Observations of secondary structures after collapse events occurring at the q = 2 magnetic surface in the TEXTOR tokamak

Collapse events at the q = 2 surface occurring during the mode locking process of an m/n = 2/1 tearing mode have been observed on TEXTOR. The plasma confinement within the q = 2 surface collapses without much influence on the width of the island (O-point). With an external rotating resonant magnetic perturbation (RMP) field induced by the dynamic ergodic divertor, secondary structures moving near the separatrix of the primary large 2/1 island with the same frequency as the RMP have been observed after the collapse events. The plasma confinement recovers when the secondary structures vanish. (letter)