with regard to the oft-invoked zonal flow shear.

Figure 1

Accurate satisfaction of the equilibrium radial force balance, at any given simulation time.

Figure 2

A cartography in ($R/L_T$, ${\nu }_{\star }$) space of the turbulent heat flux ${\chi }^{\mathrm{turb}}$. The result in Ref. 5 is shown, as is the neoclassical heat flux ${\chi }^{\mathrm{NC}}$ for the two reported collisionalities.

Figure 3

Flux-surface and time-averaged ($\Delta t={\nu }_{ii}^{-1}$) computation at low collisionality in the source-free region of the poloidal velocity (top), its turbulent and neoclassical parts along with different shears (bottom), neoclassical and $\mathbf{E}\times \mathbf{B}$.

Figure 4

Time and radial average (${\mathbb{I}}_r=[0.4,0.7]$) of $v_{\theta }$ (top) and the variance of the $\mathbf{E}\times \mathbf{B}$ shears (bottom) in the source-free region—a cartography in ($R/L_T$, ${\nu }_{\star }$) space.