The fine structure of ELMs in the scrape-off layer

Edge-localized modes (ELMs) have been observed by diagnostic systems with high spatial and temporal resolution in the ASDEX and JET tokamaks. During the ELMs, substructures exist in the scrape-off layer (SOL) with sizes of 2-5 cm and lifetimes of the order of 20-50 mus. Their poloidal-temporal evolution is directly observed, and the radial transport due to turbulent E x B drift in these substructures is estimated. A comparison is made between 'normal' fluctuations between ELMs and these substructures in terms of poloidal size and velocity and related radial transport. The increased radial transport during ELMs is shown to be mainly due to these substructures. The observed fluctuation amplitudes and velocities of the substructures are found to be compatible with the model of radial transport by plasma 'blobs' and plasma potential gradients generated by the sheath boundary conditions. The transport due to turbulent radial E x B flows is compared during and between ELMs, and the observation of ELM substructures in the main plasma SOL is put into context with the observation of substructures on the target plates.