Steady state H mode and Te ≈ Ti operation with internal transport barriers in ASDEX Upgrade

Abstract

A review is presented of the scenarios and physics of advanced tokamak discharges and the associated technical enhancements leading towards enhanced performance and steady state operation in ASDEX Upgrade. A stationary advanced tokamak scenario with internal transport barriers (ITBs) in combination with an H mode barrier and weak shear (qmin ≈ 1) was maintained for 40 confinement times and several internal skin times with HITERL-89PβN ≈ 5. By raising the triangularity of the plasma shape the performance was increased up to HITERL-89PβN ≈ 7.2, but β is still limited by neoclassical tearing modes. The density was raised to close to 50% of the Greenwald density either by edge gas fuelling, causing an increase of the threshold power to sustain an ITB and a decrease of Zeff below 2, or by improved core particle confinement with more triangular plasma shapes without changing the ITB onset conditions. Sufficient helium pumping and no temporal impurity accumulation was observed despite peaked impurity density profiles.MHD modes contribute to making the shear 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 with reversed shear (qmin ≈ 2) (2,1) fishbones can clamp the current profile development near the q = 2 surface without causing energy confinement to deteriorate. Double tearing modes act similarly, but lead to substantial confinement losses. Applying central ECRF heating and current drive to beam heated reversed shear ITB discharges shows a substantial effect on MHD stability, affecting the passage of the q profile through qmin = 2 and degrading or prolonging the reversed shear phase, depending on the current drive direction. Moreover, reactor relevant Te ≈ Ti operation with temperatures in excess of 10 keV was achieved with internal transport barriers for both electrons and ions simultaneously. For current profile 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 qmin > 2 should then be possible at a plasma current of 1 MA according to power deposition, current drive and transport calculations.