Post-mortem and in-situ investigations of magnetic dust in ASDEX Upgrade

Tokamak dust mobilization has been nowadays recognized as an important aspect of dust transport and survivability [1]. Recent investigations of dust collected in various tokamaks have provided evidence of the presence of a significant fraction of ferro-magnetic and strongly paramagnetic dust (TEXTOR [2], FTU [3,4,5], Alcator C-Mod [4,5], COMPASS [4,5], and DIII-D [4,5], up to 30wt% depending on machine conditions and operational time). Magnetic particulates, in stark contrast to non-magnetic ones, can be mobilized during, or even before, discharge start-up [6]. To date not enough attention has been paid to pre-plasma remobilization of magnetic dust. In the perspective of the use of stainless steel for the ITER diagnostic first wall [6] and of RAFM steel in future fusion plants [7], a fraction of magnetic dust could be an issue for the breakdown phase in these devices. In this work, we present a combined on-line and off-line investigation of magnetic dust in ASDEX Upgrade by means of Mie-scattering diagnostics, IR-camera observations, and collection activities. The magnetic dust fraction has been characterized by optical microscope and x-ray diffraction techniques. Post mortem collection revealed similar chemical composition and morphological features compared to magnetic dust from other tokamaks, but the overall amount was much smaller (few wt%). On the other hand, the on-line investigation has excluded the presence of fly-by dust across the beginning of plasma discharges. The experimental observations are discussed in light of the small amount of magnetic dust generated in ASDEX Upgrade despite the presence of ferromagnetic P92 still covering the central column of the vessel [8]. The detailed analysis of the temporal evolution of the magnetic field suggests that magnetic dust could still be mobilized, but well before the beginning of the plasma discharges.