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15th International Conference on Plasma Surface Interactions in Controlled Fusion Devices (PSI 15), Gifu, 2002-05-26 to 2002-05-31
Abstract:
Plasma sprayed B4C-layers considered as wall coatings for the W7X stellarator have been studied
during and after exposure to TEXTOR and after arcing experiments in vacuum. Arcing through the
B₄C layer occurred favoured by high power fluxes and not restricted to less stable phases. But this
arcing implies an especially noisy scrape-off layer (SOL). Instead of moving retrograde in the
external magnetic field, the arc spot on the B₄C-layer sticks to the same location for its whole
lifetime. Consequently, the arc erodes the entire B₄C layer, finally burning down to the Cu substrate.
In the neighbourhood of craters the surface contains Cu originating from those craters. This material,
hauled to the surface by the arc, is subject to subsequent erosion, transport, and redeposition by the
SOL-plasma. The behaviour of arcs on B₄C is most probably caused by the peculiar temperature
dependences of the electrical and heat conductivity of B₄C.
