Collective Effects and Self-Consistent Energetic Particle Dynamics in Advanced Tokamaks

In the present paper, we address the issue of fast ion and fusion product transport in conditions that are typically relevant for burning plasmas operating in so called Advanced Tokamak regimes. Our results have direct implications, e.g., on the choice of current profiles for ITER steady state operations. We demonstrate that in a Tokamak equilibrium with hollow-q profile, in general, two types of EPM (Energetic Particle Modes [1]) gap modes may exist near the minimum-q surface (q = q0), characterized by opposite signature in frequency: one with upwards chirping frequency [2] and the other with downward chirping frequency as q 0 drops. It is shown that EPM gap modes are described by the same dispersion relation of the usual resonant EPMs [1] and that they can indeed be considered as the same mode with, however, different dominant damping mechanisms. This work also presents a discussion of EPM non-linear dynamics with respect to energetic ion transport in tokamaks with hollow q-profiles. Numerical simulations based on a Hybrid MHD-Gyrokinetic Code (HMGC) [3, 4], demonstrate that, above the EPM excitation threshold, fast radial redistribution of energetic ions takes place on a time scale that is proportional to the inverse EPM growth rate (typically ≈ 100τA, τA = R0/vA being the Alfvén time). The rapid evolution of EPM mode structures and the associated fast ion transport is interpreted within the framework of the relay runner model for non-linear EPM dynamics [5]. It is found that a sensitive parameter for tokamak equilibria with hollow-q profiles is q at the minimum-q surface, higher q corresponding to larger particle transport. This fact has clear implications on the choice of current profiles in a burning plasma [6].