Modification of arc emitted W particles in a model scrape-off layer plasma

In fusion machines (like e.g. ASDEX Upgrade and JET) equipped with W-coated plasma facing components (PFCs) arc tracks are observed post-mortem and W particles are identified as a component of the dust. Results from laboratory arcing of W-coated PFC-material concerning the size, velocity and direction of macro-particles are combined with model calculations of the heating, cooling and evaporation of W particles flying through a simplified scrape-off layer (SOL) plasma to assess the role of arc-produced particles as a source of W impurities. W atoms or ions eroded from a macro-particle flying across the SOL plasma are not subject to prompt re-deposition onto a PFC. Therefore, macro-particles constitute an essential source of W in the SOL and may even result in an impurity input into the region of confined plasma if the particle succeeds in pathing the SOL and crosses the last closed flux surface. The main result of the modelling is that a W particle from the large-size end of the distribution obtained in laboratory arcing and having a typical velocity (also known from laboratory experiment) is able to pass a SOL of 10 cm thickness filled with a realistic plasma and, finally, inject a certain amount of W-material into the confined plasma region.