Internal transport barrier discharges on ASDEX upgrade: progress towards steady state

A stationary advanced tokamak scenario with an internal transport barrier (ITB) for ions and electrons. particles and momentum in combination with an H-mode barrier and flat shear (q(0) approximate to 1) was maintained for 40 confinement times and several internal skin times with HITERL-89P beta(N) approximate to 5. Raising the density by edge gas fuelling close to 50% of the Greenwald density to integrate proper exhaust conditions causes an increase of the threshold power to sustain an ITB and a decrease of Z(eff) below 2. In contrast, a density increase caused by improved core particle confinement at more triangular plasma shapes does not change the ITB onset conditions. No temporal impurity accumulation even with high-Z Ar puffing was observed despite of peaked impurity density profiles. MHD modes contribute to the stationarity of the shear profile. In the ITB/H-mode scenario (1,1) fishbones with a large reconnection area 'clamp' the q-value in the vicinity of one and avoid sawteeth during the whole high-performance phase. In ITB scenarios with reversed shear (q(min) greater than or equal to 2) fishbones can clamp the current profile development near the q = 2 surface without deteriorating energy confinement, whereas double-tearing modes, acting in a similar form, lead to substantial confinement losses. Applying central ECRF heating and current drive to beam heated reversed-shear ITB discharges shows an substantial effect on MHD stability, affecting the passage of the q-profile through q(min), = 2, and degrading or prolonging the reversed-shear phase depending on the CD direction, Moreover, reactor relevant T-e greater than or equal to T-i operation with temperatures in excess of 10 keV was achieved with internal transport barriers for both electrons and ions simultaneously.