Helium bubble formation in RAFM steel: Role of helium concentration under constant peak displacement damage dose

Reduced-activation ferritic/martensitic (RAFM) steel has been regarded as a primary candidate structural material for fusion reactors due to its excellent mechanical properties and good resistance to irradiation induced swelling. However, significant discrepancies exist regarding the influence of helium on the formation of irradiation defects in RAFM steel. In this study, He⁺ irradiation experiments at 18 keV, 50 keV and 100 keV were conducted at 450 °C to investigate the effects of varying helium concentrations on defect formation under the same peak displacement damage dose. By adjusting the irradiation fluence, the irradiations with different He⁺ energies were controlled to produce identical peak displacement damage doses but different peak helium concentrations. The results showed that compared to 50 keV He⁺ irradiation, 18 keV He⁺ irradiation, which introduced a higher helium concentration, led to larger helium bubble size, higher bubble density and greater swelling. This indicates that under the same peak displacement damage dose, a higher helium concentration significantly promotes the nucleation and growth of helium bubbles, thereby enhancing the swelling of the material. It is noteworthy that, despite the lowest helium concentration, 100 keV He⁺ irradiation resulted in the largest helium bubble size, highest bubble density and greatest swelling among the three irradiation energies. This may be attributed to the higher primary knock-on atom (PKA) energy associated with the 100 keV He⁺ irradiation, which can promote the nucleation and growth of irradiation induced defects. Thus, the promoting effect of higher PKA energy on defect formation appears to dominate in this case.