Formation criteria and positioning of internal transport barriers in ASDEX Upgrade

Advanced tokamak discharges on ASDEX Upgrade can generate an internal transport barrier (ITB) during the current ramp-up phase early in the discharge. Formation of the ITB has become more reliable with the discovery that a low density is necessary for it to form. These ITBs form in very low or negative central shear regions. There is no clear evidence of integer q magnetic surfaces triggering the ITB phase. The appearance of an integer q surface (usually q = 2) often leads to a second ITB (i.e. another steepening in the ion temperature gradient) inside the first ITB.