Stabilization of the dissipative trapped-ion instability by impurities

The influence of impurities on the dissipative trapped-ion instability is investigated considering all possible cases of ion impurities in the banana, plateau or Pfirsch-Schlüter regimes. It is found that impurities lead to very significant stabilizing effects on these modes. In particular, the instability is quenched when the impurities are in the Pfirsch-Schlüter regime, and also for plateau impurities, provided the impurity density gradients are directed as the main plasma density gradient. In the case of impurities in the banana regime, a sufficient condition for complete stabilization of the instability has been found in the case of sufficiently sharp and positive impurity gradients. In the unstable cases, there is a noticeable upward shift and shrinking of the unstable range increasing with impurity concentration. These features have been discussed quantitatively, in terms of impurity parameters, both with reference to present-day experiments and for future experiments with reduced impurity levels.