Current density features associated to Type-I and Type-III ELMs in COMPASS tokamak

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, 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 recent preliminary analysis revealed the presence of parallel current in the COMPASS ELMs and evidenced their fragmented structure [1]. Aim of this contribution is to provide a direct experimental measurement of the current density associated to different type of filaments, such as type-I and type-III ELMs including also the inter-ELM filamentary structures in the Scrape-Off Layer (SOL) region of the COMPASS tokamak. Measurements were performed in D-shaped plasmas. Discharges were performed in both ohmic and NBI heated plasmas, achieving ELMy H-mode regimes characterized by different type of ELMs. The diagnostic equipment available allows a detailed evaluation of ELM following their development along the same magnetic field line. This study exploits in particular poloidal arrays of Langmuir probes and Ball Pen Probes [2, 3] in the divertor region, intercepting the same magnetic field line of a suitably inserted probe head, the Filamentary-probe [4]. This diagnostics provides the direct measurement of the current density associated to the filaments intercepting the probe, simultaneously to the other quantities, as their associated density, temperature and electric field fluctuations. Statistical evaluation, based on advanced statistical techniques, is applied to identify the most representative behavior of each filament type. A scan of the relative distance of the probe insertion is used to evaluate their radial evolution within the SOL region. Average properties of ELM and inter-ELM filaments will be investigated as comparing ohmic and NBI heated plasmas. The plasma density effect, which is expected to affect the filaments behavior [5], will be also investigated.