Degradation of thermal conductivity of the damaged layer of tungsten irradiated by helium-plasma

Pure tungsten samples were irradiated by helium plasma in the linear plasma device PSI-2 with an ion energy of 40 eV and a flux of 1.1 × 1022 m−2s−1. The irradiation temperature was from 523 K to 773 K and the fluence was from 1.0 × 1025 to 1.0 × 1026 m−2. A damaged layer of 10 nm thickness was formed on the sample surface with a destroyed crystalline structure. Helium-bubbles and surface modification in nanoscale were observed. Thermal conductivities of the ultra-thin damaged layers were measured by the transient thermoreflectance technique. Result shows that the thermal conductivity reduced two orders of magnitude compared to the bulk value and decreased with increasing irradiation temperature and fluence. Moreover, the helium-irradiated samples were exposed to ELM-like heat load produced by electron beam on EMS-60. The pulse length was 1 ms and each sample was exposed to 5 pulses. Melting occurred under power density of 1.7 GW m−2. As the thermal conductivity of the damaged layer decreased, the molten bath of the irradiated sample deepened. The degraded thermal conductivity led to a lower melting threshold. The characterization of the thermal conductivity of the damaged layer induced by the plasma irradiation is a promising way to estimate the damage level, as well as the failure threshold, of the plasma facing components.