ELM electromagnetic fine structure in tokamak discharges

The presence of filamentary structures widely characterizes the edge region of fusion devices, independently from their magnetic configuration. These structures are generally revealed by pressure peak locally emerging on the cross-field plane from the plasma background and the name filaments is due to their extended size along the magnetic field line. Filaments emerging from turbulence background share these general features with Edge Localized Modes (ELMs), which are responsible of a large fraction of transport towards the plasma wall and divertor plates. The filament electromagnetic features were experimentally studied in the recent years due to the relevance of this aspect on the transport, considering as an example the transition between closed and open magnetic field line topology, i.e X-points and advanced divertor magnetic configurations, up to the possible magnetic field line bending effect in case of enough high current associated to ELM filaments, which could enhance their interaction with the first wall. A further aspect to be considered is the enhancement of the electromagnetic effect is expected at high beta plasmas, in particular an increasing current density characterizing turbulent was experimentally observed to characterize turbulent filaments as the beta increases. Local magnetic fluctuations were experimentally detected in presence of ELMs in different tokamak such as MAST, ASDEX-U and EAST and DII-D and JET. In this contribution a direct measurement of current density associated to ELM will be presented in the two tokamak experiments COMPASS and RFX-mod. To this end specifically designed insertable probe heads were used, properly equipped with 2D arrays of electrostatic and magnetic sensors. The diagnostic set is completed in both experiments with arrays of probes located on the divertor or first wall. The plasma analyzed are based on SN shaped tokamak discharges where the H-mode was achieved in ohmic or NBI heated plasmas, in COMPASS, or edge biasing stimulated in RFX-mod. The latter representing a newly explored scenario with this technique, where ELM electromagnetic composite filamentary structures are observed. They are characterized by clear vorticity and parallel current density patterns. On the other side, recent preliminary analysis revealed the presence of parallel current associated to the COMPASS ELMs and evidenced their fragmented structure the Scrape-Off Layer (SOL) region. The comparative analysis of the experimental results will be based on the same method applied on the two devices, aiming to the identification of insight on possible common physics.