Bursty transport in tokamak turbulence: Role of zonal flows and internal transport barriers

Large scale transport events are studied using two different 3-D simulation codes related to resistive ballooning and ion temperature gradient turbulence. The turbulence is driven by a constant incoming flux. In the case of resistive ballooning simulations, the underlying structures are found to be radially elongated on the low Held side and distorted by magnetic shear in the parallel direction (streamers). The non-linear character of these structures is emphasized. Bursty transport is investigated in the presence of zonal flows and internal transport barriers generated either by a strong shear flow or with a magnetic shear reversal. In deriving a low dimensional model that captures the main features of bursty transport dynamics, it is found that E x B shear flow is necessary to trigger the bursts.