Strong braking of m=1 impurity-induced 'snake' in EAST sawtoothing plasma

Strong braking of the m= 1 impurity-induced (e. g. molybdenum emission) density perturbation 'snake' was observed in EAST sawtoothing plasma. The shift of the peak positions of soft x-ray emissions due to the snake can be as high as 7 cm. Notable electron density fluctuations (delta n(e)/ne(0) c an be as high as 8%) originating from this m= 1 snake are measured by a hydrogen cyanide laser interferometer. Due to mode coupling, synchronous locking of the m= 2 tearing mode and m= 1 snakes is expected to happen when the width of the m= 2 island exceeds a critical value. A critical width value of m= 2 for mode locking is calculated. The forced reconnection in the q=2 surface is induced by a toroidal sideband of the m= 1 mode.