Effect of impurities on dissipative trapped ion modes

Abstract

The stability of dissipative trapped ion modes in the presence of impurity ions is investigated and compared with previous theories of these modes in pure plasma. The impurities are assumed to be either in the plateau or Pfirsch–Schluter regime as is typical of either present day or the next generation of toroidal plasmas. The important effects of the impurities are found to be different depending on whether the parallel wave phase velocity is above or below the impurity thermal velocity. However, it can, in general, be stated that when the impurity density gradients are in the same direction as the electron and hydrogen ion density gradients, the impurity ions exert stabilizing effects on the modes. The critical density for stabilization of the modes is found to be significantly reduced, even at rather small impurity concentrations. On the other hand, cases of inverse impurity gradients are likely to be unstable. Numerical applications of the theory to nominal parameters of the PLT tokamak are given.