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 turbule nce. The turbulence is driven by a constant incoming flux. In the case of r esistive 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 struc tures is emphasized. Bursty transport is investigated in the presence of zo nal flows and internal transport barriers generated either by a strong shea r flow or with a magnetic shear reversal. In deriving a low dimensional mod el that captures the main features of bursty transport dynamics, it is foun d that E x B shear flow is necessary to trigger the bursts.