The onset of a neoclassical tearing mode (NTM) depends on the existence of a large enough seed island. It is shown in the Joint European Torus that NTMs can be readily destabilized by long-period sawteeth, such as obtained by sawtooth stabilization from ion-cyclotron heating or current drive. This has important implications for burning plasma scenarios, as alpha particles strongly stabilize the sawteeth. It is also shown that, by adding heating and current drive just outside the inversion radius, sawteeth are destabilized, resulting in shorter sawtooth periods and larger beta values being obtained without NTMs.

The investigation of fluctuation phenomena in plasmas often necessitates the analysis of spatiotemporal signals. It is shown how such signals can be analyzed using the biorthogonal decomposition, which splits them into orthogonal spatial and temporal modes. The method, also referred to as the singular value decomposition, allows complex spatiotemporal patterns to be decomposed into a few coherent modes that are often easier to interpret. This is illustrated with two applications to fluctuating soft x-ray and magnetic signals, as measured in a tokamak. Emphasis is given to the physical interpretation of the biorthogonal components and their link with known physical models is discussed. It is shown how new insight can be gained in the interpretation of spatiotemporal plasma dynamics.

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.