Sawtooth activity in ohmically heated JET plasmas

Detailed studies of sawtooth activity in Ohmic plasmas in JET have revealed significant discrepancies both with previously reported observations of the phenomenon and with conventional models of the internal disruption. 'Compound' sawteeth, which display an intermediate collapse during the ramp phase, are observed in the majority of discharges. These usually exhibit no precursor activity, in contrast to observations in smaller tokamaks, but are often accompanied by successor oscillations. Furthermore, the collapse time of such sawteeth is much shorter than expected. These results suggest that the conventional model of the sawtooth is inadequate to explain sawtooth activity in large tokamaks.