Micro/nano structural analysis of mixed-material deposition layer formed on the first walls of large helical device

Nano-geological diagnosis has been applied to the deposition layer formed on the Si specimens installed at the first wall surface during 2007FY Large Helical Device (LHD) experiment. Cross-sectional views of the deposition layers with nanometer-resolution were successfully observed by using focused ion beam fabrication technique and transmission electron microscopy observations. On the specimen located close to the divertor plates, deposition layer with ∼800 nm thick was observed, and it has very fine and stratified layer structures in nanometer-level. Thickness of each layer is so various from 5 to 100 nm, and majority of the composition elements such as Fe, Cr, Ni, O and C were continuously changed towards the depth direction from the top surface. Characteristics of each layer show the operational history in the LHD and the mechanism of dusts formation by flaking of the layer. This work provides the completely new knowledge about the deposition layer in fusion devices.