Alfvénic instabilities driven by fusion generated alpha particles in ITER scenarios

Abstract

The stability of proposed ITER scenarios with respect to low-medium mode number Alfvénic instabilities driven by fusion-produced 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). All the three cases are found unstable, even though the mode growth rates are quite small. The investigation is extended to cases with larger drive, which could be realistically obtained because of a moderate increase in electron and bulk-ion temperatures as well as by inclusion of energetic particles produced by additional heating methods. It is shown that, in the SCH case, if the drive exceeds the reference value by a factor ∼1.6, fast growing energetic-particle modes (EPMs) are driven unstable by the resonance with trapped alpha particles. 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. Simulations performed with increasing drive intensity show that a strong flattening of the alpha-particle pressure profile can occur in the inner plasma region for the SC2 case, while global confinement is not significantly affected. In the SC4 and SCH scenarios nonlinear effects are more pronounced in the outer portion of the discharge, where the modes are localized, with less impact on the on-axis pressure value with respect to the SC2 case. While for SCH such effects are observed only above the threshold for EPM destabilization, in the SC4 case significant alpha particle losses can occur even at moderately increased drive.