Accelerated merging of electron vortices in background vorticity

We report Item features observed in two-dimensional interactions of discret e vortices either isolated in vacuum or immersed in a background vorticity. The vor tices are strings of electron plasma which are produced with a new ly del eloped cathode array and trapped in a Malmberg trap. We observe long -lasting orbital motion of discrete vortices in vacuum, consistent with kin etic equations of point vortices, while a rapid re-organization occurs in t he spatial distribution of vorticity when discrete vortices are immersed in an extended distribution of the background vorticity. The main features of the new observation are accounted for hy a recently-proposed theoretical m odel incorporating collective interaction between two vortices.