Hydrocarbon yield of pyrolytic graphite due to low-energy hydrogen irradiation

grantor: University of TorontoRecent developments with gaseous divertors in tokamaks have led to prospects of less energetic ion bombardment (10's of eV) in the divertor region. Previous low-energy results indicate that an H-ion induced chemical erosion process is occurring which is distinct from that observed for high impact energies (300 eV-3 keV). Furthermore, there is some discrepancy in the literature as to the magnitude of the isotopic effect due to H$\sp+$ and D$\sp+$ impact at these low energies. In the present experiments, mass spectrometry in the residual gas is used to perform a systematic study of hydrocarbon formation rates as a function of pyrolytic graphite temperature (300-1000 K) and ion energy (10-200 eV) for mass-analysed H$\sp+$ and D$\sp+$ beams (10$\sp{18}$ ions/m$\sp2$s). This provides a unique opportunity to also investigate isotopic effects. The results can be used to optimize operating conditions in a tokamak fusion reactor. The present results indicate that, as the ion impact energy is reduced, there is a reduction in the maximum chemical yield (Y$\sb{\rm m}$) and that broadening of the temperature dependence for hydrocarbon formation does, in fact, lead to significant erosion for low-energy impact at room temperature. These room-temperature methane and total chemical yields display maxima at about 50 eV and decrease as the ion energy is further reduced. Within experimental errors the isotopic effect on the chemical yield was not significant and was generally less than a factor of 1.7. Furthermore, there was strong evidence for changes in the dominating erosion mechanism(s) as the ion energy was reduced below 100 eV. A semi-empirical model has been developed, based on previously identified atomistic processes, with some modifications and additional processes incorporated. In general, this model agrees very well with the present results for methane production due to H$\sp+$ and D$\sp+$ impact. The additional processes, which account for the kinetic effects of the energetic particles, can be used to explain many of the observed erosion features.Ph.D