Plasma surface interaction on the surface of tungsten divertor tiles in LHD

Four graphite divertor tiles coated with a 100 μm thick layer of tungsten by using a vacuum plasma spray (VPS) method were installed in the LHD (Large Helical Device) in the 2008FY campaign (a total of 1700 pieces of graphite tiles are used in LHD). Tungsten emission of the plasma and the reliability of the coated tungsten layer and graphite substrate have been investigated by using real-time spectroscopic analysis and post-mortem surface analysis, respectively. The tungsten coated tiles could adequately withstand for a single experimental campaign, the heat and particle loads from the divertor plasma which were highly localized at the strike points. Tungsten accumulation in the core plasma could not be seen for the operation schemes in this campaign, and sputtered tungsten impurities did not have any influence on plasma discharges.