Active control of MHD instabilities by
ECCD in ASDEX Upgrade
M. Maraschek, G. Gantenbein, T.P. Goodman, S. Günter,
D.F. Howell, F. Leuterer, A. Mück, O. Sauter, H. Zohm,
Contributors to the EFDA-JET Workprogramme and the
ASDEX Upgrade Team
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 βN 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.