Onset of neoclassical tearing modes on JET

A detailed study has been made of the onset and behaviour of (m = 3, n = 2) neoclassical tearing modes (NTMs) on JET, validating many of the underlying concepts in NTM physics. In particular, fitting onset thresholds in terms of the correctly motivated physics parameters produces scalings closer to theoretical expectations. The use of such locally measured parameters also removes previous inconsistencies in the data between different divertor configurations. The evolution of island size with β confirms the predicted levels of bootstrap current, and highlights the role of small island size stabilization terms, which give rise to a seeding requirement for NTMs from some other instability. This seeding requirement is confirmed, and is generally provided by sawteeth in the cases studied here. Although there are a variety of possible seeding mechanisms, for the particular cases studied here, it appears doubtful that the seeds are induced by magnetic coupling. In particular the NTM always appears well below the n = 2 sawtooth precursor harmonic frequency—although they often lock together once the NTM has grown close to saturation. Also, while plasma rotation profiles are favourable for a three wave interaction involving (4, 3) and n = 1 modes, with irregular matches to the appropriate mode frequency condition, there is no clear correlation between such matches and (3, 2) NTM growth rates. Thus, other seeding models should be considered—we have discussed two possibilities: a temporary change to classical tearing stability due to the sawtooth, and flow variation leading to a change in ion polarization current polarity, but neither of these are yet confirmed by the data. A complete resolution of this issue is essential if a predictive theory of NTMs is to be obtained, but it requires further, more detailed experimental measurements, and development of theoretical models. Finally, the nature of the (4, 3) NTM has also been explored. This mode also requires a seed perturbation to be triggered (in these cases supplied by a sawtooth), and occurs at similar local parameter values as the (3, 2) NTM, but with lower heating powers and βN. A cascade process emerges in which successively lower mode number NTMs occur as β is raised.