High resolution soft-x-ray tomography of sawtooth oscillations on the Tokamak de Varennes

Up to five arrays of soft-x-ray detectors are used with the Fourier-Bessel harmonic reconstruction method to generate 2D images of sawtooth collapses during ohmic plasma discharges on the Tokamak de Varennes. For low-harmonic reconstructions (M{≤}2), a quasi-interchange behavior (Wesson model) is observed during the sawtooth crash, but a reconnectionlike behavior (Kadomtsev model) is recovered using higher harmonics (M{≥}3) on the same data. This indicates that the Kadomtsev model is a better description for the sawtooth collapse, although the collapse time remains shorter than predicted.