Energetic-ion-driven toroidal Alfven eigenmodes observed in a heliotron torsatron plasma

Toroidal Alfven eigenmodes (TAEs) of low toroidal mode number, n = 1 and 2, an observed in neutral-beam-heated plasmas in the compact helical system h eliotron/torsatron. The observed frequency is proportional to the computed TAE frequency and lies near the lower bound of the innermost TAE gap. The m odes are excited only when the beam velocity exceeds about half the central Alfven velocity and when the net plasma current induced by coinjected neut ral beams is in the required range. The modes are localized in the plasma c ore region, between 0.2 and 0.6 of the plasma minor radius.