PERFORMANCE OF B4C-CONVERTED CARBON-FIBER COMPOSITES IN HIGH-POWER NEUTRAL BEAM HEATED DIVERTOR DISCHARGES IN JT-60U

B4C conversion-coated carbon fiber composites (B4C-converted CFC) have been installed on the outboard strike point of the divertor plate in JT-60U to form a complete toroidal ring. The plasma-surface interactions of the B4C-converted CFC have been investigated in high power divertor operation with neutral beam (NB) heating power of similar to 30 MW for 2 s. It is found that the carbon impurity is reduced without a significant increase of the boron impurity using a 100 mu m thick B4C layer with a B/C ratio of 2.7. The oxygen impurity is reduced significantly by the evaporation of a 300 mu m thick B4C layer with a B/C ratio of 3.8. A reduction in the detrimental effects on plasma-surface interactions with carbon-based materials has been demonstrated for more than 1000 shots by the use of surface-boronized carbon tiles in JT-60U.