Thermo-oxidation of tokamak carbon dust

The oxidation of dust and flakes collected from the DIII-D tokamak, and various commercial dust specimens, has been measured at 350 °C and 2.0 kPa O2 pressure. Following an initial small mass loss, most of the commercial dust specimens showed very little effect due to O2 exposure. Similarly, dust collected from underneath DIII-D tiles, which is thought to comprise largely Grafoil™ particulates, also showed little susceptibility to oxidation at this temperature. However, oxidation of the dust collected from tile surfaces has led to ∼18% mass loss after 8 h; thereafter, little change in mass was observed. This suggests that the surface dust includes some components of different composition and/or structure – possibly fragments of codeposited layers. The oxidation of codeposit flakes scraped from DIII-D upper divertor tiles showed an initial 25% loss in mass due to heating in vacuum, and the gradual loss of 30–38% mass during the subsequent 24 h exposure to O2. The oxidation of these flakes was much slower than that observed for the oxidation of thinner DIII-D codeposit specimens which were still adhered to tile surfaces. This is thought to be related to structural differences.