He+ implantation into tungsten at low kinetic energy and low fluxes using a plasma source
Résumé
Tungsten was selected as a plasma facing material for the ITER reactor. It will be exposed to severe plasma conditions such as particles bombardment. High fluxes of light ions on tungsten surface are known to generate defects in the crystal. Helium ion flux interaction with a tungsten surface leads to vacancy defects generation and eventually larger defects like helium/vacancy complexes. These defects induce a deterioration of the mechanical properties of the material and impact the yield and safety of the reactor. The objective of this study is to observe the first steps of the implantation process at low ion flux and kinetic energy, when no direct W atom displacement is expected.
For this study an ICP-RF plasma source was developed at the GREMI to perform implantations at low ions fluxes, i.e. in a low electron density plasma. The ion energy is determined by the potential difference between the plasma and the biased substrate, and the gas pressure. Various plasma diagnostics were used, in some cases modified, to qualify the plasma parameters at the substrate location and, thus, to control the implantation conditions. Langmuir Probe measurements were carried out to determine the electron density and temperature, and the floating and plasma potentials. The ion energy distribution was evaluated by means of a grid system (RFA type) specially designed for measurements in low density plasma and at high accelerating voltage. A homemade energy flux diagnostic was used to measure the energy transferred to the tungsten surface during the implantation experiments.
Surface characterizations with Nuclear Reaction Analysis technique confirmed low He retention rates previously reported by P.E. Lhuillier. Positron Annihilation Spectroscopy used to investigate vacancy formation, evidenced the filling of already existing vacancies by He at low implanted dose, and the formation of new vacancies in the W lattice at the highest implanted doses. Thermal Desorption Spectroscopy was used to evidence various He trapping sites (possible formation of Vacancy/He complexes). Molecular Dynamics modeling of the implantation process was carried out and comparison with experimental results give interesting insight into the mechanisms involved in the modification of W under He+ implantation.