Mode Coupling Trigger of Tearing Modes in ECW Heated Discharges in FTU tokamak

In recent ECRH experiments on FTU tokamak the destabilization of (2,1) tearing modes, coupled to the (1,1) mode, has been observed. The dynamics of the rotating island can be described by a Rutherford-type equation, where mode coupling, inertial effects and resistive walls effects are included. TM dynamics is influenced by the position of the absorbing layer with respect to the island, to the extent that the mode can be completely suppressed if absorption occurs at the island radial position, with a precision in the order of the island size. EC absorption leading to TM stabilization is localized on the island by mechanical steering of the EC beam, and not by adjusting the toroidal field or forcing the plasma column in the right position for stabilization. Stabilization is achieved at a minimum ECRH power of 0.15POH. TM stabilization improves significantly the energy confinement in the plasma core.