Surface modification of tungsten mirrors due to low-energy helium plasma irradiation in the compact PWI simulator APSEDAS

Surface modification of a polycrystalline tungsten (W) mirror due to low-energy helium plasma irradiation has been studied in the compact plasma-wall interaction (PWI) simulator APSEDAS. Many fine irregularities of a few μm are generated on the W surface at T_s∼1400 K due to He bubble formation. On the other hand, a distinctive change in the surface morphology cannot be seen from the SEM micrograph on a W mirror exposed at T _s<710 K. In both samples, the optical reflectivity with the incident angle θ=5° decreases more at shorter wavelength. From in situ measurement of the optical reflectivity with θ=75°, it is found that the optical reflectivity at λ∼555 nm of the low T_s exposure increased by ∼6% and decreased to the original level at a fluence of ∼3×10²⁴ He⁺ m^-2, and then increased again by ∼6% with the fluence up to ∼1.6×10²⁶ He⁺ m^-2. Calculations using a model of the multiple reflection in the modification region suggest that the thickness of the modification layer of the low T_s sample exceeds 70 nm.