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International audience; Post-mortem analyses suggest that arcs in contact with metallic walls are the main cause of impurities and dust formation in tokamaks [1,2]. Such dust and impurities represent important operational and safety issues for next-step fusion devices like ITER and DEMO. If dipolar arcs, which occur in between adjacent Plasma Facing Components (PFCs), are well known and can in most of the cases be avoided by adapting the design of the PFCs, it is not the case for unipolar arcs taking place in between PFCs and the plasma. On addition to dust production, unipolar arcs also damage mirrors required by many diagnostics used for studying the plasmas as well as for ensuring a safe operation of the fusion device. Several models for explaining the triggering of unipolar arcs have been developed, but these models are based on assumptions which are difficult to assess experimentally, mainly due to the spatial and temporal scales involved (a few µm to more...

International audience; IShTAR (Ion Sheath Test ARrangement) is a linear magnetic facility developed by and operated at the Max-Planck-Institut für Plasmaphysik (IPP) in Garching in cooperation with several universities and institutes. It is equipped with a helicon plasma source and an ICRF (Ion Cyclotron Resonant Frequency) single strap antenna. It is used to investigate the interactions between the launched wave and a plasma that is representative (plasma density, temperature and wave propagation) for a tokamak edge.Whereas the device was originally meant to develop the tools to measure the electric field produced by the ICRF wave inside the sheath on the antenna limiters in order to provide data to check and benchmark the RF sheath models, it can actually provide major contributions in many other areas related to plasma production and RF-plasma interactions. Just to name a few:- helicon physics: the helicon has one of the largest diameter in the world and will soon be equipped with a major power upgrade (up to 750 kW generator power between 3.9 MHz and 26.1 MHz);- refinement of RF fields measurement methods: by comparing the modelling of the electromagnetic field distribution (including eigenmodes and superposition of the fields of helicon and RF antenna) inside the vessel with the measurements;- RF plasma production and modelling: the device was used to test plasma production for RF wall cleaning, and to investigate for which conditions the plasma is initiated inside the antenna box. It is essential to avoid such phenomenon in high power ICRF system on tokamak, but IShTAR provides the liberty for detailed investigations of the conditions under which these to-be-avoided phenomena occur.IShTAR is further designed as an experimental platform dedicated to international co-operations: to improve remote access for participants, it is equipped with an intranet to manage the operations and the documentation. The processing system has sharable notebooks and remote access to the discharge database is possible. more...