IMPURITY EFFECTS ON ETA-I MODE-STABILITY AND TRANSPORT

The effects of an impurity ion species on the linear stability of the toroidal ion temperature gradient driven mode (eta(i) mode) are investigated together with the scaling properties of the corresponding quasi-linear transport coefficients. A stabilizing influence of impurities is found for inwardly peaked or flat impurity density profiles except for small epsilon(n) (epsilon(n) = 2L(n)/L(B)), in which case destabilization is found. However, new impurity driven instabilities are introduced also below the pure eta(i) mode stability threshold. The eta(i) mode still accounts for the major part of the quasi-linear ion energy transport, which is reduced accordingly. The direction of the ion particle transport is mainly determined by the shape of the impurity density profile in relation to the background ion profile. With an inwardly peaked impurity profile, the impurity flow is usually directed towards the edge and the main ion flow is reversed owing to ambipolarity.