Internal transport barriers in ASDEX Upgrade

This paper gives an overview of all the experiments on transport barriers performed on ASDEX Upgrade. Strong (R/LTe = 25 Te(0) = 20 keV)\ntransport barriers in the electron heat channel are generated with counter electron cyclotron current drive in low density plasmas ne = 1 - 2.5x1019m-3. Ion\nbarriers are generated using neutral beam heating in the current ramp up. Correlation with plasma parameters, shows that a sufficiently low density is a\nnecessary condition for barrier formation. A new scenario with delayed heating in the current ramp has broken all the ASDEX Upgrade records of central ion\ntemperature (now 21 keV) and beta normalised (now 3.2). Pre-heating with ECRH delays the barrier formation, which forms at the same radial position.\nBroader barriers can be generated with more NBI power. A study of the barrier position with respect to the q-profile reveals that the foot of the barrier is not\nwell correlated with the minimum q position. Also turbulent fluctuations are reduced at the barrier foot, but not completely suppressed. The new scenario often\ndoes not show any external kink activity and can reach βn values of 3.2. They however, often have a (3,1) tearing mode.