Transport barrier formation induced by magnetic islands

Gyrokinetic simulation that couples self-consistent neoclassical and turbulent effects shows that internal transport barrier (ITB) formed inside a rational magnetic surface in experiments can be induced by a magnetic island (MI) at the rational surface. The key effect is associated with the ExB shear flow structure changes in the presence of MIs. An MI is shown to induce a localized neoclassical Er-well across island inner boundary. However, strong turbulence-driven Reynolds stress gradient across the island edge adds to continued growth of ExB flow beyond the neoclassical level. An MI is also shown to drive electric potential islands centred at both inner and outer edges of the island. Depending on the island width, the island-induced ExB shear layer along with the sheared vortex flows due to the low-n non-resonant potential islands can effectively facilitate ITB formation via two-fold effects: i) suppressing local turbulence in the inner core region next to the MI and ii) preventing turbulence spreading from outside the shear layer to the inside. The latter effectively decouples the plasma inside the shear layer from the outside turbulent plasma. The simulation further suggests the existence of a critical island width to trigger ITB, which underlies a connection of the ITB formation to weaker magnetic shears where MIs tend to be wider for the same perturbation amplitude. The simulation results highlight the critical role of neoclassical and turbulent physics interplay in the ITB formation process.