Hydrogen Isotopic Effects on the Erosion of Carbon

Both physical sputtering and chemical erosion take place in tokamaks. Physical sputtering occurs for all elements for incident particle energies greater than an energy threshold. For carbon targets the threshold difference for the three hydrogen isotopes is relatively small. In the energy range of 100 to 3000 eV, the physical sputtering yields are similar for D and T, and the H yields are lower by about a factor of 2 to 3. Chemical erosion studies of graphite due to H⁺ and D⁺ impact also show evidence of some isotopic effect - with the deuterium yield being larger. The isotopic yield ratios (D-yield/H-yield) observed in almost all of the chemical erosion measurements, including ion beams, laboratory plasma devices, and tokamaks, lie between 1 and 2. The recently measured chemical erosion yields due to tritium ions also fall in this range. (The notable exceptions are the mass-loss studies at the Max-Planck Institut für Plasmaphysik in Garching, Germany, where for energies <100 eV, the isotopic yield ratio was seen to increase from 4 to 7 with decreasing energy.) A nominal value of 1.5 ± 0.5 is suggested as the most appropriate value for the D/H yield ratio. This is fully consistent with the square root of mass dependence proposed for the modeling of chemical erosion.