Rapid Collapse of a Plasma Sawtooth Oscillation in the HT-7 Tokamak

The rapid collapse of a sawtooth oscillation which is characterized by the absence of any discernible precursor oscillation in the HT-7 tokamak has been observed in detail on a fast time scale. The contour plot of the raw data of soft-x-ray signals shows that sawtooth crash simultaneously on both the high field side and the low field side with symmetry. Only spatial structure of sawtooth exists in the whole sawtooth period. A larger heat outflow and profile of emission intensities flatten during sawtooth crash by means of soft x-ray tomography. The plausible interpretation is the stochastization of magnetic field lines lead to a transition from semicollisional to collisionless reconnection regimes during non-linear pre-crash phase in which the growth rate is rapidly increased and no any discernible precursor oscillation.