Hydrogen isotope (HI) retention in plasma-facing materials (PFMs) is a critical challenge for fuel self-sufficiency and operational safety in future fusion reactors. Tungsten (W), the primary candidate PFM for ITER and DEMO, will be simultaneously exposed to helium (He) ash produced by D-T fusion reactions and externally seeded impurity species such as argon (Ar), which is widely used for divertor radiative cooling and heat-load mitigation. However, the synergistic effects of He-Ar irradiation on W surface microstructure evolution and deuterium (D) retention remain insufficiently understood, particularly regarding the role of Ar in modulating He-induced nanostructures and trap sites. In this study, the effects of He-Ar mixed plasma pre-irradiation on the surface morphology, near-surface damage structure, and D retention behavior of W are systematically investigated using the Multiple Plasma Simulation Linear Device (MPS-LD). High-purity W samples are pre-irradiated with pure He and He-Ar mixed plasmas (Ar mixing ratios of 0%, 3%, 5%, and 8%) at an incident ion energy of 30 eV and a sample temperature of 1023 K. The maximum He fluence reached 8.64 × 10²⁵ He m^–2. After pre-irradiation, all samples were exposed to low-energy D plasma at 500 K with a fluence of 1.8 × 10²⁵ Dm^–2. Surface morphology and cross-sectional microstructure are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. D desorption behavior and total D retention were analyzed by thermal desorption spectroscopy (TDS).

The results indicate that, under pure He plasma irradiation, the W surface undergoes a fluence-dependent structural evolution from nanopores to ripple-like structures and finally to a dense fuzz layer. At a He fluence of 8.64 × 10²⁵ He m^–2, a continuous fuzz layer with a thickness of approximately 176.0 nm is formed, accompanied by a large number of He bubbles in the near-surface region. These He-induced bubbles and fuzz structures provide abundant high-energy trapping sites for D, leading to a significant increase in D retention from 5.1 × 10²⁰ D m^–2 in the non-pre-irradiated sample to 7.78 × 10²¹ D m^–2, corresponding to an enhancement of approximately 15 times. Meanwhile, the TDS spectra exhibit pronounced high-temperature desorption peaks, indicating that D is mainly trapped by medium- and high-binding-energy defects associated with He bubbles, vacancy clusters, and fuzz-related porous structures.

Crucially, the introduction of Ar significantly suppresses He-induced microstructure evolution. With 3% Ar addition, the thickness of the damaged layer decreases from 176.0 to 56.8 nm at the same He fluence, and the He bubbles become smaller, less dense, and more discretely distributed. Correspondingly, the high-temperature D desorption peak is weakened and shifts toward lower temperatures, while the total D retention decreases to 3.39 × 10²¹ D·m^–2. When the Ar content is further increased to 5% and 8%, fuzz formation is almost completely inhibited, and the W surface becomes much smoother, retaining only sparse nanopores or shallow surface features. At an Ar content of 8%, the total D retention decreases to 1.58 × 10²¹ D m^–2, which is approximately 80% lower than under pure He pre-irradiation. These results demonstrate that increasing the Ar content effectively reduces the density of high-energy D trapping sites by suppressing He bubble growth and fuzz formation.

The underlying mechanism is attributed to the competition between He-induced defect generation and Ar-induced surface sputtering. Under pure He irradiation, continuous He accumulation in the near-surface region promotes He bubble nucleation, growth, and coalescence, driving the formation of porous fuzz structures and producing a high density of strong D trapping sites. In contrast, Ar ions induce physical sputtering of the W surface and disturb the stress field and material transport processes required for stable He bubble growth. As a result, the development of He bubbles and fuzz structures is suppressed, the trap energy distribution shifts from high-energy traps toward lower-energy defects, and D retention is significantly reduced. This study clarifies the regulatory role of Ar in He-induced W surface damage and D retention under mixed impurity plasma irradiation. The results reveal that moderate Ar addition can effectively suppress He-induced fuzz formation and reduce D retention in W, providing new experimental evidence for understanding impurity-controlled plasma-wall interactions and tritium retention in future fusion devices.