A POSSIBLE HYBRID DISSIPATIVE TRAPPED ELECTRON-ION TEMPERATURE-GRADIENT MODE

The effects of dissipative trapped electrons on ion temperature gradie nt-driven instability (eta(i) mode) in tokamak plasmas are considered. A sheared slab geometry is adopted and a linearized fluid model of io n temperature gradient (ITG) mode including dissipative trapped electr ons, which are described by means of the well-known formula of the non adiabatic electron response [P. L. Similon and P. H. Diamond, Phys. Fl uids 27, 916 (1984)], is presented. Results show that in tokamak plasm as not only is there a modification of the dissipative trapped electro ns on the eta(i), mode, but also there may exist an intrinsic oscillat ion mode, namely, a hybrid dissipative trapped electron ion temperatur e gradient mode. The higher the dissipative trapped electron fraction, the more it drives the eta(i) mode, that is, when the trapped electro n fraction is sufficient high and the trapped electrons are dissipated strongly, the mode is dominated by the trapped electron dynamics and propagates in the electron diamagnetic direction. These analytical res ults can be reduced to the usual predictions of the ion temperature gr adient-driven instability in the absence of the dissipative trapped el ectron. Numerical results further show that (a) there may be a hybrid dissipative trapped electron ITG mode and the dissipative trapped elec tron effect is a destabilizing effect on this mode; and (b) when the t rapped electron fraction is sufficiently high and the trapped electron s are dissipated strongly, the mode is determined by the trapped elect ron dynamics. These conclusions are in agreement with the experimental observations in the latest simulated tokamak experiment on the Columb ia Linear Machine [J. Chen and A. K. Sen, Phys. Rev. Lett. 72, 3997 (1 994)]. (C) 1996 American Institute of Physics.