Temperature-Dependent Irradiation-History Effects in Tungsten Under Sequential Helium and Hydrogen Plasma Exposure

Tungsten plasma-facing components in fusion devices may experience sequential exposure to helium and hydrogen, making irradiation history an important factor in their subsequent structural and hydrogen-release behavior. This study investigated polycrystalline tungsten exposed to He-only, H₂-only, and sequential He → H₂ plasmas at 700, 900, 1100, and 1300 °C. Surface and cross-sectional scanning electron microscopy, X-ray diffraction, instrumented indentation, and thermal desorption spectroscopy (TDS) were used to characterize the resulting changes. SEM observations showed that the largest differences among the H₂-only, He-only, and sequential He → H₂ exposure conditions occurred at 1100 °C, whereas predominantly grain-scale surface morphology was observed for all three conditions at 1300 °C. The bcc-W structure was retained under all investigated conditions, with no detectable secondary crystalline phases. The apparent coherent diffraction domain size, estimated from the W(110) reflection using the Scherrer equation, increased monotonically with irradiation temperature, from approximately 48-55 nm at 700 °C to approximately 126 nm at 1300 °C. At 1100 °C, the H₂-only specimen exhibited a larger apparent coherent diffraction domain size than the He-containing specimens. The nanomechanical response also depended on temperature and exposure history. TDS revealed distinct high-temperature hydrogen-desorption behavior for the sequential He → H₂ specimens, particularly at 1100 and 1300 °C. Overall, the results indicate that the tungsten response under the present sequential protocol depends on both temperature and exposure history, and that structural convergence at elevated temperature does not necessarily correspond to equivalent hydrogen-release behavior.