Abstract: Background: Due to their high melting point, high thermal conductivity, low sputtering yield, tungsten (W) and molybdenum (Mo) have been regarded as the potential candidates for plasma facing materials in fusion reactors. Purpose: This study aims to understand the irradiation damage behavior of W and Mo for the development of key fusion reactors materials. Methods: Polycrystalline W and Mo materials have been irradiated by 120-eV He ions with the irradiation temperature at 873 K. In situ analysis of these samples was carried out using nano-identation masked technique and non-destructive conductive atomic force microscopy (CAFM). The morphology, microscopic evolution and distribution of defects of these samples before and after irradiation are compared. Results: Analysis indicated that a large number of nanometer-sized helium bubbles were formed near the sample surface. Over-high internal pressure of nanometer-sized helium bubbles may result in forming protuberances of irradiated W and Mo. The section analysis of nano-identation showed that an obvious etching effect was founded on Mo materials. Conclusion: This work is of great significance for further understanding of the low-energy ion irradiation damage on plasma facing materials.

Key words: Irradiation damage, Tungsten, Molybdenum, Nano-identation, CAFM