Turbulence-generated stepped safety factor profiles in tokamaks with low magnetic shear

Nonlinear local and global gyrokinetic simulations of tokamak plasmas demonstrate that turbulence-generated currents flatten the safety factor profile near low-order rational surfaces when magnetic shear is low, even when the plasma β is small. A large set of flux tube simulations with different safety factor profiles (e.g., linear and nonlinear safety factor profiles) and global simulations with reversed magnetic shear profiles show that such stepped safety factor profiles dramatically reduce the heat transport and are a robust phenomenon. This mechanism may play a key role in the triggering of internal transport barriers and more generally reveal novel strategies for improving confinement in devices with low magnetic shear.