Active control of MHD instabilities by ECCD in ASDEX upgrade
The modification of the stability and the behaviour of core MHD with local electron cyclotron current drive (ECCD) is presented. Starting from the innermost resonant surface, the q = 1 surface, the stability and hence the sawtooth period and the size of sawteeth is controlled with local on/off-axis co-/counter-ECCD. The sawteeth themselves can serve as a trigger for neoclassical tearing modes (NTMs) and therefore the excitation of NTMs can be influenced. Once these NTMs get excited they can be fully stabilized at high beta(N) with co-ECCD at the resonant surface. Detailed experiments on the dependence of the stabilization on the ECCD deposition width and the total driven current have shown ways to improve the stabilization efficiency both for the (3/2) and the (2/1)-NTM significantly. In the presence of (3/2)-NTMs the impact on the confinement can be reduced by triggering the so-called frequently interrupted regime (FIR-NTM) with the current drive in the vicinity of the (4/3) surface leading also to a clearer understanding of the FIR-NTM.