Effect of Biasing Voltage on Fiber-Form Nanostructured Tungsten Formation by Collisional Helium Arc Plasma Irradiation

Recently, it has been found that fiber-form nanostructures are formed on metal surfaces such as tungsten (W) by irradiation with helium (He) plasma. We are conducting experiments to produce fiber-form nanostructures on W surfaces by He arc discharge plasma irradiation under a gas pressure of 5 kPa. The biasing voltage (V_b) applied between the W substrate and the vacuum vessel was varied from -52 to -242 V to control the incident energy E_i of He ions to the W substrate. Under such high neutral gas pressure, collisions between ions and neutral particles in the sheath cannot be neglected and E_i has a distribution. The results show that as V_b increases, the diameter of the W fiber at the top surface of the fiber-form nanostructure layer becomes smaller. Furthermore, surface blackening due to the nanostructure formation appeared more quickly when V_b was increased. These results suggest that the thickness of the fiber-form nanostructured layer increased with increasing in V_b. Although He ions with E_i above the sputtering threshold of W were also irradiated at V_b = -162 and -242 V, it is suggested that the growth of fiber-form nanostructures is dominant over sputtering erosion in the present experiment.