Electromagnetic filaments detected in the COMPASS Scrape Off Layer

Filamentary structures have been observed in all magnetic configurations with very similar features despite the difference in the magnetic geometry: theory and experiments suggest they exhibit a radial convective motion across the SOL, and the interest in blob dynamics is further motivated by their interaction with first wall and divertor. Despite their possible different generation mechanisms, turbulent structures and Edge Localized Mode (ELM) filaments share some common physical features, as the localization in the cross-field plane and the associated parallel current, with a convective radial velocity component somehow related to their dimension. The electromagnetic (EM) effects on filament structures deserve particular interest, among the others for the implication they could have for ELM, related for instance to their dynamics in the transition region between closed and open field lines or to the possibility, at high beta regimes, of causing line bending which could enhance the interaction of blobs with the first wall. The presence of ELMs and inter-ELM EM filaments was revealed in the SOL of COMPASS tokamak, where a new probe head was recently developed and commissioned [1, 2].The diagnostic, based on the U-probe concept [3], allows the simultaneous measurements of electrostatic and magnetic fluctuations with high time resolution suitable for the identification of EM features of filaments, providing in particular the direct measurement of the current density associated to filaments. The probe head was inserted at different radial positions in the SOL of D-shaped diverted discharges. The COMPASS experiment was operated in these discharges in ohmic and NBI induced H-mode, with the clear presence of different type of ELMs. In addition, the complete diagnostic setup used allowed monitoring filamentary structures in different toroidal positions along the magnetic field line.