HELIUM INTERACTION WITH TUNGSTEN SURFACE AT LOW INCIDENT ENERGY AND LOW ION FLUXES
Résumé
Tungsten was selected as a potential plasma facing material for the new experimental nuclear fusion reactor. This metal will be exposed to severe plasma conditions and in particular, particles bombardment. High fluxes of light ions (He, D, T) on tungsten surface generate defects in the crystal. Those are vacancies, implanted ions or even larger defects as gas bubbles or vacancy clusters. These damages can lead to a deterioration of the mechanical properties of tungsten and impact the yield and safety of the reactor. The objective of this study is to realize the insertion of Helium ions in a tungsten surface by plasma process at low energy and low ion flux in order to observe the first steps of the formation of defects without any sputtering of the substrate. For this study an ICP-RF plasma source was developed at the GREMI laboratory in Orleans, France. Langmuir probe and energy flux diagnostic are used to determine the Helium plasma parameters and the global energy transfer from the plasma to the surface. Then tungsten samples, at temperature ranging from 80 to 875K, are immersed in the plasma ignited in 3He gas. Ion fluxes varying from 1011 to 1015 at.cm-2.s-1 can be obtained by the ICP-RF source. From measurements of plasma parameters (as plasma and floating potentials, electron temperature and density etc.), the mean free path of the helium ions was evaluated. Implantation conditions were chosen to ensure that the sheath is non-collisional. The kinetic energy of the ions is thus mainly determined by the substrate bias voltage and can be varied between 50 eV and 500 eV. At low substrate bias (below -100V), it appears that the main part of the global energy flux is delivered to the surface by He+ ions. This shows that the incident He+ ions are the major interacting species. No heating of the ICP source, that would induce emission of IR radiations, was evidenced. Nuclear reaction analysis measurements showed low He retention rates (below 1021 at/m2), similar to the ones presented by P.E. Lhuillier in a previous study [1]. From Scanning Electron Microscopy observations the evolution of the material surface characteristics after implantation and annealing was followed. Positron Annihilation Spectroscopy and Thermal Desorption Spectroscopy will be used to characterize respectively the vacancy defects induced in the tungsten surface and determine amount of helium released after annealing. References [1] P.E. Lhuillier et al, J. Nuc. Mater 433 (2013) 305-313