Observation of arc spots initiated on nanostructured tungsten

When tungsten is exposed to helium plasmas, nanostructures are grown on the surface. Field emission properties of the tungsten samples are measured; it is revealed that the field emission was significantly increased as changing the surface morphology by the plasma irradiation. Demonstration of arcing and unipolar arcing on the nanostructured tungsten was conducted in the linear plasma device NAGDIS-II, which simulates the fusion relevant conditions. Arcing can be easily initiated in response to a transient heat load with a pulsed laser, which demonstrates transient heat loads in fusion devices. It is thought that the increase in the field emission and the decrease in the thermal diffusivity near the surface lead to the initiation of arcing. The behavior of the free running arc spots are observed with fast framing camera. Initial behavior of initiated arc spots and an interesting behavior, bifurcation, random motion, etc. are shown.