Accelerated Merging of Electron Vortices in Background Vorticity

We report new features observed in two-dimensional interactions of discrete vortices either isolated in vacuum or immersed in a background vorticity. The vortices are strings of electron plasma which are produced with a newly developed cathode array and trapped in a Malmberg trap. We observe long-lasting orbital motion of discrete vortices in vacuum, consistent with kinetic equations of point vortices, while a rapid re-organization occurs in the 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 by a recently-proposed theoretical model incorporating collective interaction between two vortices.