Collisional effects on positive and negative triangularity through local and global temperature gradient-driven and flux-driven gyrokinetic simulations

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Abstract
In this work, we explore the effects of collisions on turbulent transport, focusing on two different TCV magnetic equilibria featuring positive and negative triangularity. The investigation is conducted using gyrokinetic modeling. Both gradient-driven and flux-driven approaches are employed using the global code ORB5, and a comparison with GENE flux-tube modeling is also carried out. Linear and nonlinear simulations show that negative triangularity retains its beneficial effect on turbulent transport in both collisionless and collisional regimes. However, flux tube and global nonlinear simulations show contrasting trends on the impact of collisions on ion transport. Flux-driven simulations confirm that edge stiffness is significantly reduced in negative compared to positive triangularity, and a spontaneous pedestal-like logarithmic gradient develops for negative triangularity in the collisionless setup. However, without proper realistic profiles and physical boundary conditions, it is not possible to fully replicate the experimental differences observed in the temperature profiles between positive and negative triangularity. A collisionality scan reveals that the ion and electron transport coefficients do not change monotonically with collisionality, stressing how deep the interplay is between turbulent features and collisions. The general picture of collisions stabilizing TEM and damping zonal flows is confirmed, but it is shown that the effects on transport cannot be predicted without a numerical assessment.

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