Statistical analysis of a large set of current profile data obtained for sawtoothing discharges in the DIII-D tokamak indicates that the safety factor (inverse rotational transform) in the plasma center, ${\mathit{q}}_0$, is close to 1.0. The increase of ${\mathit{q}}_0$ associated with the sawtooth crash is found to be of the order of 0.05. The results of time resolved measurements of the current profile and the soft x-ray emissions support a model of the sawtooth instability based on total reconnection of the m=1 magnetic island.

• Received 26 April 1993

DOI:https://doi.org/10.1103/PhysRevLett.71.859