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 ºC 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 hours; 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 form 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 hours exposure to O2. This behavior is significantly different from that observed for the oxidation of thinner DIII-D codeposit specimens which were still adhered to tile surfaces, and this is thought to be related to the low deuterium content (D/C ~ 0.03 – 0.04) of the flakes

Thermo-oxidation of laboratory-produced undoped and W-doped carbon films: A reaction product analysis

Thermo-oxidation is a technique where O2 gas is used to remove carbon co-deposits at elevated temperatures from plasma-facing materials of fusion devices. Following thermo-oxidation, the reaction products produced will need to be managed by a reactor's gas-handling system, including the tritium plant. Thus, it is important to know what molecular species are produced during oxidation. The reaction products of thermo-oxidation have previously been studied for undoped films indicating CO, CO2 and D2O as the reaction products. The present study directly compares the reaction product evolution of undoped films to that of films doped with tungsten.A glow discharge was used to produce sets of undoped and W-doped films on stainless steel (SS) foil substrates. X-ray photoelectron spectroscopy (XPS) found the W-doped films to contain an average 0.1 at.% W/(W+C). Laser thermal desorption spectroscopy (LTDS) was used to measure the areal D-concentration. Reaction products during oxidation were measured using a quadrupole mass spectrometer (QMS). Both the undoped and W-doped specimens were oxidized in 2Torr O2, at 350 °C for 4h, total. LTDS was then used to determine how much D had been removed during oxidation (93% and 32% of the D was removed after oxidation for the undoped and W-doped cases respectively). The reduced D loss for the W-doped specimens is attributed to a change in film structure due to the incorporation of W in the film. Oxidation reaction products for both undoped and W-doped films include CO, CO2, and D2O, with no finding of D2 or CD4 within experimental uncertainty. Particle accounting was performed for oxygen, deuterium and carbon atoms. Keywords: Plasma-surface interactions, Tritium, Carbon, Carbon co-deposition, Oxidation, Reaction product