Effects of particles with large gyroradii on resistive magnetohydrodynamic stability

Fast ions in tokamaks are known to have a significant influence on global plasma instabilities. In normal mode analyses for tokamaks, the perturbed electric and magnetic fields have been evaluated at the position of the particle's guiding center. The effect of spatial variation of the perturbed fields within the gyroradius for resistive internal magnetohydrodynamic (MHD) modes is considered in the present paper. The resulting tearing mode stability for the reversed field pinch (RFP) is investigated. Such effects are important for neutral beam injected particles in current RFP experiments and for fusion-generated alpha particles. The fast particle dielectric response is evaluated from the linearized Vlasov equation, and inserted into a cylindrical MHD model for the bulk plasma. The response is found for the simplified particles distribution function assuming that equilibrium magnetic field is uniform within the gyro-orbit. The effect of large gyroradii is strong, and can be either stabilizing or destabilizing (depending on the radial distribution of the fast particle density). The effect is maximal when the fast particles reside near the tearing-resonant surface