Effects of impurities on the trapped ion dissipative instability

Low frequency instabilities on trapped ions are expected on theoretical grounds to occur in the future high temperature Tokamak experiments. Instability regions, in terms of plasma parameters, have been derived for the trapped ion dissipative instability (Rosenbluth et al., 1972). Evaluations of anomalous particle and energy losses across the magnetic field have stressed the danger that this instability represents for good plasma confinement, leading possibly to anomalous fluxes several orders of magnitude larger than the neoclassical fluxes. It is therefore of extreme interest to investigate any possible mechanism of stabilization of these unstable waves. It is pointed out that a very efficient stabilizing factor for the ion dissipative modes may be naturally present in the plasma owing to its impurity content.