Alfvénic Instabilities Driven by Alpha Particles in ITER Scenarios

Abstract. The stability of proposed ITER scenarios with respect to Alfvénic instabilities driven by fusionproduced alpha particles is investigated by hybrid MHD-particle simulations. Three cases are considered: the monotonic safety-factor scenario (SC2), the reversed-shear one (SC4) and a flat safety-factor profile “hybrid” scenario (SCH). Though all the cases are found unstable, growth rates are quite small. The effects of nonlinear mode dynamics on the alpha-particle confinement at the reference drive are negligible for the SC2 and the SCH cases. For the SC4 scenario, some broadening of the alpha-particle pressure profile is observed, indicating inconsistency problems of the scenario itself. Such problems could arise for the other scenarios as well in the presence of a stronger drive (e.g., because of a moderate increase in electron and bulk-ion temperatures, or the inclusion of energetic particles produced by additional heating methods). The SCH scenario, however, does not present relevant profile broadening, unless the threshold for Energetic Particle Mode (EPM) destabilization (a factor ∼ 1.6 greater than the reference pressure gradient) is exceeded.