Arcing at B4C-covered limiters exposed to a SOL-plasma
Laux, M.
W7-X: Physics (PH), Max Planck Institute for Plasma Physics, Max Planck Society;
Schneider, W.
Tokamak Theory (TOK), Max Planck Institute for Plasma Physics, Max Planck Society;
Wienhold, P.
Plasma Diagnostics Group (HUB), Max Planck Institute for Plasma Physics, Max Planck Society;
Jüttner, B.
Balden, M.
Material Research (MF), Max Planck Institute for Plasma Physics, Max Planck Society;
Herrmann, A.
Experimental Plasma Physics 1 (E1), Max Planck Institute for Plasma Physics, Max Planck Society;
Hildebrandt, D.
Bolt, H.
Laux, M., Schneider, W., Wienhold, P., Jüttner, B., Huber, A., Balden, M., et al. (2003). Arcing at B4C-covered limiters exposed to a SOL-plasma. Journal of Nuclear Materials, 313-316, 62-66. doi:10.1016/S0022-3115(02)01333-8.
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.