Dust remobilization experiments on the COMPASS tokamak

Dust transport is among important issues for ITER and DEMO, where material erosion will be significant. One of possible mechanisms how material is eroded from plasma facing surfaces is the remobilization of dust particles linked to their lifetime there and to the formation of dust accumulation sites. On the COMPASS tokamak, dust remobilization experiments have been performed using a tungsten surface with well-defined seeded dust particles exposed to L-mode and ELMy H-mode discharges as well as to a disruption. On small flat tungsten blocks, a dust of the particle size up to 25 mm was deposited by a low speed gas gun. Dust particles were prepared as sub-millimetre spots positioned on the top and on the side surfaces of the block. Dust particle positions for all prepared spots were mapped using a scanning electron microscope (SEM). Then, the samples were mounted on a manipulator allowing insertion to a fixed position close to the divertor region and were exposed to a discharge of the COMPASS tokamak. After the removal of the blocks from the vacuum vessel, positions of the seeded tungsten dust particles were again mapped by SEM and compared with the original ones, deducing movements of individual particles as well as a pattern-like behaviour. Remobilization of dust particles were monitored by a fast visible light camera with a pixel resolution of 0.3 mm at 15-40 kfps. Heat flux conditions were derived from measurements of the divertor probes (0.02-0.5 MW/m22 in L-mode and inter-ELM periods of H-mode) and Langmuir and ball-pen probes (<10 MW/m22 during ELMs). The contribution shows details on dust remobilization observations (direct one by the camera and indirect one via SEM comparisons), methods and instruments used for the experiment, and also introduces possible physical mechanisms responsible for the observed collective grain transport (unipolar arcs, eddy currents).