Pre-plasma mobilization of magnetic dust and possible interference with tokamak operations

Recent investigations on dust collected in tokamaks (TEXTOR [1], Globus-M [2], FTU [3]) confirmed the presence of an unexpected significant fraction of ferromagnetic dust (?30 wt% in FTU, 15 wt% in TEXTOR, 10% in Globus-M). Magnetic dust particles could be mobilized during, or even before, the discharge start-up phase, preventing a positive evolution or even inhibiting a plasma discharge. To correctly describe the motion of the mobilized particles, the time dependent magnetic dipole force must be included in the physical model of dust tracking codes. Moreover, especially in the perspective of the use of stainless steel for the ITER diagnostic first wall [4] and RAFM steel in future fusion plants [5], this fraction of magnetic dust could be an issue. In this work we present some experimental results, obtained during the last FTU campaign, proving the presence of mobilized dust before and during the discharge start-up phase.