Modelling of W dust dynamics related to radiating plasmoid formation

Transient Impurity Events (TIEs) leading to intense radiation spikes are often observed in tokamaks with tungsten (W) lined (e.g. JET with the ITER-Like Wall, ILW), namely after disruptions [1-2]. In JET-ILW, TIEs with a significant increase of radiated power (1.5 MW on average) could be associated to polluting W ions resulting from dust ablation [2]. The interpretation of these events brings to key questions concerning the total amount of dust mobilised from the Plasma Facing Components (PFCs) and the tokamak vessel, and the contribution of the dust ablated mass to the contamination of the plasma. Here we present some applications of the dust trajectory code DUSTTRACK [3-4] to the qualitative and quantitative description of the mobilisation and fate of bunches of isolated spherical W dust particles, subject to dynamic interactions with the Scrape-Off Layer (SOL) plasma and the tokamak first-wall. Particular emphasis is placed on the thermodynamic phase transformations of W dust particles which can bring to their full ablation. The neutral ablated cloud thus formed, interacting with the surrounding plasma, ionises leading to a high dense and cold W plasma, plasmoid in the following, probably responsible of TIEs. Inspired by researches on the injection of fuel pellets in tokamaks [5-6], an MHD model has been developed to study the time evolution of the shape of the radiating W plasmoid and it is here described.