Excitation of Alfvn eigenmodes by low energy beam ions in the DIII-D and JET tokamaks

Core localized Alfv́n eigenmodes in DIII-D [J. L. Luxon, Nucl. Fusion 42, 614 (2002)] and Joint European Torus (JET) [P. H. Rebut and B. E. Keen, Fusion Technol. 11, 13 (1987)] plasmas are driven by deuterium neutral beam ions traveling well below the Alfv́n speed. Modes are observed in reverse magnetic shear discharges with deuterium ion velocities as low as 0.23 and 0.16 of the Alfv́n speed parallel to the magnetic field in DIII-D and JET plasmas, respectively. Ellipticity-induced Alfv́n eigenmodes in DIII-D and toroidicity-induced Alfv́n eigenmodes in JET are excited by deuterium ions traveling well below the fundamental passing ion resonance condition, indicating the role of high-order resonances in driving these modes. NOVA-K analysis reveals many high-order resonances as contributing to the mode drive at high central safety factor due to the correspondingly large poloidal orbit width and the decrease in the perpendicular scale length of the modes.