NUMERICAL-SIMULATION OF ION-TEMPERATURE-GRADIENT-DRIVEN MODES

Ion-temperature-gradient-driven modes in the presence of ion-ion collisions in a toroidal geometry with trapped ions have been studied by using a one-and-one-half-dimensional (1 1/2-D) linearized gyrokinetic particle simulation code in the electrostatic limit. The purpose of the investigation is to try to understand the physics of flat density discharges, in order to test the marginal stability hypothesis. Results giving threshold conditions of L(Ti)/R0, and linear growth rates and mode frequencies over all wavelengths for the collisionless ion-temperature-gradient-driven modes are obtained. The behavior of ion-temperature-gradient-driven instabilities in the transition from slab to toroidal geometry, with trapped ions, is shown. A Monte-Carlo scheme for the inclusion of ion-ion collisions, in which ions can undergo Coulomb collisional dynamical friction, velocity space diffusion, and random walk of guiding centers, has been constructed. The effects of ion-ion collisions on the long wavelength limit of the ion modes is discussed.