Semicollisional microinstabilities in the presence of a magnetic separatrix

An improved confinement regime, known as H-mode, has been observed since 1982 in several auxiliary-heated Tokamaks. Common features of these experiments seem to be, among the others, the presence of a magnetic separatrix quite close to the plasma boundary, the existence of a threshold in the edge electron temperature for the achievement of the H-mode, and the observation of electron density and temperature profiles rather flat over the whole radius with marked pedestals at the edge. These features indicate that the role of good confinement region is possibly played by the region beyond the q=2 surface. Therefore, in order to understand the global confinement properties of the discharges, it is necessary to investigate the mechanisms which can determine the transport in such region. Due to the high edge temperature values, the role of microinstabilities in contributing to the anomalous transport coefficients may be important, in principle, differently from the ohmic and L-mode cases. Moreover microinstabilities may still be relevant in determining transport in the internal region where the reduced gradients may be consistent with the locally improved confinement. The aim of this paper is to examine if the stability of toroidal semicollisional modes, belonging to the electrostatic drift branch, destabilized by ion temperature gradients and trapped electron dynamics, may be affected by the presence of a magnetic separatrix. (author) 2 refs., 4 figs