MHD activity in FTU plasmas with reversed magnetic shear

Abstract. The MHD activity of plasma configurations with reversed magnetic shear has been investigated on the FTU tokamak. In the presence of pairs of surfaces with the same rational value q D m=n of the safety factor, double-tearing modes are excited which give rise in most cases to bursts of sawtooth-like profile rearrangements. More stable regimes have also been found, in which the activity is dominated by rotating saturated modes. In a particular case with q.0 / 2:8 and qmin 2:5 a discharge without any detectable MHD activity during the current flat-top has been obtained. In high-temperature regimes (Te 8 keV at ne 0:81020 m−3), an irregular activity has been detected near the plasma centre which could be due to the excitation of resistive interchange modes. 1