What regulates the improved confinement in negative triangularity tokamaks—the physics of zonal flow saturation.

Gyrokinetic simulations reveal new insights into the collisionless saturation of zonal flows in ITG turbulence while illuminating the dynamics of the Dimits shift and improved confinement in negative triangularity (NT) tokamaks. Time series of the zonal entropy transfer function exhibits intermittent bursts of back-transfer from zonal modes to fluctuations. The zonal shear lifetime correlates with the zonal back-transfer fraction. Increased back-transfer leads to shorter zonal shear lifetimes at higher temperature gradients. Near marginality, a reduction in back-transfer drastically enhances zonal flow coherence. NT mitigates ITG turbulence and transport by extending the lifetime of and increasing the strength of the zonal ExB shear, through suppression of these zonal back-transfer events. The reduced zonal back-transfer is likely linked to the local magnetic shear reversal characteristic of the NT magnetic geometry. NT exhibits lower zonal kinetic energy (ZKE) but higher zonal E×B shear, whereas positive triangularity (PT) plasmas show the opposite trend—higher ZKE but lower zonal shear. The ZKE fraction rises sharply while the shearing rates decline as the system approaches marginality from above. Consequently, turbulent heat diffusivity is lower for NT than for PT, though their difference diminish at lower temperature gradients. The Dimits shift is smaller in NT than in PT because, although the nonlinear heat flux critical gradient is similar, linear critical gradient is higher for NT.