Surface modification of F82H steel exposed to low energy, high flux He plasmas

Abstract

Polished F82H steel samples were exposed to He plasmas with an ion incident energy of ∼80 eV at 773-873 K to three central exposure fluences: 6 × 1024, 1 × 1025 and 4 × 1025 He m-2. Pinholes with diameters less than 70 nm are densely distributed on all exposed samples, resulting from the formation and bursting of blisters. Further, tendril-like features in maze-like patterns on the top of terrace-like ridges are visible for the sample exposed to 4 × 1025 He m-2 in the beam center. EDS indicates that the tendrils are tungsten enriched. Voids with different sizes and shapes with a depth up to 3.5 μm are observed across the sample exposed to 1 × 1025 He m-2. Over the area exposed to a fluence of up to 2 × 1024 He m-2, variations of wavy morphology and blisters are also observed, which could correlate with the crystallographic orientation at the surface. These He pre-exposed F82H steel samples were subsequently subjected to the conditions near the plasma edge in the EAST tokamak for 367 D plasma pulses of varied durations. It is found that the tendril-like features are partially destroyed by these plasma exposures. These results suggest that the properties of the material likely undergo substantial changes due to expected plasma exposure in a tokamak environment. The bursting of blisters and erosion of W-enriched tendrils would introduce additional impurities that could negatively influence the operation of a fusion device.