Two-ion ion cyclotron resonance (ICR) heating in a tokamak can produce superthermal minority ions having orbital radii comparable with the plasma radius. The drift motion causes the trapped fast ions to precess around the torus, thus forming a fast ion current. A semi-analytic model and a Monte Carlo radiofrequency (RF) orbit code have been used to calculate the direction, magnitude and profile of this current. For typical high power central ICR heating of JET, a fast ion current density of j|| ~ 0.7 MA/m2 is found which is peaked at a radius ~ 30 cm from the centre where it enhances the local current density. The effect of the fast ion current is to flatten the q-profile within the region of the inversion radius of MHD sawteeth, where the sawtooth stability is expected theoretically to be sensitive to the radial profile of the current density. The fast ion current could explain the observed sawtooth stabilization during ICR heating. Data for RF heating of JET have been examined for evidence of a correlation between the natural duration of sawtooth-free periods and the expected magnitude of the off-axis fast ion current density. The data show a threshold current density below which sawteeth are not stabilized. Above this threshold, the duration of sawtooth stable periods increases monotonically with the magnitude of the off-axis RF fast ion current density
Sawtooth oscillations and fast-ion ejection in tokamaks