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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

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.

This work investigates the problematic of hot spots induced by accelerated particle fluxes in tokamaks. It is shown that the polarization due to sheaths in the edge plasma in which an electron beam at a high level of energy is injected, can reach several hundreds volts and thus extend the deposition area. The notion of obstructed sheath is introduced and explains the acceleration of energy deposition by decreasing of the sheath potential. Then, a 2D fluid modeling of flux tubes in front of ICRF antennae allows us to calculate the rectified potentials taking into account RF polarization currents transverse to magnetic field lines. The 2D fluid code designed validates the analytical results which show that the DC rectified potential is 50% greater with polarization currents than without. Finally, the simultaneous application of an electron beam and a RF potential reveals that the potentials due to each phenomenon are additives when RF potential is much greater than beam polarization. The density depletion of polarized flux tubes in 2D PIC simulations is characterized but not yet explained., Ces travaux abordent la problématique des points chauds induits par des flux de particules accélérées dans les tokamaks. Il est montré que la polarisation due aux gaines du plasma de bord soumis à un faisceau électronique à haute énergie peut atteindre plusieurs centaines de volts et ainsi allonger la zone de dépôt. La notion de gaine entravée est introduite et explique l'accélération de ce dépôt par réduction du potentiel de gaine. Ensuite, une modélisation fluide 2D des tubes de flux devant les antennes ICRF a permis de calculer les potentiels rectifiés en tenant compte des courants de polarisation transverses aux lignes de champ magnétique. Le code fluide 2D mis au point valide les résultats analytiques qui montrent que ces courants de polarisation peuvent augmenter de 50% la valeur DC du potentiel rectifié par rapport aux modèles classiques sans courant. Enfin, l'application simultanée d'un faisceau et d'un potentiel RF révèle que les potentiels induits, propres à chaque phénomène, s'additionnent pour des potentiels RF bien plus grands que la polarisation due uniquement au faisceau. La déplétion de densité des tubes de flux polarisés dans les simulations PIC 2D est caractérisée mais non expliquée.