Mixed and high-Z plasma-facing materials in TEXTOR

Abstract

The main concerns with graphite in a fusion device are associated with its large chemical erosion by hydrogen isotopes and oxygen and by the formation of hydrogen-rich carbon layers, which in the case of tritium, results in a high tritium inventory. Laboratory experiments show a reduced erosion yield of doped carbon materials. Therefore, test limiter experiments have been performed in TEXTOR to investigate alternative materials under plasma exposure. The erosion and redeposition results of some mixed materials, such as amorphous Si/C-films ("siliconization"), Si-doped CFC (NS31), B4C-coated copper, are reviewed in comparison to carbon from test limiter investigations in TEXTOR discharges. In the first two cases, the C and O flux on the toroidal limiter was reduced and no SiDx formation at a limiter temperature of 550 K was observed. The surface temperature of the B4C coated copper limiter reached 1200 K for a power load of 4MW/m2. No severe delamination of the B4C coating occurred after 80 exposures. For a direct comparison twin limiter experiments on C/W and W/Ta were performed. The Dγ line of the re-emitted D flux was about 25% larger on C than on W. The re-deposition of C on initially pure W limiter was modeled by the ERO-TEXTOR code. The build-up of a net-deposition zone and a net-erosion zone on the W limiter was in agreement with the experimental observations.