Internal transport barriers (ITBs) can be produced in JET by the application of strong additional heating during the current rise of plasma discharges. These `so-called' optimized shear experiments with low positive magnetic shear have revealed a strong dependence between the formation of the barrier and the integer q magnetic surfaces (q = 2 or q = 3). Further analysis also shows a correlation between the emergence of the ITB and the edge external MHD activity which is triggered when an integer q surface occurs at the edge of a strongly heated plasma (q = 4, q = 5 or q = 6). Mode coupling is the prime candidate to explain the link between the internal integer flux surfaces, where the ITB is triggered, and the plasma edge. Modelling of the MHD behaviour confirms the possibility of such a mechanism. Once coupled, this destabilized mode is thought to enhance locally the E×B shearing rate, either by magnetic braking or by the radial transport losses resulting from the modification of the field line topology. This could then trigger an ITB inside the internal integer surface at q = 2 or q = 3.
Formation criteria and positioning of internal transport barriers in ASDEX Upgrade