Effects of dust on start-up phase of tokamaks discharges

Dust in tokamaks is recognized to be an issue for, both, safety and operations [1, 2] issues. Recent investigations have proved the existence of magnetic dust in tokamaks [3-8] despite the fact that no magnetic material had ever been introduced in tokamaks. In fact this magnetic dust is the result of a change of the crystalline phase, from the austenite to the ferrite ones, of dust coming from nonmagnetic stainless steel or inconel plasma-facing components and/or diagnostics, change caused by the extreme harsh environment present in tokamaks during operations [6]. Although the presence of dust in tokamaks has been widely investigated [9, 10], magnetic dust, on the contrary of non-magnetic one, is prone to be mobilized by the external magnetic and transformer ramp-up field before the beginning of plasma discharges, leading to the presence of mobilized dust in the vacuum vessel during the start-up stage of discharges. The evidence of the presence of dust in vacuum vessel during start-up has recently been documented in FTU [11] where an average dust density of 2 - 30 × 10-3 cm-3 before the start-up phase was estimated. On the presence of magnetic dust and its possible impact on the discharge start-up phase in tokamaks have already been speculated in the past [3, 12]. In this report, we present a model describing the gas breakdown and plasma current ramp-up phases in presence of dust. This investigation could also be important in the perspective of using steel components and RAFM materials in ITER [13] and future fusion power plants [14, 15]. The impact on tokamaks operations of flying dust in vacuum chamber, across the plasma volume during start-up phase, could be summarized in essentially three phases, namely: i) perturbing the breakdown phase of discharges; ii) perturbing the current ramp-up phase; iii) terminating the discharge upon the full plasma is established due to the dust vaporization. In this report we focus our attention on the first two mentioned mechanisms