Localized reversal of the perpendicular velocity in Tore Supra ohmic, L-mode, limited plasmas

In Tore Supra plasmas, the perpendicular velocity measured by Doppler reflectometry was observed to reverse in a localized zone close to a normalized radius  ∼0.5–0.6, changing from a negative value (corresponding to a negative radial electric field Er) to a positive value (). This occurs in L-mode, ohmic plasmas with a negligible external momentum input, a non-circular limited cross-section, and an edge safety factor close to 3. This reversal is favoured by a decrease in the magnetic field, or an increase in density. It is accompanied by a characteristic behaviour of the MHD activity signal, whose amplitude decrease during a ramp-down of the edge safety factor as it approaches –3.2. A m/n  =  2/1 mode is involved in the mechanism causing these observations.