The question of sawtooth stabilization at very high fast-ion energy contentshas been addressed in discharges carried out in the JET tokamak with ioncyclotron resonance frequency (ICRF) heating and varying plasma density,controlled by deuterium gas puffs. In these experiments dramatic differencesin the sawtooth behaviour have been observed. When the plasma densityne decreases below a certain threshold, the sawtooth frequency and thecrash duration time increase by a factor of five. Since the fast-ion energycontent increases with decreasing ne due to the inverseproportionality of the fast-ion slowing-down time on ne, the thresholdin ne corresponds to a threshold in the fast-ion energy content. Inthe present experiments, this threshold is reached when the fast-ion energycontribution to the total plasma diamagnetic energy content becomes largerthan 45%. The sawtooth activity with short sawtooth free period isaccompanied by MHD activity, with a toroidal mode number n = 1 at frequenciesbetween 55 and 65 kHz. This activity is interpreted as an energetic particlefishbone mode that is resonant with the ICRF-driven fast ions. Theexperimental results appear to be consistent with the stability diagram forsawtooth and fishbone modes (White 1989 Theory of Tokamak Plasmas(Amsterdam: North-Holland)), exploring the part of thediagram with a very large fast-ion population.
Experimental studies of instabilities and confinement of energetic particles on JET and MAST