Melting dynamics of femtosecond laser-irradiated tungsten with non-intrinsic defects

Understanding the structural dynamics of fs laser-induced melting is crucial in the study of a wide range of applications from laser micro-machining to high energy density physics experiments. While much attention has been given to melting dynamics of bulk crystals and thin films, little work has been done on understanding the melting dynamics in materials containing non-intrinsic defects, such as materials in the extreme radiation environments of a fusion reactor. Here we present an ultrafast-electron-diffraction study of fs laser-induced melting in solids with highly populated non-intrinsic defects. The incorporation of these non-intrinsic defects were obtained through high-energy ion irradiation of W. Diffraction experiments show that W subjected to 10 displacement per atom of damage undergoes a melting transition within 10ps. In contrast, the un-irradiated target still displays considerable crystallinity on timescales in excess of 20ps. The observed rapid melting of radiation-damaged W is confirmed by two temperature molecular dynamics simulations revealing the crucial role of defect clusters, particularly nanovoids, in driving the ultrafast melting process. These studies provide atomic-level insights into the ultrafast melting of materials with highly-populated defects.