Rapid Collapse of a Plasma Sawtooth Oscillation in the JET Tokamak

The rapid collapse of a sawtooth oscillation in the JET tokamak has been observed in detail on a fast time scale. Tomographic reconstruction of data from two x-ray cameras and electron-cyclotron-emission temperature profiles show that during the sawtooth collapse the central hot region is rapidly (in approximately 100 \ensuremath{\mu}s) displaced off axis with an $m=1$ component and is then redistributed around a surface of constant minor radius. The theoretical implications of the measurements are discussed.