Clear observations of early triggering of neo-classical tearing modes by
sawteeth with long quiescent periods have motivated recent efforts to control,
and in particular destabilize, sawteeth. One successful approach explored
in TCV utilizes electron cyclotron heating in order to locally increase the
current penetration time in the core. The latter is also achieved in various
machines by depositing electron cyclotron current drive or ion cyclotron current
drive close to the q = 1 rational surface. Crucially, localized current drive
also succeeds in destabilizing sawteeth which are otherwise stabilized by a
co-existing population of energetic trapped ions in the core. In addition, a
recent reversed toroidal field campaign at JET demonstrates that counter-neutral
beam injection (NBI) results in shorter sawtooth periods than in the Ohmic
regime. The clear dependence of the sawtooth period on the NBI heating
power and the direction of injection also manifests itself in terms of the toroidal
plasma rotation, which consequently requires consideration in the theoretical
interpretation of the experiments. Another feature of NBI, expected to be
especially evident in the negative ion based neutral beam injection (NNBI)
heating planned for ITER, is the parallel velocity asymmetry of the fast ionpopulation. It is predicted that a finite orbit effect of asymmetrically distributed
circulating ions could strongly modify sawtooth stability. Furthermore, NNBI
driven current with non-monotonic profile could significantly slow down the
evolution of the safety factor in the core, thereby delaying sawteeth.
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