Effect of Helium Concentration on the Structural and Mechanical Degradation of Tungsten in High-Temperature Plasma

This paper presents a study of the structural and mechanical degradation of tungsten under steady-state mixed hydrogen–helium plasma (He/H2). The experiments were carried out on the KAZ-PSI linear plasma simulator at a surface temperature of 1100 °C, while the helium fraction in the mixture was varied from 5% to 50%. Changes in surface morphology, roughness, phase composition, micromechanical response, and gas retention were analyzed using profilometry, scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS), X-ray diffraction (XRD), nanoindentation, and thermal desorption spectroscopy (TDS). The results show that increasing the helium fraction promotes the formation of a porous, defect-rich near-surface layer and modifies the gas-trapping behavior of tungsten. The surface roughness increases moderately from 0.031 μm for the initial polished state to 0.065 μm after exposure to a 50% He/50% H2 plasma. EDS and XRD confirm that the observed degradation is not associated with detectable oxidation, carburization, or the formation of secondary crystalline phases. The TDS results indicate that helium-related vacancy complexes and gas-filled pores act as deep trapping sites for hydrogen. Therefore, the helium-modified near-surface layer should be considered as a trapping barrier that localizes hydrogen in the radiation-damaged layer rather than as a quantitatively proven diffusion barrier blocking hydrogen penetration into the bulk.