Staircase Safety Factor Profiles in Gyrokinetic Simulations at Low Magnetic Shear Speaker Name: Arnas Volcokas, EPFL
In this work, we investigate the impact of electromagnetic effects on plasma turbulence self-organization at low magnetic shear using nonlinear gyrokinetics. Our previous electrostatic studies showed that turbulent eddies extend along magnetic   field lines for hundreds of poloidal turns when the magnetic shear s is low or zero. Such ``ultra-long'' eddies have significant consequences on turbulent transport due to parallel self-interaction. At low magnetic shear, parallel self-interaction induces strong corrugations in plasma profiles at low-order rational surfaces, including the formation of stationary current layers. When electromagnetic effects are considered, turbulence-generated currents lead to the development of stationary zonal magnetic potential, locally flattening the safety factor profile to form staircase structures or broaden the safety factor profile minimum. This represents a crucial feedback mechanism between turbulence and the imposed safety factor profile, resulting in a reduction in turbulent transport. We study the corrugated safety factor profiles using both the local flux tube code GENE and the global particle-in-cell code ORB5. To further explore this interaction, we employed a novel extension of the flux tube model, allowing simulations of non-uniform magnetic shear profiles, including minimum-q   profiles relevant for Internal Transport Barrier (ITB) formation. Our findings indicate that turbulence-generated current layers can flatten the imposed non-uniformity across the entire domain or substantially widen rational surface regions, consistent with global simulation results. We believe these results are relevant for understanding ITB formation and inform long-standing experimental observations.   |