Impurity drift modes at the plasma edge

The linear stability of modes, due to the presence of impurities and driven unstable by the combination of the free energy contained in the ion temperature gradient and the dissipation due to collisions, is investigated for parameters appropriate to the edge of the plasma in toroidal magnetic confinement experiments. These modes are low frequency (compared to the drift wave frequency), propagate in the electron diamagnetic direction (even though ηi= d ln Ti/d ln n>0), and are sensitive to the overall charge concentration, ZInI/ne, the collisionality of the plasma, and the relative peaking of the density gradients of the different ion species. They are not stabilized by magnetic shear. (orig.)