Experimental and numerical study of asymmetric vortex merger in a pure electron plasma

The merging of an intense localized vortex with an extended vortex is investigated by means of an experimental analysis performed on a Malmberg-Penning trap with photocathode, and numerical simulations with a two-dimensional (2D) particle in cell (PIC) code. The study is restricted to highly nonlinear conditions, where the perturbative approach does not hold. A very good agreement between experimental results and simulations is obtained. It is found that the localized vortex is firstly wrapped around by the extended vortex, then moves towards the center of the system, eventually approaching an almost stationary state of rotation characterized by the formation of vorticity holes. During the whole evolution the extended vortex gains energy from the field surrounding the vortices, while the energy of the localized vortex remains constant.