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. Investigation of possible turbulence suppression mechanisms suggests that something is missing from the generally accepted model of sufficient ExB sheared flow suppressing the turbulence since the ExB shearing rate was not greater than, or comparable with, the maximum linear growth rate in about half of the ITB discharges analysed. Thus, it does not explain the reduced radial transport observed in these discharges. (author)