Radially global gyrokinetic simulation studies of transport barriers

Improvements in tokamak transport have recently been obtained in a variety of operational modes through the formation of transport barriers, or good confinement radial zones. Here global nonlinear three‐dimensional toroidal gyrokinetic simulation is used to study three effects that are linearly stabilizing and may cause the formation of transport barriers, namely, sheared toroidal rotation, reversed magnetic shear, and peaked density profiles. The effect of toroidal shear flow on ion heat diffusivity is found to be relatively weak compared to mixing‐length expectations based on linear calculations. In contrast, it is found that weak or negative magnetic shear (s<1/2) in combination with a peaked density profile relative to the temperature profile greatly suppresses ion‐temperature‐gradient‐driven turbulence in the central region of global nonlinear simulations. Similar features are seen experimentally in reversed magnetic shear tokamak plasmas. There is some nonlocal penetration (∼20–30ρ_i) of the turbulence into the subcritical core region.