Flux pumping studies and Ti peaking by q-profile shaping using ECCD in ASDEX Upgrade

Abstract

On ASDEX Upgrade a prioritized ‘advanced Tokamak’ program has been run during the last two experimental campaigns, focusing on the effect of non-standard current profiles on the behavior of high beta plasmas. Two lines of non-standard (i.e. non-sawtoothing) q-profiles are followed: (1) plasma self-organization of centrally flat q-profiles (qmin ≈ 1) via ‘flux pumping’, here based on a naturally occurring continuous (1,1) mode and (2) externally shaped q-profiles with qmin > 1, allowing for more experimental freedom of the q-profile to be established. The advantages and requirements of both concepts are discussed. The invited presentation at the workshop focuses on specific effects of Electron Cyclotron Current Drive (ECCD). With respect to flux pumping it could be shown that increasing the plasma beta raises the flux pumping capability. More central coECCD can be redistributed if beta is increased. This is in line with qualitative theory. For the case with qmin > 1, it is shown that small variations of the ECCD profile can modify the peaking of the central ion temperature. For larger radii no significant changes of the kinetic profiles have been observed as the q-profile is changed. Hfactors have not exceeded 1.2, well below the assumptions of several models for steady state tokamak operation. To achieve these changes of q at larger radii we have used ctr-ECCD in the plasma center, increasing the ohmic current globally. The net effect of central ctr-ECCD and increased ohmic current is a strong off-axis current drive, allowing H-modes with Te ≈ Ti ≈ 8 keV and n¯e ≈ 6 · 1019m−3 ≈ 0.5 nGW at q95 = 4 to be run stationary with qmin ≈ 1.4. The design of these discharges was strongly supported by inter-shot model based optimization.