Coupling of ηi and trapped electron modes in plasmas with negative magnetic shear

Abstract

In toroidal collisionless plasmas, the ion temperature gradient (ITG or ηi) and the trapped electron (TE) modes are shown to be weakly (strongly) coupled when both the temperature gradients and the driving mechanism of the trapped electrons are moderate to strong (weak but finite). In the regime of strong coupling, there is a single hybrid mode, unstable for all ηi 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 simultaneously excite the two modes in a plasma with negative shear. The results of this study are significant for tokamak experiments.