Morphology Classification and Video Tracking of Dust Particles in ASDEX Upgrade

Full text: Due to the relevance of dust particles in respect to safety and plasma performance, the morphological properties of dust particles as well as their generation, mobilization, transport and deposition pattern have to be investigated. The main investigations at ASDEX Upgrade (AUG) related to dust are in-situ detection of plasma wall interaction areas and migration of strongly radiating particles during plasma operation using infrared, standard and fast cameras and post-mortem analysis of collected particles using optical light microscopy and automated analytical scanning electron microscopy (SEM with energy dispersive X-ray spectrometry (EDX)) assisted by cross-sectioning with a focused ion beam. The automated EDX analysis of particles collected since the transition of AUG to a full tungsten plasma-facing wall allows to obtain statistically data containing thousands of particles down to particles size of ∼ 100 nm and to classify the particles. Even classes of particles with low abundance but distinct configuration (e.g. spherical particles from diagnostic mirrors) can be separated from morphologically comparable particle fractions, e.g. tungsten droplets produced by arcing. Furthermore, particles below 5 μm are dominantly spherically and show W as their main constituent, while above 5 μm complex agglomerates were found dominated by C and B matrix with embedded W spheres. Sampling method dependent collection efficiencies were considered as an important influencing factor for data interpretation. The in-situ registered event trajectories are automatically classified into stationary hot spots, single-frame events (e.g. by neutrons) and real dust particle fly-bys with velocities in the range of 10 - 100 m/s. A considerable decrease of detected events with progressing plasma operation was observed. After ∼ 1800 s operation, even deliberately induced disruptions do not induce more dust events. This indicates an absence of mobilizable dust at that time of operational period. In addition to the above described investigations, the over-all strategy of investigations at AUG related to dust will be presented briefly and the rule of arcs on dust production will be discussed. (author)