Sawtooth control using off-axis NBI

It has previously been observed that neo-classical tearing modes can be triggered by long period sawteeth even at low values of β. Since fusion-born α particles are predicted to invoke long period sawteeth, mechanisms to control sawteeth are important for ITER. Here, sawtooth behaviour in off-axis neutral beam injection (NBI) heated plasmas in JET is presented. It is found that the off-axis NBI can be used to destabilize sawteeth even in discharges with concurrent stabilizing on-axis neutral beams. Drift kinetic modelling of such discharges qualitatively shows that the passing fast ions born due to the NBI determine the stability of the n = 1 internal kink mode, thought to be related to sawtooth oscillations. In particular, when the beam ions are deposited outside the q = 1 surface, they can result in more frequent sawteeth. It is proposed that off-axis neutral beams could be used as an actuator for sawtooth control in ITER. Provided that the q = 1 surface is sufficiently core localized, the strongly passing population induced by the negative-ion NBI planned for ITER may be able to significantly destabilize the sawteeth.

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