Effects of dust on plasma discharges during tokamaks start-up phase

Recent investigations have proved the existence of magnetic dust in tokamaks [1-3] Dust in tokamaks has been widely investigated [4], but magnetic dust can be mobilized in the vacuum vessel by the external magnetic field before and during the start-up stage of discharges. Recently in FTU [5] the presence of an estimated average dust density of 2 – 30 × 10-3 cm-3 before the start-up phase was documented. The impact on tokamaks operations of flying dust, during the plasma breakdown phase, could be summarized in essentially three phases: i) perturbing the breakdown phase of discharges; ii) affecting the current ramp-up phase; iii) terminating the discharge due to the dust vaporization. In this paper we focus our attention on the first two mechanisms. This investigation could be important in the perspective of using steel components and RAFM materials in ITER and future fusion power plants [6].