Sawtooth behaviour in highly elongated TCV plasmas

The effect of plasma elongation κ on the sawtooth behaviour is studied in ohmically heated TCV plasmas up to values of κ = 2.8. Above a certain value of κ, which varies from 2.2 to 2.6 and depends on the current profile, the sawtooth oscillations disappear completely. The sawteeth are then replaced by continuous oscillations with frequencies in the range from 5 to 20 kHz, which are resonant at the q = 1 surface and seem to prevent a peaking of the temperature and pressure profiles in the plasma core. It is suggested that the transition is a consequence of an ideal internal kink mode becoming unstable, even with a flat pressure profile inside the q = 1 surface, if κ is sufficiently high