BEHAVIOR OF BORON-CARBIDE MATERIALS IN TEXTOR AND UNDER ELECTRON-BEAM IRRADIATION

Boron-carbon materials have been exposed to high heat fluxes in the TEXTOR tokamak and in the JUDITH electron beam facility. The materials were a B4C Coat Mix bulk material, a conversion-type B4C coating, and a plasma-sprayed B4C coating on graphite EK98. The boron-carbide Coat Mix limiter showed, due to the low thermal conductivity of 11 W/mK, surface melting already at a heat flux of about 1.5 kW/cm(2) after 0.4 s. Both in the tokamak experiments and the electron beam tests the surface temperature stayed close to the melting temperature under continued heating. The conversion coated limiter and specimens had a very irregular coating thickness and surface structure. At heat fluxes of 2.6 kW/cm(2) melting of the coating was reached after 0.5 s. The thermal behaviour and evaporation of impurities into the plasma were comparable to the Coat Mix limiter but shifted to higher heat fluxes. Both limiters fractured under the applied heat loads. The plasma sprayed coating showed early melting of the coating and agglomeration of the molten carbide to droplets on the graphite.