A new type of MHD activity in JET ICRF-only discharges with high fast-ion energy contents

The question of sawtooth stabilization at very high fast-ion energy content s has been addressed in discharges carried out in the JET tokamak with ion cyclotron resonance frequency (ICRF) heating and varying plasma density, co ntrolled by deuterium gas puffs. In these experiments dramatic differences in the sawtooth behaviour have been observed. When the plasma density n(e) decreases below a certain threshold, the sawtooth frequency and the crash d uration time increase by a factor of five. Since the fast-ion energy conten t increases with decreasing n(e) due to the inverse proportionality of the fast-ion slowing-down time on n(e), the threshold in n(e) corresponds to a threshold in the fast-ion energy content. In the present experiments, this threshold is reached when the fast-ion energy contribution to the total pla sma diamagnetic energy content becomes larger than 45%. The sawtooth activi ty with short sawtooth free period is accompanied by MHD activity, with a t oroidal mode number n = 1 at frequencies between 55 and 65 kHz. This activi ty is interpreted as an energetic particle fishbone mode that is resonant w ith the ICRF-driven fast ions. The experimental results appear to be consis tent with the stability diagram for sawtooth and fishbone modes (White 1989 Theory of Tokamak Plasmas (Amsterdam: North-Holland)), exploring the part of the diagram with a very large fast-ion population.