Coupling of (eta)(sub i) and trapped electron modes in plasmas with negative magnetic shear

摘要

In toroidal collisionless plasmas, the ion temperature gradient (ITG or (eta)(sub i)) and the trapped electron (TE) modes are shown to be weakly (strongly) coupled when both the temperature gradient and the driving mechanisms of the trapped electrons are moderate or strong (weak but finite). In the regime of strong coupling, there is a single hybrid mode, unstable for all (eta)(sub i) in plasmas with positive magnetic shear. For the weak coupling case, two independent unstable modes, one in the ion and the other in the electron diamagnetic direction, are found to coexist, in either situation, the negative magnetic shear exerts a strong stabilizing influence; the stabilizing effect is considerably enhanced by the presence of trapped particles. It is predicted that for a given set of plasma parameters, it will be much harder to excite the two modes simultaneously in a plasma with negative shear. The results of this study are significant for tokamak experiments. (author). 18 refs, 7 figs. (Atomindex citation 28:022645)