Unipolar arc is a unique phenomenon, which many people have been trying to investigate experimentally and theoretically. In this paper, we show that unipolar arcing is promptly initiated on tungsten in response to a pulsed heat load when a nanostructure is formed on the surface by exposure to a helium plasma. The behavior of the arc spot is investigated from the observation with a fast framing camera and from the arc trail recorded on the material, and comparisons are made between the observation and numerical modeling. It is revealed that the arc trail has a self-affine fractal nature under magnetized conditions, and it significantly alters with the magnetic field strength.