Effect of high-flux, low-energy He⁺ ion irradiation on Ta as a plasma-facing material

The goal of this work is to assess Ta as a potential plasma-facing material for future fusion reactors in terms of its response to high-flux, low-energy He⁺ ion irradiation. Ta samples were irradiated with 100 eV He⁺ ions at various fluences up to 3.5 × 10²⁵ ions m^-2 while simultaneously heated at constant temperatures in the range 823-1223 K. SEM studies show that irradiated Ta surfaces undergo significant morphology changes that have a strong dependence on both ion fluence and sample temperature. Optical reflectivity complements SEM and demonstrates a vertical growth of surface structures with increasing fluence. Ex situ XPS and XRD both show significant oxidation of the irradiated Ta surfaces, giving further qualitative information on the extent of surface modification. Overall, these irradiation-induced structures on Ta are similar to early-stage "fuzz" structures observed in W. However, Ta exhibits a higher fluence threshold for structure formation. While Ta may have less desirable bulk properties (e.g., thermal conductivity) when compared to W, its higher resilience to He⁺ ion-induced surface modification suggests that surface thermal and mechanical properties may not degrade as quickly in extreme fusion environments; this quality may be a redeeming factor for Ta as a plasma-facing material.