Plasma isotopic effect on the damping rate of toroidal Alfvén eigenmodes with intermediate toroidal mode numbers

This paper reports on the results of recent experiments performed on the JET tokamak on toroidal Alfvén eigenmodes (TAEs) with toroidal mode number (n) in the range up to |n| = 7. The stability properties of these medium-n TAEs are investigated experimentally using a set of compact in-vessel antennas, providing a direct and real-time measurement of the frequency, damping rate and amplitude for each individual toroidal mode number. The measurements, reported here, of the damping rate (γ/ω) for these medium-n modes were obtained during a deuterium to helium to hydrogen changeover experimental campaign, and are used to infer the effect of the plasma effective isotopic composition (AEFF) on the stability properties of these medium-n TAEs. We find that the damping rate of n = 1 TAEs decreases approximately as γ/ω ∼ 1/AEFF as reported previously, but only for modes whose frequency is close to the centre of the n = 1 toroidal gap and for density and current profiles giving an open gap structure. Conversely, for n ⩾ 3 TAEs we find that their damping rate approximately increases as γ/ω ∼ AEFF. (paper)