Bombardment of graphite with hydrogen isotopes: A model for the energy dependence of the chemical sputtering yield

Carbon materials inside magnetic-confinement nuclear fusion devices are eroded by hydrogen ions and atoms. Even at room temperature, where thermal chemical erosion is negligible, erosion does not cease below the energy threshold for physical sputtering. The effect, which requires both chemical reactivity and hyperthermal energy of the eroding species, is known as 'chemical sputtering'. Based on a concept of the underlying microscopic mechanism we have previously developed a model for the energy-dependent yield of chemical sputtering due to simultaneous impact of thermal atomic hydrogen and energetic noble gas ions. In the present article the model is adapted to the case of pure hydrogen ion bombardment of graphite yielding good quantitative and qualitative agreement with published data. Especially, the large isotope effect below 100 eV comparing deuterium and hydrogen ion bombardment is well reproduced and its origin is discussed. Additionally, the chemical sputtering yield is calculated for tritium ions.