Oxygen gettering on graphite in the SOL of TEXTOR after wall conditioning with boron

Abstract

Starting with carbonized walls as a reference condition, the TEXTOR vessel has first been flushed with reactive diborane gas (B 2H 6), later the inner plasma facing surface was boronized with a-C/B:H (∼ 100 nm). After the introduction of the boron into the machine a drastic decrease of the oxygen contamination of the plasma could be observed. Surprisingly, the amount of oxygen which has been collected on graphite targets in the scrape-off layer (SOL) showed just the opposite trend : it increased. This behaviour is explained with gettering of the oxygen ions by the simultaneously collected boron. The oxygen forms stable oxides with the boron due to energetic ion deposition keeping the plasma contamination low. The observations allow also the quantification of two other contributing terms : sticking of oxygen ions and the oxygen absorption by uptake from air. For the ex situ analysis, the graphite targets were studied by ion beam analysis (O, B, D, metals). XPS investigations showed the bonding state between oxygen and boron.