Coherent structures in the edge turbulence of the CASTOR tokamak

A two-dimensional array of electric probes placed on the poloidal plane has been inserted in the edge region of the CASTOR tokamak. The floating potential patterns measured by the probes show the presence of localized structures which are convected poloidally by the E x B plasma motion. The lifetime of these structures has been found to be large enough to allow them to complete several full poloidal rotations before dying away. Furthermore, the eddy turnover time of the plasma around the potential blobs is smaller than the lifetime, so that these structures can be called coherent.