Double tearing reconnection and the off-axis sawteeth crash

A theoretical model is developed for the onset of the off-axis sawteeth crash observed in TFTR reversed magnetic shear experiments. The dispersion relation of the double tearing mode is obtained from the solution structure of the ideal external kink equation. The onset of 'annular crash' is due to the fast reconnection of the hot and cold islands, triggered by the interaction of both branches of the double tearing mode. The onset of 'core crash' is mainly due to the coalescence between the hot islands, triggered by the explosive growth of the inner branch and the rapid expansion of the hot islands. (author)