Dynamics and frequency behaviour of the MARFE instability on FTU

FTU is a circular, high field tokamak with limiter, which can operate at high electron density regimes, of the order of 1020 m-3, where the MARFE instability is present at various current and magnetic field values. MARFE oscillations cause continuous density fluctuations affecting the line-integrated density measurements. On the other hand, in the presence of a well developed MARFE, a peaking of density profiles is observed, this occurrence favors the exceeding of the Greenwald density limit [1,2]. The amplitude of density fluctuations decreases with edge safety factor and with electron density [3]. The dependence of the frequency on plasma parameters is investigated using spectrograms, for high q discharges, whereas the MARFE oscillations are well revealed by the high resolution interferometer. A study of the E? ×B? drift effect on the MARFE instability has been performed in a dedicated experimental session on FTU, including some discharges with reversed toroidal magnetic field; the field sign has provoked the MARFE positioning in the upper midplane or lower, respectively. Finally the plasma was moved on equatorial plane from high field side to low field side. The MARFE localization in respect of the drift, due to the influence of the radial electric field Er on the stable and unstable positions [4] is discussed.