Sawtooth Control Relying on Toroidally Propagating ICRF Waves
Dedicated JET experiments have been devised in order to test the predictions of a recent theory indicating that sawteeth can be destabilised by ICRH in reactor relevant conditions. Energetic passing ions influence the MHD internal kink mode instability when they are distributed asymmetrically in parallel velocity, which is a natural feature of minority ion populations in resonance with toroidally co or counter propagating ICRF waves. Reported here are theory and dedicated JET experiments which have been devised in order to neutralise an alternative sawtooth control mechanism, involving changes in the equilibrium current due to ICRF, and permit comparison with recent theory across physical parameters. Negligible change to the net equilibrium current was assured by choosing ITER relevant 3He minority ICRF. Depending on the antenna phasing, the sawtooth period can remain as short as those of Ohmic pulses, despite being close to the L-H threshold. A change of antenna phasing can produce sawteeth so long that NTMs are triggered and saturated even in L-mode. The sawtooth stability properties are explained by the effect of the wide drift orbits of fast ions intersecting the q = 1 radius. Sophisticated numerical modelling agrees well with an analytical solution involving the distribution of energetic particles at q = 1, and with salient experimental observation.