Merging and directional drift of electron spots during the nonlinear evolution of the current filamentation instability.

The transport and energy deposition of relativistic electron beams in the radially non-uniform plasmas are investigated with two-dimensional electromagnetic particle-in-cell simulations. For the beam with the radius much larger than plasma skin depth, the current filamentation instability excited by the relativistic electron beam can be clearly observed, which breaks the electron beam into a large number of filaments and leads to the formation of strong magnetic field consequently. As the beam-plasma system evolves self-consistently, asymmetric transverse magnetic field, which is associated with plasma density gradient, contributes to the directional drift of electron focal spots and thus can enhance the merge effectively. The effects of different plasma distributions on transport and energy deposition of the eletron beam are compared. Furthermore, an axial electric field is generated in the middle of two filaments and causes the corresponding energy step change when they merge under the action of the magnetic field. The energetic plasma jet, vertical to the merging direction, is shown to result from the defocusing effect of the magnetic field.