Staircase Safety Factor Profiles in Gyrokinetic Simulations at 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.

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 to 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.