Sawteeth and m=1 mode evolution during ECRH/ECCD on FTU tokamak

Localized ECRH/ECCD has been performed on FTU with the specific aim of shaping the temperature/current density profile for controlling MHD activity. In particular, m=1 mode dynamics and stability are studied during the heating phase, when the steady-state, sawtoothing conditions of the ohmic plasma are modified by a strong off-axis electron heating. Depending on the position of the absorbing layer and on the density of the ohmic target plasma, sawteeth are temporarily suppressed, still in the presence of a slowly growing m=1 oscillation, or a complete quenching of any m=1 activity can be achieved. When direct current, driven by EC waves launched at +/-10 degrees from the perpendicular in the up-shifted scheme, is added to the pure beating effect, different shapes and periods of relaxation phenomena in the plasma core are observed. Although low values of I-ECCD/I-p are expected because of rather high values of Z(eff) during heating, (I-ECCD/I(p)approximate to 0.04 for P-EC =800KW, Z(eff)=5 and I-p=360kA), different local changes in the magnetic shear are thought to be responsible of these differences. Time-dependent calculations of current profiles, including transport and diffusion effects, and an analysis of the onset and stabilization of sawtooth oscillations based on the time evolution of sl(the magnetic shear value at q=1) compared with that of a suitable critical value are performed.