Dynamical calculation of low energy hydrogen reflection

A current critical question in plasma/first wall interactions is that of low energy (< 100 eV) hydrogen reflection. Experiments in this energy regime are extremely difficult. Previous to this work, calculations have not been performed with an interaction potential of sufficient quality to simulate the chemical interactions which may dominate reflection at low energies. The Embedded Atom Method (EAM) [1] has recently been developed to simulate such chemical interactions with a free surface. Using EAM the reflection coefficient for hydrogen on a clean (100) nickel surface has been calculated as a function of incident energy from 0.1 to 100 eV. At the high end of the range the reflection coefficient is similar to that predicted by TRIM, a binary collision model, but below 3 eV, EAM predicts considerably lower reflection than does TRIM. Additional calculations have been performed to investigate the effects of crystal orientation and angle of incidence. These results will have a significant effect on the predictions made by neutral gas codes such as DEGAS [2] on the modelling of pump limiters and divertor ducts.