| Graphite is one of the prime candidates for first-wall use in fusion devices. Its main advantages are its low Z and excellent thermomechanical properties, while its drawbacks are plasma-induced erosion and hydrogen retention. In addition to normal physical sputtering, graphite shows regimes of enhanced erosion at elevated temperatures. Exposure to hydrogen—the fusion fuel—in the temperature range 300–1000K leads to the formation of volatile hydrocarbons. Furthermore, since oxygen is often one of the main intrinsic impurities in the plasma of current fusion devices with carbon walls, the reaction of oxygen-containing ions with carbon materials also plays an important role in the complex process of plasma wall interaction. This thesis investigates the synergistic effect of H+ and O+ irradiation of graphite.; Experiments of simultaneous O+ and H+ irradiation on graphite showed that water is formed as the main synergistic product, along with reductions in the CO/O+, CO2/O+ and CH4/H+ yields. Similar to CH4 yields, water yields (H2O/O+) are temperature dependent with maximum yield occurring at ∼800K. Beam range separation experiments of O+ and H+ irradiations indicated that water is produced at the end of the O+ ion range. Furthermore, water formation was shown to be dependent on the mobile hydrogen concentration on internal surfaces, and independent of ion range separation. Reduction of CO and CO2 yields was found to depend on the reduction of oxygen supply due to water formation. CH4 yield reduction was found to be dependent on the incident oxygen, and the “methane break-up” mechanism was postulated and confirmed by Ne+-H+ → C experiments. Experiments of simultaneous O+ and H+ irradiation on boron-doped graphite showed that the methane yield reduction due to break-up may be more than compensated for by the methane yield due to the combined presence of oxygen and boron. The reduction of methane yields from the H+ → C/B reaction due to boron can also be partially nullified by the incident oxygen. Furthermore, O+-H + → C/B irradiations showed that water formation was reduced when compared to O+-H+ → C irradiations. These observations provided additional details in the CH4 and H 2O formation mechanisms. Finally, a semi-empirical mathematical model was formulated based on the emerging mechanisms. |
