Sawtooth oscillations

The recent observations of sawtooth oscillations having a very fast collapse phase call for a re-examination of the theoretical description of sawtooth behaviour. One possibility is that the instability involved is an ideal m=1 ''quasi-interchange'' mode. A calculation of the stability of this mode shows that flattening of the q profile reduces the critical βp required for instability. Thus, contrary to previous expectations, the ideal mode can be unstable even at the low values of βp observed experimentally. Non-linear simulations show the predicted cellular convection. The distinction between the behaviour of the quasi-interchange mode and a tearing mode is described and a comparison is made with experimental results from JET. Finally, attention is drawn to a fundamental difficulty in explaining the rapid collapse in precursorless sawtooth oscillations. (author)