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A new scenario to delay or prevent neoclassical tearing mode (NTM) onset is presented. By active sawtooth destabilization, short period and low amplitude sawteeth are generated, such that the sawtooth produced NTM seed island is reduced and the threshold normalized plasma pressure for triggering of NTMs, β_Nonset, is increased. The scenario has been explored experimentally in the Joint European Torus (JET). Ion cyclotron resonance frequency (ICRF) waves tuned to the 2nd harmonic H-minority resonance have been used for sawtooth control. Whereas ICRF waves generally induce sawtooth stabilization, favouring the triggering of NTMs and reducing β_Nonset, the present experiments show that by toroidally directed waves, ion cyclotron current drive is produced, and that sawteeth can be destabilized by careful positioning of the 2nd harmonic H resonance layer with respect to the sawtooth inversion radius. As a result, NTM onset is delayed and β_Nonset is increased above its value obtained in discharges with additional heating from neutral beam injection alone.

The non-linear evolution of low mode number tearing modes is analysed, emphasizing the need for a threshold condition, to account for observations in tokamaks. The discussion is illustrated by two models recently introduced in the literature. The models can be compared with the available data and/or serve as a basis for planning experiments in order either to test theory (by means of beta -limit scaling laws, as proposed in this paper) or to attempt to control undesirable tearing modes. Introducing a threshold condition in the tearing mode stability analysis is found to reveal some bifurcation points and thus domains of intrinsic stability in the island dynamics operational space