OT 7 Particle Simulations of Alfvén Modes in Reversed-Shear DIII-D Discharges Heated by Neutral Beams∗

A rich spectrum of oscillations in the Alfvénic range has been observed in DIII-D tokamak reversedshear discharges heated by neutral beams. During the discharge phase characterized by Alfvénic activity, the energetic particle density profile, as calculated by a classical deposition model (TRANSP), appears to be much more peaked than that observed experimentally, as inferred, e.g., from the equilibrium reconstruction. In this paper the results of particle simulations of Alfvénic modes driven unstable by energetic ions in reversed-shear DIII-D discharges, performed by the HMGC code, are presented. Single toroidal mode number simulations (n = 2÷ 4), which fully retain energetic particle nonlinearities, are considered. An investigation of the sensitivity of the simulated-mode frequency to the q-profile variations is presented, along with a comparison with the experimental results. On the basis of our simulations the following interpretation of the DIII-D Alfvénic/energetic-particle phenomenology can be drawn: a) if the Alfvén modes were ineffective, the energetic ion density profile would be close to that obtained by TRANSP simulations; b) with Alfvén mode dynamics included in the simulations, the TRANSP fast ion profile would generate strongly driven modes (EPM, with frequencies mainly determined by the energetic ions), inducing significant transport and flattening such profile on a time scale τ ≤ 100μs; c) as the drive is reduced by the flattening of the energetic ion density profile and/or by the modification of their velocity-space distribution function, residual Alfvén modes exist, close to marginal stability, with frequencies strongly influenced by the q-profile evolution.