Turbulence-Modified Safety Factor Profile in Tokamaks with Low Magnetic Shear

In this work, we investigate the impact of electromagnetic effects on plasma turbulence self-organization at low magnetic shear using nonlinear gyrokinetics simulations. Previous electrostatic studies showed that turbulent eddies can extend along magnetic field lines for hundreds of poloidal turns when the magnetic shear ŝ is low or zero. Under these conditions, parallel self-interaction induces strong corrugations in plasma profiles at multiple low-order rational surfaces, including the formation of stationary current layers. When magnetic shear is low and electromagnetic effects are considered, these turbulence-generated currents locally flatten the safety factor profile to form staircase structures. This feedback mechanism between turbulence and the imposed safety factor profile results in a significant reduction of turbulent transport. To further explore this interaction, we employed a novel extension of the flux tube model, allowing simulations of non-uniform magnetic shear profiles, including q profiles with reversed magnetic shear relevant to Internal Transport Barrier (ITB) formation. Our findings indicate that turbulence-generated current layers “pull” the safety factor profile toward rational values, thereby broadening the safety factor profile minimum and enhancing the turbulent stabilization. We believe these results are key in ITB formation, can inform long-standing experimental observations, and are important for the design of low magnetic shear fusion devices.