STABILITY OF THE IDEAL M=1 MODE IN A TOKAMAK
The observation of a very fast collapse phase in JET sawtooth oscillations has led us to reconsider the stability of the ideal m = 1 mode. With a resonant surface in the plasma this mode is unstable in cylindrical geometry but can be stable in toroidal geometry. The calculation described uses the procedure given by Bussac "it et al/". and recalculates beta"rm pc" for current profiles which, outside the q = 1 surface, take the form given by equation (2) but are flattened to a chosen degree inside this surface. The critical value of beta"rm p" is determined as a function of the safety factor on axis, q"rm o", and the radius, r1, of the q = 1 surface. It is found that the value of beta"rm p" required to produce instability is much reduced and for q"rm o" < 1 it is found that the critical beta"rm p" rightarrow 0 as 1-q"rm o"rightarrow 0. These results lead to the possibility that the ideal m = 1 mode could be the cause of the rapid sawtooth collapse. The detailed implications of this result for the theory of sawtooth oscillations are explored elsewhere. Here we describe the linear stability calculation.
Bibliographic Reference: REPORT JET-P(87)-31, 1987, AVAILABILITY: JET, ABINGDON, UK
Record Number: 1989126067600 / Last updated on: 1989-05-01
Available languages: en