Effect of impurity ions on the geodesic acoustic mode

A dispersion relation of the geodesic acoustic mode with the effect of impurity ions is systematically derived. It is found that the frequency of the geodesic acoustic mode for a plasma with impurity ions is lower than that without impurity ions, which are mainly due to the polarization of impurity ions. It is also found that the damping rate of the mode increases with the increase in effective charge in the small effective charge limit due to the polarization currents of impurity ions, and decreases in the large effective charge limit mainly due to the effect of the curvature drift of impurity ions. A maximum damping rate is found in the intermediate effective charge regime.