Effects of He+ energy and irradiation temperature on W sputtering yields under fusion-relevant conditions

The sputtering yields of W coatings irradiated with He+ at different irradiation temperatures under fusion-relevant conditions have been measured as the function of He+ energy ranging from 18 to 300 eV. The sputtering yield of W coatings irradiated at temperature of 673 K increases by one order of magnitude from ∼10−3 to ∼10−2 W/He+ when He+ energy varies from ≤100 eV to ≥150 eV, indicating the existence of W sputtering threshold. The sputtering yield of W coatings irradiated at He+ energy of 300 eV rapidly increases from 4.56 × 10−3 to 1.44 × 10−2 W/He+ when the temperature increases from 293 to 973 K. Measurements by conductive atomic force microscopy show that plenty of nanometer-sized defects are formed after He+ irradiation at a relatively high He+ energy. The serious etching of W coatings at He+ energies of ≥150 eV can be also related to He trapping into defects in the sub-surface layer and their growth, leading to an increase in the thermal instability of W.