Dissipative trapped-electron modes in the presence of impurities

The effect of impurities on low-frequency drift modes of a toroidally confined plasma is investigated by the gyro-kinetic equation. It is assumed that the electrons are in the banana and the ions in the plateau regime. Impurity collision damping is found to be significant in the usual trapped-electron mode. A new instability due to the impurities can occur for normal profiles and impurities peaked at the center. Quasi-linear considerations show that impurities will be driven outward if such an instability occurs.