Post-mortem analyses of limiter tiles of FTU

During the 2013 shut-down of FTU, some tiles of the toroidal limiter were collected. In particular two tiles, located in front of the poloidal limiter, were discovered to be covered by craters surrounded of deposited material (haloed craters). The tiles have been morphologically and chemical analyzed by means of electron microscope and surface analyses techniques showing that the halos were formed, upon impact with Mo dust, inside a co-deposited region of molybdenum oxide MoO3. Based on the results of the analysis and on the information available in literature it seems that the haloed craters are due to the impact of dust ejected by the poloidal limiter surface upon a disruption and hitting the closest available surface. In order to support the above rationale we have conducted an experimental campaign simulating the dust projectile, by means of a light gas gun, reproducing the impacts of Mo grains, with different size, on Mo targets. The results suggested that the responsible of the haloed crated are those particles with size within 60-100?m diameter, travelling at, about, 500m/s.