Steady state H mode and T-e approximate to T-i operation with internal transport barriers in ASDEX Upgrade

A review is presented of the scenarios and physics of advanced tokamak disc harges and the associated technical enhancements leading towards enhanced p erformance and steady state operation in ASDEX Upgrade. A stationary advanc ed tokamak scenario with internal transport barriers (ITBs) in combination with an H mode barrier and weak shear (q(min) approximate to 1) was maintai ned for 40 confinement times and several internal skin times with H(ITERL-8 9P)beta(N) approximate to 5. By raising the triangularity of the plasma sha pe the performance was increased up to H(ITERL-89P)beta(N) approximate to 7 .2, but beta is still limited by neoclassical tearing modes. The density wa s raised to close to 50% of the Greenwald density either by edge gas fuelli ng, causing an increase of the threshold power to sustain an ITB and a decr ease of Z(eff) below 2, or by improved core particle confinement with more triangular plasma shapes without changing the ITB onset conditions. Suffici ent helium pumping and no temporal impurity accumulation was observed despi te peaked impurity density profiles. MHD modes contribute to making the she ar profile stationary. In the ITB/H mode scenario (1,1) fishbones clamp the q value to the vicinity of 1 and avoid sawteeth, while in ITB scenarios wi th reversed shear (q(min) approximate to 2) (2,1) fishbones can clamp the c urrent profile development near the q = 2 surface without; causing energy c onfinement to deteriorate. Double tearing modes act similarly, but lead to substantial confinement losses. Applying central ECRF heating and current d rive to beam heated reversed shear ITB discharges shows a substantial effec t 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 c urrent drive direction. Moreover, reactor relevant T-e approximate to T-i o peration with temperatures in excess of 10 keV was achieved with internal t ransport barriers for both electrons and ions simultaneously. For current p rofile control ECCD will be supplemented by on-axis fast wave ICCD and off- axis current drive up to 400 kA using NBI (available in 2001) and ICRF mode conversion. Stationary discharges with reversed shear and q(min) > 2 shoul d then be possible at a plasma current of 1 MA according to power depositio n, current drive and transport calculations.